Snow Owl® is a highly scalable, open source terminology server and collaborative authoring platform. It allows you to store, search and author high volumes of terminology artifacts quickly and efficiently.

Here are a few use-cases that Snow Owl could be used for:

• You work in the healthcare industry and are interested in using a terminology server for browsing, accessing and distributing components of various terminologies and classifications to third-party consumers. In this case, you can use Snow Owl to load the necessary terminologies and access them via FHIR and proprietary APIs.

• You are responsible for maintaining and publishing new versions of a particular terminology. In this case, you can use Snow Owl to collaboratively access and author the terminology content and at the end of your release schedule publish it with confidence and zero errors.

• You have an Electronic Health Record system and would like to capture, maintain and query clinical information in a structured and standardized manner. Your Snow Owl terminology server can integrate with your EHR server via standard APIs to provide the necessary access for both terminology binding and data processing and analytics.

In this tutorial, you will be guided through the process of getting Snow Owl up and running, taking a peek inside it, and performing basic operations like importing SNOMED CT RF2 content, searching, and modifying your data. At the end of this tutorial, you should have a good idea of what Snow Owl is, how it works, and hopefully be inspired to see how you can use it for your needs.

There are a few concepts that are core to Snow Owl. Understanding these concepts from the outset will tremendously help ease the learning process.

Terminology / Code System

A terminology (also known as code system, classification and/or ontology) defines and encapsulates a set of terminology components (eg. set of codes with their meanings) and versions. A terminology is identified by a unique name and stored in a repository. Multiple code systems can exist in a single repository besides each other as long as their name is unique.

Terminology Component

A terminology component is a basic element in a code system with actual clinical meaning or use. For example in SNOMED CT, the Concept, Description, Relationship and Reference Set Member are terminology components.

Version

A version that refers to an important snapshot in time, consistent across many terminology components, also known as tag or label. It is often created when the state of the terminology is deemed to be ready to be published and distributed to downstream customers or for internal use. A version is identified by its version ID (or version tag) within a given code system.

Repository

A repository manages changes to a set of data over time in the form of revisions. Conceptually very similar to a source code repository (like a Git repository), but information stored in the repository must conform to a predefined schema (eg. SNOMED CT Concepts RF2 schema) as opposed to storing it in pure binary or textual format. This way a repository can support various full-text search functionalities, semantical queries and evaluations on the stored, revision-controlled terminology data.

A repository is identified by a name and this name is used to refer to the repository when performing create, read, update, delete and other operations against the revisions in it. Repositories organize revisions into branches and commits.

Revision

A revision is the basic unit of information stored in a repository about a terminology component or artifact. It contains two types of information:

• one is the actual data that you care about, for example a single code from a code system with its meaining and properties.

• the other is revision control information (aka revision metadata). Each revision is identified by a random Universally Unique IDentifier (UUID) that is assigned when performing a commit in the repository. Also, during a commit each revision is associated with a branch and timestamp. Revisions can be compared, restored, and merged.

Branch

A set of components under version control may be branched or forked at a point in time so that, from that time forward, two copies of those components may develop at different speeds or in different ways independently of each other. At later point in time the changes made on one of these branches can be merged into the other.

Branches are organized into hierarchies like directories in file systems. A child branch has access to all of the information that is stored on its parent branch up until its baseTimestamp, which is the time the branch was created. Each repository has a predefined root branch, called MAIN.

Commit

A commit represents a set of changes made against a branch in a repository. After a successful commit, the changes made by the commit are immediately available and searchable on the given branch.

Merge / Rebase

A merge/rebase is an operation in which two sets of changes are applied to set of components. A merge/rebase always happens between two branches, denoting one as the source and the other as the target of the operation.

Now that we have our instance up and running, the next step is to understand how to communicate with it. Fortunately, Snow Owl provides very comprehensive and powerful APIs to interact with your instance.

REST API

Among the few things that can be done with the API are as follows:

• Check your instance health, status, and statistics

• Administer your instance data

• Perform CRUD (Create, Read, Update, and Delete) and search operations against your terminologies

• Execute advanced search operations such as paging, sorting, filtering, scripting, aggregations, and many others

Now that we have a code system, let's take a look at its content! We can list concepts using either the SNOMED CT API tailored to this tooling, or the FHIR API for a representation that is uniform across different kinds of code systems. For the sake of simplicity, we will use the former in this example.

To list all available concepts in a code system, use the following command (just as with importing, the second SNOMEDCT in the request path represents the code system identifier):

curl http://localhost:8080/snowowl/snomedct/SNOMEDCT/concepts?pretty

The expected response is:

{
  "items": [ ],
  "limit": 50,
  "total": 0
}

The concept list is empty, indicating that we haven't imported anything into Snow Owl - yet.

Snow Owl requires Java 11 or newer version. Specifically as of this writing, it is recommended that you use JDK (Oracle of OpenJDK is preferred) version 11.0.2. Java installation varies from platform to platform so we won’t go into those details here. Oracle’s recommended installation documentation can be found on Oracle’s website. Suffice to say, before you install Snow Owl, please check your Java version first by running (and then install/upgrade accordingly if needed):

java -version
echo $JAVA_HOME

Once we have Java set up, we can then download and run Snow Owl. The binaries are available at the Releases pages. For each release, you have a choice among a zip or tar archive, a DEB or RPM package.

Installation example with zip

For simplicity, let's use a zip file.

Let's download the most recent Snow Owl release as follows:

curl -L -O https://github.com/b2ihealthcare/snow-owl/releases/download/<version>/snow-owl-oss-<version>.zip

Then extract it as follows:

unzip snow-owl-oss-<version>.zip

It will then create a bunch of files and folders in your current directory. We then go into the bin directory as follows:

cd snow-owl-oss-<version>/bin

And now we are ready to start the instance:

./startup

Successfully running instance

If everything goes well with the installation, you should see a bunch of log messages that look like below:

TODO example output

Let’s start with a basic health check, which we can use to see how our instance is doing. We’ll be using curl to do this but you can use any tool that allows you to make HTTP/REST calls. Let’s assume that we are still on the same node where we started Snow Owl on and open another command shell window.

We will be using Snow Owl's Core API to check its status. You can run the following command by clicking the "Copy" link on the right side and pasting it into a terminal.

curl http://localhost:8080/snowowl/info?pretty

And the response:

{
  "version": "<version>",
  "description": "You Know, for Terminologies",
  "repositories": {
    "items": [ {
      "id" : "snomed",
      "health" : "GREEN",
      "diagnosis" : "",
      "indices" : [ {
        "index" : "snomed-relationship",
        "status" : "GREEN"
      }, {
        "index" : "snomed-commit",
        "status" : "GREEN"
      }, ...
    } ]
  }
}

We can see the installed version along with available repositories, their overall health (eg. "snomed" with health "GREEN"), associated indices and status (eg. "snomed-relationship" with status "GREEN").

Repository indices store content for any number of code systems that share the same data structure and API, in the case of "snomed" the International Edition of SNOMED CT and its extensions.

Whenever we ask for repository status, we either get GREEN, YELLOW, or RED and an optional diagnosis message.

• GREEN - everything is good (repository is fully functional)

• YELLOW - some data or functionality is not available, or diagnostic operation is in progress (repository is partially functional)

• RED - diagnostic operation required in order to continue (repository is not functional)

Now let's take a peek at our code systems:

curl http://localhost:8080/snowowl/codesystems?pretty

The response:

{
  "items" : [ ],
  "limit" : 0,
  "total" : 0
}

...it sure looks empty! This is expected, as Snow Owl does not contain any predefined code system metadata out of the box. We can create the first code system with the following request:

curl -X POST \
-H "Content-type: application/json" \
http://localhost:8080/snowowl/codesystems \
-d '{
  "id": "SNOMEDCT",
  "url": "http://snomed.info/sct/900000000000207008",
  "title": "SNOMED CT International Edition",
  "language": "en",
  "description": "SNOMED CT International Edition",
  "status": "active",
  "copyright": "(C) 2022 International Health Terminology Standards Development Organisation 2002-2022. All rights reserved.",
  "owner": "snowowl",
  "contact": "https://snomed.org",
  "oid": "2.16.840.1.113883.6.96",
  "toolingId": "snomed",
  "settings": {
    "moduleIds": [
      "900000000000207008",
      "900000000000012004"
    ],
    "locales": [
      "en-x-900000000000508004",
      "en-x-900000000000509007"
    ],
    "languages": [
      {
        "languageTag": "en",
        "languageRefSetIds": [
          "900000000000509007",
          "900000000000508004"
        ]
      },
      {
        "languageTag": "en-us",
        "languageRefSetIds": [
          "900000000000509007"
        ]
      },
      {
        "languageTag": "en-gb",
        "languageRefSetIds": [
          "900000000000508004"
        ]
      }
    ],
    "publisher": "SNOMED International",
    "namespace": "373872000",
    "maintainerType": "SNOMED_INTERNATIONAL"
  }
}'

The request body includes:

• The code system identifier "SNOMEDCT"

• Various pieces of metadata offering a human-readable title, ownership and contact information, code system status, URL and OID for identification, etc.

• The tooling identifier "snomed" that points to the repository that will store content

• Additional code system settings stored as key-value pairs

If everything goes well, the command will run without any errors (the server returns a "204 No Content" response). We can double-check that code system metadata has been registered correctly with the following request:

curl http://localhost:8080/snowowl/codesystems/SNOMEDCT?pretty

The expected response is:

{
  "id": "SNOMEDCT",
  "url": "http://snomed.info/sct/900000000000207008",
  "title": "SNOMED CT International Edition",
  "language": "en",
  ...
  "branchPath": "MAIN/SNOMEDCT",
  ...
}

In addition to the submitted values, you will find that additional administrative properties also appear in the output. One example is branchPath which specifies the working branch of the code system within the repository.

Snow Owl is both a simple and complex product. We’ve so far learned the basics of what it is, how to look inside of it, and how to work with it using some of the available APIs. Hopefully this tutorial has given you a better understanding of what Snow Owl is and more importantly, inspired you to further experiment with the rest of its great features!

This section includes information on how to setup Snow Owl and get it running, including:

• Downloading

• Installing

• Starting

• Configuring

Java (JVM) Version

Snow Owl is built using Java, and requires at least Java 11 in order to run. Only Oracle’s Java and the OpenJDK are supported. The same JVM version should be used on all Snow Owl nodes and clients.

We recommend installing Java version 11.0.x or a later version in the Java 11 release series. We recommend using a supported LTS version of Java.

The version of Java that Snow Owl will use can be configured by setting the JAVA_HOME environment variable.

Snow Owl® is a highly scalable, open source terminology server with revision-control capabilities and collaborative authoring platform features. It allows you to store, search and author high volumes of terminology artifacts quickly and efficiently. If you’d like to see Snow Owl in action, the Snowray Terminology Service™ provides a managed terminology server and high-quality terminology content management from your web browser.

Introduction

Features include:

• Revision-controlled authoring

  • Maintains multiple versions (including unpublished and published) for each terminology artifact and provides APIs to access them all

  • Independent work branches offer work-in-process isolation, external business workflow integration and team collaboration

• SNOMED CT and others

  • SNOMED CT terminology support

    • RF2 Release File Specification as of 2023-09-01

    • Support for Relationships with concrete values

    • Official and Custom Reference Sets

    • Expression Constraint Language v2.1.0 spec, implementation

    • Compositional Grammar 2.3.1 spec, implementation

    • Expression Template Language 1.0.0 spec, implementation

  • With its modular design, the server can maintain multiple terminologies (including local codes, mapping sets, value sets)

• Various set of APIs

  • SNOMED CT API (RESTful and native Java API)

  • FHIR API R4 v4.0.1 spec

  • CIS API 1.0 see reference implementation

• Highly extensible and configurable

  • Simple to use plug-in system makes it easy to develop and add new terminology tooling/API or any other functionality

• Built on top of Elasticsearch (highly scalable, distributed, open source search engine)

  • Connect to your existing cluster or use the embedded instance (supports up to Elasticsearch 8.x)

  • All the power of Elasticsearch is available (full-text search support, monitoring, analytics and many more)

Download

• WINDOWS - sha

• LINUX - sha

• RPM - sha

• DEB - sha

View the detailed release notes here.

Not the version you're looking for? View past releases.

Install and Run

NOTE: You need to have version 17 of the JDK installed for local builds and running the development environment. Official releases include the runtime.

Once you have downloaded the appropriate package:

• Run bin/snowowl.sh on unix, or bin/snowowl.bat on windows

• Run curl http://localhost:8080/snowowl/info to access server health status information

• Run curl http://localhost:8080/snowowl/fhir/metadata to access FHIR terminology capabilities

• Navigate to http://localhost:8080/snowowl to access the REST API documentation page

• See more documentation at SNOMED CT API docs and at FHIR API docs

Learn Snow Owl

• Getting Started

• Set up Snow Owl

• Configuring Snow Owl

• FHIR API

• SNOMED CT API

• Admin API

Building from source

Snow Owl uses Maven for its build system. In order to create a distribution, simply run the following command in the cloned directory.

./mvnw clean package

The distribution packages can be found in the releng/com.b2international.snowowl.server.update/target folder, when the build is complete.

To run the test cases, use the following command:

./mvnw clean verify

Development

These instructions will get Snow Owl up and running on your local machine for development and testing purposes.

Prerequisites

Snow Owl is an Equinox-OSGi based server. To develop plug-ins for Snow Owl you need to use Eclipse as IDE:

• Download Eclipse IDE for Eclipse Committers 2023-12 package from here: https://www.eclipse.org/downloads/packages/release/2023-12/r/eclipse-ide-eclipse-committers

Required Eclipse plug-ins in order (install the listed features via Help → Install New Software...):

Note: you may have to untick the Show only the latest versions of the available software checkbox to get older versions of a feature. Please use the exact version specified below, not the latest point release.

• Groovy Development Tools (https://groovy.jfrog.io/ui/native/plugins-release/e4.30 or https://groovy.jfrog.io/artifactory/plugins-release-local/org/codehaus/groovy/groovy-eclipse-integration/5.2.0/e4.30)

  • Eclipse Groovy Development Tools - 5.2.0 (in category "Main Package")

• MWE2 (https://download.eclipse.org/modeling/emft/mwe/updates/releases/2.16.0/)

  • MWE SDK 1.10.0 (MWE)

• Xtext/Xtend (https://download.eclipse.org/modeling/tmf/xtext/updates/releases/2.33.0/)

  • Xtend IDE 2.33.0 (Xtext)

  • Xtext Complete SDK 2.33.0 (Xtext)

Eclipse Preferences

Make sure you have the following preferences enabled/disabled.

• Plug-in development API baseline errors is set to Ignored (Preferences > Plug-in Development > API Baselines)

• The Plugin execution not covered by lifecycle configuration: org.apache.maven.plugins:maven-clean-plugin:2.5:clean type of errors can be ignored or changed to Warnings in Preferences→Maven→Errors/Warnings.

• Set the workspace encoding to UTF-8 (Preferences→General→Workspace)

• Set the line endings to Unix style (Preferences→General→Workspace)

• Set the number of imports and static imports wildcard limit to 5 (Preferences→Java→Code Style→Organize Imports)

Git configuration

• Make sure the Git line endings are set to input (Preferences→Team→Git→Configuration - add key if missing core.autocrlf = input)

Maven Settings

• Make sure the settings.xml in your ~/.m2/settings.xml location is updated with the content from the settings.xml from this repository's root folder.

First steps

1. Import all projects into your Eclipse workspace and wait for the build to complete

2. Select all projects and hit Alt + F5 and trigger an update to all Maven projects manually (to download dependencies from Maven)

3. Open the target-platform/target-platform.target file

4. Wait until Eclipse resolves the target platform (click on the Resolve button if it refuses to do so) and then click on Set as Active Target platform

5. Wait until the build is complete and you have no compile errors

6. Launch snow-owl-oss launch configuration in the Run Configurations menu

7. Navigate to http://localhost:8080/snowowl

Contributing

Please see CONTRIBUTING.md for details.

Versioning

Our releases use semantic versioning. You can find a chronologically ordered list of notable changes in CHANGELOG.md.

License

This project is licensed under the Apache 2.0 License. See LICENSE for details and refer to NOTICE for additional licensing notes and uses of third-party components.

Acknowledgements

In March 2015, SNOMED International generously licensed the Snow Owl Terminology Server components supporting SNOMED CT. They subsequently made the licensed code available to their members and the global community under an open-source license.

In March 2017, NHS Digital licensed the Snow Owl Terminology Server to support the mandatory adoption of SNOMED CT throughout all care settings in the United Kingdom by April 2020. In addition to driving the UK’s clinical terminology efforts by providing a platform to author national clinical codes, Snow Owl will support the maintenance and improvement of the dm+d drug extension which alone is used in over 156 million electronic prescriptions per month. Improvements to the terminology server made under this agreement will be made available to the global community.

Many other organizations have directly and indirectly contributed to Snow Owl, including: Singapore Ministry of Health; American Dental Association; University of Nebraska Medical Center (USA); Federal Public Service of Public Health (Belgium); Danish Health Data Authority; Health and Welfare Information Systems Centre (Estonia); Department of Health (Ireland); New Zealand Ministry of Health; Norwegian Directorate of eHealth; Integrated Health Information Systems (Singapore); National Board of Health and Welfare (Sweden); eHealth Suisse (Switzerland); and the National Library of Medicine (USA).

GET the ROOT concept:

And the response:

Search by ECL:

And the response:

Let's import an RF2 release in SNAPSHOT mode so that we can further explore the available SNOMED CT APIs! To do so, use the appropriate request from the as follows (the second SNOMEDCT in the request path represents the code system identifier):

Curl will display the entire interaction between it and the server, including many request and response headers. We are interested in these two (response) rows in particular:

The first one indicates that the file was uploaded successfully and a resource has been created to track import progress, while the second row indicates the location of this resource.

The process itself is asynchronous and its status can be checked by periodically sending a GET request to the location indicated by the response header:

The expected response while the import is running:

Upon completion, you should receive a different response which lists component identifiers visited during the import as well as any defects encountered in uploaded release files:

Snow Owl is provided in the following package formats:

Snow Owl is provided as a .zip and as a .tar.gz package. These packages can be used to install Snow Owl on any system and are the easiest package format to use when trying out Snow Owl.

The latest stable version of Snow Owl can be found on the page.

Download and install the zip package

The .zip archive for Snow Owl can be downloaded and installed as follows:

Download and install the .tar.gz package

The .tar.gz archive for Snow Owl can be downloaded and installed as follows:

Running Snow Owl from the command line

Snow Owl can be started from the command line as follows:

By default, Snow Owl runs in the foreground, prints its logs to the standard output (stdout), and can be stopped by pressing Ctrl-C.

Checking that Snow Owl is running

You can test that your instance is running by sending an HTTP request to Snow Owl's status endpoint:

which should give you a response like this:

Running in the background

You can send the Snow Owl process to the background using the combination of nohup and the & character:

Log messages can be found in the $SO_HOME/serviceability/logs/ directory.

To shut down Snow Owl, you can kill the process ID directly:

or using the provided shutdown script:

Directory layout of .zip and .tar.gz archives:

The .zip and .tar.gz packages are entirely self-contained. All files and directories are, by default, contained within $SO_HOME — the directory created when unpacking the archive.

This is very convenient because you don’t have to create any directories to start using Snow Owl, and uninstalling Snow Owl is as easy as removing the $SO_HOME directory. However, it is advisable to change the default locations of the config directory, the data directory, and the logs directory so that you do not delete important data later on.

Next steps

You now have a test Snow Owl environment set up. Before you start serious development or go into production with Snow Owl, you must do some additional setup:

Snow Owl uses SLF4J and Logback for logging.

The logging configuration file (serviceability.xml) can be used to configure Snow Owl logging. The logging configuration file location depends on your installation method, by default it is located in the ${SO_HOME}/configuration folder.

Extensive information on how to customize logging and all the supported appenders can be found on the Logback documentation.

You should rarely need to change Java Virtual Machine (JVM) options. If you do, the most likely change is setting the heap size.

The preferred method of setting JVM options (including system properties and JVM flags) is via the the SO_JAVA_OPTS environment variable. For instance:

export SO_JAVA_OPTS="$SO_JAVA_OPTS -Djava.io.tmpdir=/path/to/temp/dir"
./bin/startup

When using the RPM or Debian packages, SO_JAVA_OPTS can be specified in the system configuration file.

Snow Owl ships with good defaults and requires very little configuration.

Config files location

Snow Owl has three configuration files:

• snowowl.yml for configuring Snow Owl

• serviceability.xml for configuring Snow Owl logging

• elasticsearch.yml for configuring the underlying Elasticsearch instance in case of embedded deployments

These files are located in the config directory, whose default location depends on whether or not the installation is from an archive distribution (tar.gz or zip) or a package distribution (Debian or RPM packages).

For the archive distributions, the config directory location defaults to $SO_PATH_HOME/configuration. The location of the config directory can be changed via the SO_PATH_CONF environment variable as follows:

SO_PATH_CONF=/path/to/my/config ./bin/startup

Alternatively, you can export the SO_PATH_CONF environment variable via the command line or via your shell profile.

For the package distributions, the config directory location defaults to /etc/snowowl. The location of the config directory can also be changed via the SO_PATH_CONF environment variable, but note that setting this in your shell is not sufficient. Instead, this variable is sourced from /etc/default/snowowl (for the Debian package) and /etc/sysconfig/snowowl (for the RPM package). You will need to edit the SO_PATH_CONF=/etc/snowowl entry in one of these files accordingly to change the config directory location.

Config file format

The configuration format is YAML. Here is an example of changing the path of the data directory:

path:
    data: /var/lib/snowowl

Settings can also be flattened as follows:

path.data: /var/lib/snowowl

Environment variable substitution

Environment variables referenced with the ${...} notation within the configuration file will be replaced with the value of the environment variable, for instance:

repository.host: ${HOSTNAME}
repository.port: ${SO_REPOSITORY_PORT}

Snow Owl is also available as Docker images. The images use centos:7 as the base image.

A list of all published Docker images and tags is available at Docker Hub.

These images are free to use under the Apache 2.0 license. They contain open source features only.

Pulling the image

Obtaining Snow Owl for Docker is as simple as issuing a docker pull command against the Docker Hub registry.

docker pull snow-owl-oss:latest

Running Snow Owl from the command line

Development mode

Snow Owl can be quickly started for development or testing use with the following command:

docker run -p 8080:8080 snow-owl-oss:latest

Production mode

The following example brings up Snow Owl instance with its dedicated Elasticsearch node. To bring up the cluster, use the docker-compose.yml and just type:

docker-compose up

The node snowowl listens on localhost:8080 while it talks to the elasticsearch node over a Docker network.

To stop the cluster, type docker-compose down. Data volumes/mounts will persist, so it's possible to start the stack again with the same data using docker-compose up`.

Configuring Snow Owl with Docker

Snow Owl loads its configuration from files under /usr/share/snowowl/config/. These configuration files are documented in the Configure Snow Owl pages.

The image offers several methods for configuring Snow Owl settings with the conventional approach being to provide customized files, that is to say, snowowl.yml. It's also possible to use environment variables to set options:

• A. Bind-mounted configuration Create your custom config file and mount this over the image's corresponding file. For example, bind-mounting a custom_snowowl.yml with docker run can be accomplished with the parameter:

-v full_path_to/custom_snowowl.yml:/usr/share/snowowl/configuration/snowowl.yml

• B. Customized image In some environments, it may make more sense to prepare a custom image containing your configuration. A Dockerfile to achieve this may be as simple as:

FROM snow-owl-oss:{version}
COPY --chown=snowowl:snowowl snowowl.yml /usr/share/snowowl/configuration/

You could then build and try the image with something like:

docker build --tag=snow-owl-oss-custom .
docker run -ti -v /usr/share/snowowl/resources snow-owl-oss-custom

Notes for production use and defaults

We have collected a number of best practices for production use. Any Docker parameters mentioned below assume the use of docker run.

By default, Snow Owl runs inside the container as user snowowl using uid:gid 1000:1000.

• If you are bind-mounting a local directory or file, ensure it is readable by this user, while the <<path-settings,data and log dirs>> additionally require write access. A good strategy is to grant group access to gid 1000 or 0 for the local directory. As an example, to prepare a local directory for storing data through a bind-mount:

  mkdir sodatadir
  chmod g+rwx sodatadir
  chgrp 1000 sodatadir

• It is important to ensure increased ulimits for nofile and nproc are available for the Snow Owl containers. Verify the init system for the Docker daemon is already setting those to acceptable values and, if needed, adjust them in the Daemon, or override them per container, for example using docker run:

  --ulimit nofile=65535:65535

NOTE: One way of checking the Docker daemon defaults for the aforementioned ulimits is by running:

  docker run --rm centos:7 /bin/bash -c 'ulimit -Hn && ulimit -Sn && ulimit -Hu && ulimit -Su'

• Swapping needs to be disabled for performance and stability. This can be achieved through any of the methods mentioned in the system settings.

• The image exposes TCP ports 8080 and 2036.

• Use the SO_JAVA_OPTS environment variable to set heap size. For example, to use 16GB use SO_JAVA_OPTS="-Xms16g -Xmx16g" with docker run.

• Pin your deployments to a specific version of the Snow Owl OSS Docker image. For example, snow-owl-oss:7.2.0.

• Consider centralizing your logs by using a different https://docs.docker.com/engine/admin/logging/overview/[logging driver]. Also note that the default json-file logging driver is not ideally suited for production use.

Snow Owl (with embedded Elasticsearch) uses a lot of file descriptors or file handles. Running out of file descriptors can be disastrous and will most probably lead to data loss. Make sure to increase the limit on the number of open files descriptors for the user running Snow Owl to 65,536 or higher.

For the .zip and .tar.gz packages, set ulimit -n 65536 as root before starting Snow Owl, or set nofile to 65536 in /etc/security/limits.conf.

RPM and Debian packages already default the maximum number of file descriptors to 65536 and do not require further configuration.

The RPM for Snow Owl can be downloaded from the Downloads section. It can be used to install Snow Owl on any RPM-based system such as OpenSuSE, SLES, Centos, Red Hat, and Oracle Enterprise.

Download and install

wget https://github.com/b2ihealthcare/snow-owl/releases/download/<version>/snow-owl-oss-<version>.rpm
wget https://github.com/b2ihealthcare/snow-owl/releases/download/<version>/snow-owl-oss-<version>.rpm.sha512
shasum -a 512 -c snow-owl-oss-<version>.rpm.sha512 # Compares the SHA of the downloaded RPM and the published checksum, which should output `snow-owl-oss-<version>.rpm: OK`.
sudo rpm --install snow-owl-oss-<version>.rpm

Running Snow Owl with SysV init

Use the chkconfig command to configure Snow Owl to start automatically when the system boots up:

sudo chkconfig --add snowowl

Snow Owl can be started and stopped using the service command:

sudo -i service snowowl start
sudo -i service snowowl stop

If Snow Owl fails to start for any reason, it will print the reason for failure to STDOUT. Log files can be found in /var/log/snowowl/.

Running Snow Owl with systemd

To configure Snow Owl to start automatically when the system boots up, run the following commands:

sudo /bin/systemctl daemon-reload
sudo /bin/systemctl enable snowowl.service

Snow Owl can be started and stopped as follows:

sudo systemctl start snowowl.service
sudo systemctl stop snowowl.service

These commands provide no feedback as to whether Snow Owl was started successfully or not. Instead, this information will be written in the log files located in /var/log/snowowl/.

Checking that Snow Owl is running

You can test that your Snow Owl instance is running by sending an HTTP request to:

curl http://localhost:8080/snowowl/admin/info

which should give you a response something like this:

{
  "version": "7.2.0",
  "description": "You Know, for Terminologies",
  "repositories": {
    "items": [
      {
        "id": "snomedStore",
        "health": "GREEN"
      }
    ]
  }
}

Configuring Snow Owl

Snow Owl defaults to using /etc/snowowl for runtime configuration. The ownership of this directory and all files in this directory are set to root:snowowl on package installation and the directory has the setgid flag set so that any files and subdirectories created under /etc/snowowl are created with this ownership as well (e.g., if a keystore is created using the keystore tool). It is expected that this be maintained so that the Snow Owl process can read the files under this directory via the group permissions.

Snow Owl loads its configuration from the /etc/snowowl/snowowl.yml file by default. The format of this config file is explained in Configuring Snow Owl.

Directory layout of RPM

The RPM places config files, logs, and the data directory in the appropriate locations for an RPM-based system:

Next steps

You now have a test Snow Owl environment set up. Before you start serious development or go into production with Snow Owl, you must do some additional setup:

• Learn how to configure Snow Owl.

• Configure important Snow Owl settings.

• Configure important system settings.

The Debian package for Snow Owl can be downloaded from the Downloads section. It can be used to install Snow Owl on any Debian-based system such as Debian and Ubuntu.

Download and install

wget https://github.com/b2ihealthcare/snow-owl/releases/download/<version>/snow-owl-oss-<version>.deb
wget https://github.com/b2ihealthcare/snow-owl/releases/download/<version>/snow-owl-oss-<version>.deb.sha512
shasum -a 512 -c snow-owl-oss-<version>.deb.sha512 # Compares the SHA of the downloaded Debian package and the published checksum, which should output `snow-owl-oss-<version>.deb: OK`.
sudo dpkg -i snow-owl-oss-<version>.deb

Running Snow Owl with SysV init

Use the update-rc.d command to configure Snow Owl to start automatically when the system boots up:

sudo update-rc.d snowowl defaults 95 10

Snow Owl can be started and stopped using the service command:

sudo -i service snowowl start
sudo -i service snowowl stop

If Snow Owl fails to start for any reason, it will print the reason for failure to STDOUT. Log files can be found in /var/log/snowowl/.

Running Snow Owl with systemd

To configure Snow Owl to start automatically when the system boots up, run the following commands:

sudo /bin/systemctl daemon-reload
sudo /bin/systemctl enable snowowl.service

Snow Owl can be started and stopped as follows:

sudo systemctl start snowowl.service
sudo systemctl stop snowowl.service

These commands provide no feedback as to whether Snow Owl was started successfully or not. Instead, this information will be written in the log files located in /var/log/snowowl/.

Checking that Snow Owl is running

You can test that your Snow Owl instance is running by sending an HTTP request to:

curl http://localhost:8080/snowowl/admin/info

which should give you a response something like this:

{
  "version": "7.2.0",
  "description": "You Know, for Terminologies",
  "repositories": {
    "items": [
      {
        "id": "snomedStore",
        "health": "GREEN"
      }
    ]
  }
}

Configuring Snow Owl

Snow Owl defaults to using /etc/snowowl for runtime configuration. The ownership of this directory and all files in this directory are set to root:snowowl on package installation and the directory has the setgid flag set so that any files and subdirectories created under /etc/snowowl are created with this ownership as well (e.g., if a keystore is created using the keystore tool). It is expected that this be maintained so that the Snow Owl process can read the files under this directory via the group permissions.

Snow Owl loads its configuration from the /etc/snowowl/snowowl.yml file by default. The format of this config file is explained in Configuring Snow Owl.

Directory layout of Debian package

The Debian package places config files, logs, and the data directory in the appropriate locations for a Debian-based system:

Next steps

You now have a test Snow Owl environment set up. Before you start serious development or go into production with Snow Owl, you must do some additional setup:

• Learn how to configure Snow Owl.

• Configure important Snow Owl settings.

• Configure important system settings.

By default, Snow Owl is starting and connecting to an embedded Elasticsearch cluster available on http://localhost:9200. This cluster has only a single node and its discovery method is set to single-node, which means it is not able to connect to other Elasticsearch clusters and will be used exclusively by Snow Owl.

This single node Elasticsearch cluster can easily serve Snow Owl in testing, evaluation and small authoring environments, but it is recommended to customize how Snow Owl connects to an Elasticsearch cluster in larger environments (especially when planning to scale with user demand).

You have two options to configure Elasticsearch used by Snow Owl.

Configure the embedded instance

The first option is to configure the underlying Elasticsearch instance by editing the configuration file elasticsearch.yml, which depending on your installation is available in the configuration directory (you can create the file, if it is not available, Snow Owl will pick it up during the next startup).

Connect to a remote cluster

The second option is to configure Snow Owl to use a remote Elasticsearch cluster without the embedded instance. In order to use this feature you need to set the repository.index.clusterUrl configuration parameter to the remote address of your Elasticsearch cluster. When Snow Owl is configured to connect to a remote Elasticsearch cluster, it won't boot up the embedded instance, which reduces the memory requirements of Snow Owl slightly.

You can connect to self-hosted clusters or hosted solutions provided by and for example.

Most operating systems try to use as much memory as possible for file system caches and eagerly swap out unused application memory. This can result in parts of the JVM heap or even its executable pages being swapped out to disk.

Swapping is very bad for performance, and should be avoided at all costs. It can cause garbage collections to last for minutes instead of milliseconds and can cause services to respond slowly or even time out.

There are two approaches to disabling swapping. The preferred option is to completely disable swap, but if this is not an option, you can minimize swappiness.

Disable all swap files

Usually Snow Owl is the only service running on a box, and its memory usage is controlled by the JVM options. There should be no need to have swap enabled.

On Linux systems, you can disable swap temporarily by running:

To disable it permanently, you will need to edit the /etc/fstab file and comment out any lines that contain the word swap.

Configure swappiness

Another option available on Linux systems is to ensure that the sysctl value vm.swappiness is set to 1. This reduces the kernel’s tendency to swap and should not lead to swapping under normal circumstances, while still allowing the whole system to swap in emergency conditions.

Snow Owl uses a mmapfs directory by default to store its data. The default operating system limits on mmap counts is likely to be too low, which may result in out of memory exceptions.

On Linux, you can increase the limits by running the following command as root:

sysctl -w vm.max_map_count=262144

To set this value permanently, update the vm.max_map_count setting in /etc/sysctl.conf. To verify after rebooting, run sysctl vm.max_map_count.

The RPM and Debian packages will configure this setting automatically. No further configuration is required.

While Snow Owl requires very little configuration, there are a number of settings which need to be considered before going into production.

The following settings must be considered before going to production:

Elasticsearch settings

By default, Snow Owl includes the OSS version of Elasticsearch and runs it in embedded mode to store terminology data and make it available for search. This is convenient for single node environments (eg. for evaluation, testing and development), but it might not be sufficient when you go into production.

To configure Snow Owl to connect to an Elasticsearch cluster, change the clusterUrl property in the snowowl.yml configuration file:

repository:
  index:
    clusterUrl: http://your.es.cluster:9200 # the ES cluster URL
    clusterUsername: snowowl # Optional username to connect to a protected ES cluster
    clusterPassword: snowowl_password # Optional password to connect to a protected ES cluster

The value for this setting should be a valid HTTP URL point to the HTTP API of your Elasticsearch cluster, which by default runs on port 9200.

Path settings

If you are using the .zip or .tar.gz archives, the data and logs directories are sub-folders of $SO_HOME. If these important folders are left in their default locations, there is a high risk of them being deleted while upgrading Snow Owl to a new version.

In production use, you will almost certainly want to change the locations of the data and log folders.

path:
  data: /var/data/snowowl

The RPM and Debian distributions already use custom paths for data and logs.

Network settings

To allow clients to connect to Snow Owl, make sure you open access to the following ports:

• 8080/TCP:: Used by Snow Owl Server's REST API for HTTP access

• 8443/TCP:: Used by Snow Owl Server's REST API for HTTPS access

• 2036/TCP:: Used by the Net4J binary protocol connecting Snow Owl clients to the server

Setting the heap size

By default, Snow Owl tells the JVM to use a heap with a minimum and maximum size of 2 GB. When moving to production, it is important to configure heap size to ensure that Snow Owl has enough heap available.

To configure the heap size settings, change the -Xms and -Xmx settings in the SO_JAVA_OPTS environment variable.

# Set the minimum and maximum heap size to 12 GB.
SO_JAVA_OPTS="-Xms12g -Xmx12g" ./bin/startup

The value for these setting depends on the amount of RAM available on your server and whether you are running Elasticsearch on the some node as Snow Owl (either embedded or as a service) or running it in its own cluster. Good rules of thumb are:

• Set the minimum heap size (Xms) and maximum heap size (Xmx) to be equal to each other.

• Too much heap can subject to long garbage collection pauses.

• Set Xmx to no more than 50% of your physical RAM, to ensure that there is enough physical RAM left for kernel file system caches.

• Snow Owl connecting to a remote Elasticsearch cluster requires less memory, but make sure you still allocate enough for your use cases (classification, batch processing, etc.).

Snow Owl uses a number of thread pools for different types of operations. It is important that it is able to create new threads whenever needed. Make sure that the number of threads that the Snow Owl user can create is at least 4096.

This can be done by setting ulimit -u 4096 as root before starting Snow Owl, or by setting nproc to 4096 in /etc/security/limits.conf.

The package distributions when run as services under systemd will configure the number of threads for the Snow Owl process automatically. No additional configuration is required.

Ideally, Snow Owl should run alone on a server and use all of the resources available to it. In order to do so, you need to configure your operating system to allow the user running Snow Owl to access more resources than allowed by default.

The following settings must be considered before going to production:

• Disable swapping

• Increase file descriptors

• Ensure sufficient virtual memory

• Ensure sufficient threads

• Tweaking for performance

Configuring system settings

Where to configure systems settings depends on which package you have used to install Snow Owl, and which operating system you are using.

When using the .zip or .tar.gz packages, system settings can be configured:

• temporarily with ulimit, or

• permanently in /etc/security/limits.conf.

When using the RPM or Debian packages, most system settings are set in the system configuration file. However, systems which use systemd require that system limits are specified in a systemd configuration file.

ulimit

On Linux systems, ulimit can be used to change resource limits on a temporary basis. Limits usually need to be set as root before switching to the user that will run Snow Owl. For example, to set the number of open file handles (ulimit -n) to 65,536, you can do the following:

sudo su # Become `root`
ulimit -n 65536 # Change the max number of open files
su snowowl # Become the `snowowl` user in order to start Snow Owl

The new limit is only applied during the current session.

You can consult all currently applied limits with ulimit -a.

/etc/security/limits.conf

On Linux systems, persistent limits can be set for a particular user by editing the /etc/security/limits.conf file. To set the maximum number of open files for the snowowl user to 65,536, add the following line to the limits.conf file:

snowowl  -  nofile  65536

This change will only take effect the next time the snowowl user opens a new session.

# session    required   pam_limits.so

Sysconfig file

When using the RPM or Debian packages, system settings and environment variables can be specified in the system configuration file, which is located in:

However, for systems which uses systemd, system limits need to be specified via systemd.

Systemd configuration

When using the RPM or Debian packages on systems that use systemd, system limits must be specified via systemd.

The systemd service file (/usr/lib/systemd/system/snowowl.service) contains the limits that are applied by default.

To override them, add a file called /etc/systemd/system/snowowl.service.d/override.conf (alternatively, you may run sudo systemctl edit snowowl which opens the file automatically inside your default editor). Set any changes in this file, such as:

[Service]
LimitMEMLOCK=infinity

Once finished, run the following command to reload units:

sudo systemctl daemon-reload

Scheduler

An orderly shutdown of Snow Owl ensures that Snow Owl has a chance to cleanup and close outstanding resources. For example, an instance that is shutdown in an orderly fashion will initiate an orderly shutdown of the embedded Elasticsearch instance, gracefully close and disconnect connections and perform other related cleanup activities. You can help ensure an orderly shutdown by properly stopping Snow Owl.

If you’re running Snow Owl as a service, you can stop Snow Owl via the service management functionality provided by your installation.

If you’re running Snow Owl directly, you can stop Snow Owl by sending Ctrl-C if you’re running Snow Owl in the console, or by invoking the provided shutdown script as follows:

The method for starting Snow Owl varies depending on how you installed it.

Archive packages (.tar.gz, .zip)

If you installed Snow Owl with a .tar.gz or zip package, you can start Snow Owl from the command line.

Running Snow Owl from the command line

Snow Owl can be started from the command line as follows:

By default, Snow Owl runs in the foreground, prints some of its logs to the standard output (stdout), and can be stopped by pressing Ctrl-C.

Running as a daemon

To run Snow Owl as a daemon, use the following command:

Log messages can be found in the $SO_HOME/serviceability/logs/ directory.

RPM packages

Snow Owl is not started automatically after installation. How to start and stop Snow Owl depends on whether your system uses SysV init or systemd (used by newer distributions). You can tell which is being used by running this command:

Running Snow Owl with SysV init

Use the chkconfig command to configure Snow Owl to start automatically when the system boots up:

Snow Owl can be started and stopped using the service command:

If Snow Owl fails to start for any reason, it will print the reason for failure to STDOUT. Log files can be found in /var/log/snowowl/.

Running Snow Owl with systemd

To configure Snow Owl to start automatically when the system boots up, run the following commands:

Snow Owl can be started and stopped as follows:

These commands provide no feedback as to whether Snow Owl was started successfully or not. Instead, this information will be written in the log files located in /var/log/snowowl/.

Debian packages (Coming Soon)

Docker images (Coming Soon)

Snow Owl security features enable you to easily secure your terminology server. You can password-protect your data as well as implement more advanced security measures such as role-based access control and auditing.

Realms

By default Snow Owl comes without any security features enabled and all read and write operations are unprotected. To configure a security realm, you can choose from the following built-in identity providers:

Authentication

After configuring at least one security realm, Snow Owl will authenticate all incoming requests to ensure that the sender of the request is allowed to access the terminology server and its contents. To authenticate a request, the client must send an HTTP Basic or Bearer Authorization header with the request. The value should be a user/pass pair in case of using Basic authentication or a token generated by Snow Owl if using the Bearer method.

Snow Owl sends an HTTP 401 Unauthorized response if a request needs to be authenticated.

Authorization

If supported by the security realm, Snow Owl will also check whether an authenticated user is permitted to perform the requested action on a given resource.

Within an organization, roles are created for various job functions. The permissions to perform certain operations are assigned to specific roles. Members, staff or other system users are assigned particular roles, and through those role assignments acquire the permissions needed to perform particular system functions. Since users are not assigned permissions directly, but only acquire them through their role (or roles), management of individual user rights becomes a matter of simply assigning appropriate roles to the user's account; this simplifies common operations, such as adding a user, or changing a user's department.

Rules

1. Role assignment: A subject can exercise a permission only if the subject has selected or been assigned a role.

2. Permission authorization: A subject can exercise a permission only if the permission is authorized for the subject's active role.

With rules 1 and 2, it is ensured that users can exercise only permissions for which they are authorized.

S = Subject = A person or automated agent R = Role = Job function or title which defines an authority level P = Permissions = An approval of a mode of access to a resource

Permissions

In Snow Owl a permission is a single value that represents both the operation the user would like to perform and the resource that is being accessed. The format is the following: <operation>:<resource>

Currently there are 7 operations supported by Snow Owl:

• browse - read the contents of a resource

• edit - write the contents of the resource, delete the resource

• import - import from external content and formats

• export - export to external content and formats

• version - create a version in a Code System, create a release

• promote - merge content from isolated branch environments to a Code System's development version

• classify - run classifiers and save their results

Resources represent the content that is being accessed by a client. A resource can be anything that can be resolved to a database entry. Currently, the following resource formats are allowed to be used in a permission:

• <repositoryId> - access the entire content available in a terminology repository

• <repositoryId>/<branch> - access the content available on a branch in a terminology repository

• <codeSystemId> - access all content of a Code System, including both the latest development and all previous releases

• <codeSystemId>/<versionId> - access a specific release of a Code System

There is a special * wild card character that can be used for both the operation and resource parts in a permission value to allow any operation to be performed on any or selected resources, or to allow certain operations to be performed on any available resources.

Examples:

• browse:snomedStore - browse all SNOMED CT Code Systems and their content

• edit:SNOMEDCT-UK-CL - edit the SNOMEDCT-UK-CL Code System

• export:SNOMEDCT-US/2019-03-01 - export the 2019-03-01 US Extension release

• *:SNOMEDCT - allow any operations to be performed on the SNOMEDCT Code System

• browse:* - allow read operations on all available resources

• *:* - administrator permission, the user can do anything with any of the available resources

Configuring Authorization

To configure authorization, please consult the security realm specific documentation:

You can manage and authenticate users with the built-in file realm. All the data about the users for the file realm is stored in the users file. The file is located in SO_PATH_CONF and is read on startup.

You need to explicitly select the file realm in the snowowl.yml configuration file in order to use it for authentication.

identity:
  providers:
    - file:
        name: users

In the above configuration the file realm is using the users file to read your users from. Each row in the file represents a username and password delimited by : character. The passwords are BCrypt encrypted hashes. The default users file comes with a default snowowl user with the default snowowl password.

Users Command

To simplify file realm configuration, the Snow Owl CLI comes with a command to add a user to the file realm (snowowl users add). See the command help manual (-h option) for further details.

Authorization

The file security realm does NOT support the Authorization formats at the moment. If you are interested in configuring role-based access control for your users, it is recommended to switch to the LDAP security realm.

This section describes the use case scenarios present in the world of SNOMED CT and how Snow Owl can be used in those scenarios to maximize its full potential. Each scenario comes with a summary and a pros/cons section to help your decision making process when selecting the appropriate scenario for your use case.

• Single Edition

• Single Extension

• Multi Extension

Snow Owl is a multi-purpose terminology server with a main focus on SNOMED CT International Edition and its Extensions. Whether you are a producer of a SNOMED CT Extension or a consumer of one, Snow Owl has you covered. As always, feel free to ask your questions regarding any of the content you read here (raise a ticket on GitHub Issues).

Snow Owl Concepts

Snow Owl uses the following basic concepts to provide authoring and maintenance support for SNOMED CT Extensions.

Code Systems

From the getting started page, we've learned what is a Repository and how Code Systems are defined as part of a Repository.

SNOMED CT Extensions in Snow Owl are basically Code Systems with their own set of properties and characteristics. With Snow Owl's Code System API, a Code System can be created for each SNOMED CT Extension to easily identify the Code System and its components with a single unique identifer, called the Code System short name. The recommended naming approach when selecting the unique short name identifier is the following:

• SNOMED CT International Edition: SNOMEDCT - often included in other editions for distribution purposes

• National Release Center (single maintained extension) - SNOMEDCT-US - represents the SNOMED CT United States of America Extension

• National Release Center (multiple maintained extensions) - SNOMEDCT-UK-CL, SNOMEDCT-UK-DR - United Kingdom Clinical and Drug Extensions, respectively

• Care Provider with a special extension based on a national extension - SNOMEDCT-US-UNMC - University of Nebraska Medical Center's extension builds on top of the SNOMEDCT-US extension

The primary namespace identifer and set of modules and languages can be set during the creation of the Code System, and can be updated later on if required. These properties can be used when users are accessing the terminology server for authoring purposes to provide a seamless authoring experience for the user without them needing to worry about selecting the proper namespace, modules, language tags, etc. (NOTE: this feature is not available yet in the OSS version of Snow Owl)

Extension Of

A Snow Owl Code System can be marked as an extensionOf another Code System, which ties them together, forming a dependency between the two Code Systems. A Code System can have multiple Extension Code Systems, but a Code System can only be extensionOf a single Code System.

Branching

In Snow Owl, a Repository maintains a set of branches and Code Systems are always attached to a dedicated branch. For example, the default root Code Systems are always tied to the default branch, called MAIN. When creating a new Code System, the "working" branchPath can be specified and doing so assigns the branch to the Code System. A Code System cannot be attached to multiple branches at the same time, and a branch can only be assigned to a single Code System in a Repository. Snow Owl's branching infrastructure allows the use of isolated environments for both distribution and authoring workflows, therefore they play crucial role in SNOMED CT Extension managament as well. They also provide the support for seamless upgrade mechanism, which can be done whenever there is a new version available in one of your SNOMED CT Extension's dependent Code Systems.

Versions

As in real life, a Code System can have zero or more versions (or with another name, releases). A version is a special branch that is created during the versioning process and makes the currently available latest content accessible later in its current form. Since SNOMED CT Extensions can have releases as well, creating a Code System Version in Snow Owl is a must in order to produce the release packages.

Examples

The following image shows the repository content rendered from the available commits, after a successful International Edition import.

Dots represent commits made with the commit message on the right. Green boxes represent where the associated branch's HEAD is currently located. Blue tag labels represent versions created during the commit.

If your use case would be to import the SNOMED CT US Extension 2019-09-01 version into this repository, then ideally it would look like this:

The next section describes the use case scenarios in the world of SNOMED CT and the recommended approaches for deploying these scenarios in Snow Owl.

Introduction

The Snow Owl Terminology Server is capable of managing multiple SNOMED CT extensions for both distribution and authoring purposes in a single deployment. This guide describes the typical scenarios, like creating, managing, releasing and upgrading SNOMED CT Extensions in great detail with images. If you are unfamiliar with SNOMED CT Extensions, the next section walks you through their logical model and basic characteristics, while the following pages describe distribution and authoring scenarios as well as how to use the Snow Owl Terminology Server for SNOMED CT Extensions.

What is a SNOMED CT Extension?

Common Structure

SNOMED CT is a multilingual clinical terminology that covers a broad scope. However, some users may need additional concepts, relationships, descriptions or reference sets to support national, local or organizational needs.

The extension mechanism allows SNOMED CT to be customized to address the terminology needs of a country or organization that are not met by the International Edition.

A SNOMED CT Extension may contain components and/or derivatives (e.g. reference sets used to represent subsets, maps or language preferences). Since the international edition and all extensions share a common structure, the same application software can be used to enter, store and process information from different extensions. Similarly, reference sets can be constructed to refer to content from both the international release and extensions. The common structure also makes it easier for content developed by an extension producer to be submitted for possible inclusion in a National Edition or the International Edition.

Therefore, a SNOMED CT Extension uses the same Release Format version 2 as the International Edition, they share a common structure and schema (see ).

Namespace

Extensions are managed by SNOMED International, and Members or Affiliate Licensees who have been issued a namespace identifier by SNOMED International. A namespace identifier is used to create globally unique SNOMED CT identifiers for each component (i.e. concept, description and relationship) within a Member or Affiliate extension. This ensures that references to extension concepts contained in health record data are unambiguous and can be clearly attributed to a specific issuing organization.

A national or local extension uses a namespace identifier issued by SNOMED International to ensure that all extension components can be uniquely identified (across all extensions).

Therefore, a SNOMED CT Extension uses a single namespace identifier to identify all core components in the SNOMED CT Extension (see ).

Modules

Every SNOMED CT Extension includes one or more modules, and each module contains either SNOMED CT components or reference sets (or both). Modules may be dependent on other modules. A SNOMED CT Edition includes the contents of a focus module together with the contents of all the modules on which it depends. This includes the modules in the International Edition and possibly other modules from a national and/or local extension.

An edition is defined based on a single focus module. This focus module must be the most dependent module, in that the focus module is dependent on all the other modules in the edition.

Language

SNOMED CT extensions can support a variety of use cases, including:

Translating SNOMED CT, for example

• Adding terms used in a local language or dialect

• Adding terms used by a specific user group, such as patient-friendly terms

Representing language, dialect or specialty-specific term preferences is possible using a SNOMED CT extension. The logical design of SNOMED CT enables a single clinical idea to be associated with a range of terms or phrases from various languages, as depicted in Figure 3.1-1 below. In an extension, terms relevant for a particular country, speciality, hospital (or other organization) may be created, and different options for term preferences may be specified. Even within the same country, different regional dialects or specialty-specific languages exist may influence which synonyms are preferred. SNOMED CT supports this level of granularity for language preferences at the national or local level.

Dependency

A SNOMED CT extension is a set of components and reference set members that add to the SNOMED CT International Edition. An extension is created, structured, maintained and distributed in accordance with SNOMED CT specifications and guidelines. Unlike, the International Edition an extension is not a standalone terminology. The content in an extension depends on the SNOMED CT International Edition, and must be used together with the International Edition and any other extension module on which it depends.

Versions

A specific version of an extension can be referred to using the date on which the extension was published.

There are many use cases that require a date specific version of an edition, including specifying the substrate of a SNOMED CT query, and specifying the version of SNOMED CT used to code a specific data element in a health record. A versioned edition includes the contents of the specified version of the focus module, plus the contents of all versioned modules on which the versioned focus module depends (as specified in the |Module dependency reference set|). The version of an edition is based on the date on which the edition was released. Many extension providers release their extensions as a versioned edition, using regular and predictable release cycles.

Characteristics

To summarize, a SNOMED CT Extension has the following characteristics:

• Uses the same RF2 structure as the SNOMED CT International Edition

• Uses a single namespace identifer to globally identify its content

• Uses one or more modules to categorize the content into groups

• Uses one or more languages to support specific user groups and patient-friendly terms

• Depends on the SNOMED CT International Edition

• Uses versions (effective times) to identity its content across multiple releases

Now that we have a clear understanding of what SNOMED CT Extensions are, let's take a look at how can we use them in Snow Owl.

Coming soon!

You can configure security to communicate with a Lightweight Directory Access Protocol (LDAP) server to authenticate and authorize users.

To integrate with LDAP, you configure an ldap realm in the snowowl.yml configuration file.

identity:
  providers:
    - ldap:
        uri: <ldap_uri>
        bindDn: cn=admin,dc=snowowl,dc=b2international,dc=com
        bindDnPassword: <adminpwd>
        baseDn: dc=snowowl,dc=b2international,dc=com
        roleBaseDn: {baseDn}
        userFilter: (objectClass={userObjectClass})
        roleFilter: (objectClass={roleObjectClass})
        userObjectClass: inetOrgPerson
        roleObjectClass: groupOfUniqueNames
        userIdProperty: uid
        permissionProperty: description
        memberProperty: uniqueMember
        usePool: false

Configuration

The following configuration settings are supported:

The default configuration values are selected to support both OpenLDAP and Active Directory without needing to customize the default schema that comes with their default installation.

Configure Authentication

When users send their username and password with their request in the Authorization header, the LDAP security realm performs the following steps to authenticate the user:

1. Searches for a user entry in the configured baseDn to get the DN

2. Authenticates with the LDAP instance using the received DN and the provided password

If any of the above-mentioned steps fails for any reason, the user is not allowed to access the terminology server's content and the server will respond with HTTP 401 Unauthorized.

To configure authentication, you need to configure the uri, baseDn, bindDn, bindDnPassword, userObjectClass and userIdProperty configuration settings.

Adding a user

To add a user in the LDAP realm, create an entry under the specified baseDn using the configured userObjectClass as class and the userIdProperty as the property where the user's username/e-mail address is configured.

Example user entry:

dn: cn=John Doe+sn=Doe+uid=johndoe@b2international.com,dc=snowowl,dc=b2international,dc=com
objectClass: inetOrgPerson
objectClass: organizationalPerson
objectClass: person
objectClass: top
cn: John Doe
sn: Doe
uid: johndoe@b2international.com
userPassword: <encrypted_password> 

Configure Authorization

On top of the authentication part, the LDAP realm provides configuration values to support full role-based access control and authorization.

When a user's request is successfully authenticated with the LDAP realm, Snow Owl authorizes the request using the user's currently set roles and permissions in the configured LDAP instance.

Adding a role

To add a role in the LDAP realm, create an entry under the specified baseDn using the configured roleObjectClass as class and the configured permissionProperty and memberProperty properties for permission and user mappings, respectively.

Example read-only role:

dn: cn=Browser,dc=snowowl,dc=b2international,dc=com
objectClass: top
objectClass: groupOfUniqueNames
cn: Browser
description: browse:*
description: export:*
uniqueMember: cn=John Doe+sn=Doe+uid=johndoe@b2international.com,dc=snowowl,dc=b2international,dc=com 

The most common use case to consume a SNOMED CT Release Package is to import it directly into a Terminology Server (like Snow Owl) and make it available as read-only content for both human and machine access (via REST and FHIR APIs).

SNOMED CT International Edition

Since Snow Owl by default comes with a pre-initialized SNOMED CT Code System called SNOMEDCT, it is just a single call to import the official RF2 package using the The import by default creates a Code System Version for each SNOMED CT Effective Date available in the supplied RF2 package. After a successful import the content is immediately available via REST and FHIR APIs.

SNOMED CT Extension Edition

Summary

The single edition scenario without much effort provides access to any SNOMED CT Edition directly on the pre-initialized SNOMEDCT Code System. It is easy to set up and maintain. Because of its flat structure, it is good for distribution and extension consumers. Although it can be used for authoring in certain scenarios, due to the missing distinction between the International Edition and the Extension, it is not the best choice for extension authoring and maintenance.

#

Pros:

• Good for maintaining the SNOMED CT International Edition

• Good for distribution

• Simple to set up and maintain

Cons:

• Not recommended for extension authoring and maintenance

• Not recommended for multi-extension distribution scenarios

Multi Extension Authoring and Distribution

On top of single Edition/Extension distribution and authoring, Snow Owl provides full support for multi-SNOMED CT distribution and authoring even if the Extensions depend on different versions of the SNOMED CT International Edition.

To achieve a deployment like this you need to perform the same initialization steps for each desired SNOMED CT Extension as if it were a single extension scenario (see single extension). Development and maintenance of each managed extension can happen in parallel without affecting one or the other. Each of them can have their own release cycles, maintenance and upgrade schedules, and so on.

Next steps

After you have initialized your Snow Owl instance with the Extensions you'd like to maintain the next steps are:

• Development

• Release

• Upgrade

• Integrate

A typical extension scenario is the development of the extension itself. Whether you are starting your extension from scratch or already have a well-developed version that you need to maintain, the first choice you need to make is to identify the dependencies of your SNOMED CT Extension.

Extending the International Edition

If your Extension extends the SNOMED CT International Edition directly, then you need to pick one of the available International Edition versions:

• If you are starting from scratch, it is always recommended to select the latest International Release as the starting point of your Extension.

• If you have an existing Extension then you probably already know the International Release version your Extension depends on.

When you have identified the version you need to depend on then you need to import that version (or a later release packages that also includes that version in its FULL RF2 package) first into Snow Owl. Make sure that the createVersion feature of the RF2 import process is enabled, so it will automatically create the versions for each imported RF2 effectiveTime value.

After you have successfully imported all dependencies into Snow Owl, the next step is to create a Code System that represents your SNOMED CT Extension (see Core API). When creating the Code System, besides specifying the namespace and optional modules and languages, you need to enter a Code System shortName, which will serve as the unique identifier of your Extension and select the extensionOf value, which represents the dependency of the Code System.

After you have successfully created the Code System representing your Extension, you can import any existing content from a most recent release or start from scratch by creating the module concept of your extension.

#

Extending another Extension

If your Extension needs to extend another Extension and not the International Edition itself, then you need to identify the version you'd like to depend on in that Extension (that indirectly will select the International Edition dependency as well). When you have identified all required versions, then starting from the International Edition recursively traverse back and repeat the RF2 Import and Code System creation steps described in the previous section until you have finally imported your extension. In the end your extension might look like this, depending on how many Extensions you are depending on.

Summary

Setting up a Snow Owl deployment like this is not an easy task. It requires a thorough understanding of each SNOMED CT Extension you'd like to import and their dependencies as well. However, after the initial setup, the maintenance of your Extension becomes straightforward, thanks to the clear distinction from the International Edition and from its other dependencies. The release process is easier and you can choose to publish your Extension as an extension only release, or as an Edition or both (see Release). Additionally, when a new version is available in one of the dependencies, you will be able to upgrade your Extension with the help of automated validation rules and upgrade processes (see Upgrade). From the distribution perspective, this scenario shines when you need to maintain multiple Extensions/Editions in a single deployment.

Pros:

• Excellent for authoring and maintenance

• Good for distribution

Cons:

• Harder to set up the initial deployment

Authoring is the process by which content is created in an extension in accordance with a set of authoring principles. These principles ensure the quality of content and referential integrity between content in the extension and content in the International Edition (the principles are set by SNOMED International, can be found here).

During the extension development process authors are:

• creating, modifying or inactivating content according to editorial principles and policies

• running validation processes to verify the quality and integrity of their Extension

• classifying their authored content with an OWL Reasoner to produce its distribution normal form

The authors directly (via the available REST and FHIR APIs) or indirectly (via user interfaces, scripts, etc.) work with the Snow Owl Terminology Server to make the necessary changes for the next planned Extension release.

Workflow and Editing

Authors often require a dedicated editing environment where they can make the necessary changes and let others review the changes they have made, so errors and issues can be corrected before integrating the change with the rest of the Extension. Similarly to how SNOMED CT Extensions are separated from the SNOMED CT International Edition and other dependencies, this can be achieved by using branches.

• Branching API - to create and merge branches

• Compare API - to compare branches

Authoring APIs

To let authors make the necessary changes they need, Snow Owl offers the following SNOMED CT component endpoints to work with:

• Concept API - to create, edit SNOMED CT Concepts

• Description API - to create, edit SNOMED CT Descriptions

• Relationship API - to create, edit SNOMED CT Relationships

• Reference Set API - to create, edit SNOMED CT Reference Sets

• Reference Set Member API - to create, edit SNOMED CT Reference Set Members

Validation

To verify quality and integrity of the changes they have made, authors often generate reports and make further fixes according to the received responses. In Snow Owl, reports and rules can be represented with validation queries and scripts.

• Validation API - to run validation rules and fetch their reported issues on a per branch basis

Classification

Last but not least, authors run an OWL Reasoner to classify their changes and generate the necessary normal form of their Extension. The Classification API provides support for running these reasoner instances and generating the necessary normal form.

When an Extension reaches the end of its current development cycle, it needs to be prepared for release and distribution.

Workflows and Authoring Branches

All planned content changes that are still on their dedicated branch either need to be integrated with the main development version or removed from the scope of the next release.

Prepare the Release

After all development branches have been merged and integrated with the main work-in-progress version, the Extension needs to be prepared for release. This usually involves last minute fixes, running quality checks and validation rules and generating the final necessary normal form of the Extension.

Release

When all necessary steps have been performed successfully, a new Code System Version needs to be created in Snow Owl to represent the latest release. The versioning process will assign the requested effectiveTime to all unpublished components, update the necessary Metadata reference sets (like the Module Dependency Reference Set) and finally create a version branch to reference this release later.

Packaging

Maintenance of a SNOMED CT Extension is essential to ensure that

• it incorporates changes requested by terminology consumers

• it remains aligned with the SNOMED CT International Edition

While both of these maintenance related tasks are potentially assigned to one of the upcoming Extension development cycles, there is a clear distinction between the two maintenance tasks.

#

Change requests

Changes requested by your terminology consumers are typically content authoring tasks that you would assign to an Extension authoring team. They usually come with a well-described problem you need to address in the terminology as you would do in the usual development cycle.

See the Extension Development section on how you can address change requests and incorporate them as regular tasks into the main version of your Extension.

International Edition Changes

Aligning content to the SNOMED CT International Edition is one of the main responsibilities of an Extension maintainer. However, keeping up with the changes introduced in SNOMED CT International Edition biannually (on January 31st and July 31st) can be an overwhelming task, especially if:

• you are under pressure from your terminology consumers to make the requested changes ASAP, especially in mission critical scenarios.

• the changes introduced in the International Edition are conflicting with your local changes and/or causing maintenance related issues after the upgrade.

To address SNOMED CT International Edition upgrade tasks in a reliable and reproducible way, Snow Owl offers an upgrade flow for SNOMED CT Extensions.

Upgrades

A Code System upgrade in Snow Owl is a complex workflow with states and steps. The workflow involves a special Upgrade Code System, a series of automated migration processes and validation rules to ensure the quality and reliability of the operation. The upgrade can be done quickly if there were no conflicts between the Extension and the International Edition. However, updates can also be a long-running process spanning over many months when significant structural changes (e.g. in substances, anatomy, or modeling approach) are made in the International Edition.

Starting the Upgrade

In Snow Owl, SNOMED CT Extension are linked to their SNOMED CT dependency with the extensionOf property. This property describes the International Edition and its version that the Extension depends on. For example, the SNOMEDCT/2019-07-31 value specifies that our Extension depends on the 2019-07-31 version of the International Edition.

Extension upgrades can be started when there is a new version available in the Extension/Edition we have selected as our dependency in the extensionOf property. When fetching a SNOMED CT Code System via the Code System API, Snow Owl will check if there are any upgrades availables and return them in the availableUpdates array property. If there are no upgrades available the array will be empty.

To start an Extension upgrade to a newer International Edition (or to a newer Extension dependency version), you can use the Upgrade API. The only thing that needs to be specified there is the desired new version of the Extension's extensionOf dependency.

When the upgrade is started, Snow Owl creates a special <codeSystemShortName>-UP-<newExtensionOf> (eg. SNOMEDCT-MYEXT-UP-SNOMEDCT-2020-01-31) Code System to allow authors and the automated processes to migrate the latest development version of the Extension to the new dependency.

Regular Maintenance

Regular daily Extension development tasks still need to be resolved and pushed somewhere in order to continue the development of the Extension, even if an upgrade process is in progress. Each Extension still has an active development version, even if an upgrade is in progress, which can be used to push daily maintenance changes and business as usual tasks.

Changes pushed to the development area will regularly need to be synced with the upgrade until the upgrade completes, so the upgrade team will be able to resolve all remaining conflicts and issues.

Upgrade Checks

Upgrade Checks ensure the quality of the upgrade process and execute certain tasks/checks automatically. An Upgrade Check can be any logic or function to be run during the upgrade. Upgrade Checks can access the underlying upgrade Code System's content and report any issues (validation rules) or fix content automatically (migration rules). For example, a validation rule (like Active relationships must have active source, type, destination references) can be executed after each change pushed to the upgrade branch to verify whether there is any potentially invalid relationship left to fix or you are ready to go.

Completing the Upgrade

Once the upgrade authoring team is done with the necessary changes to align the Extension with the new International Edition and all the checks are completed successfully the upgrade can be completed. Completing the upgrade performs the following steps:

• Creates a <codeSystemShortName>-DO-<previousExtensionOf> Code System to refer to the previous state of the Extension

• Changes the current working branch of the Extension Code System to the branch that was used during the upgrade process

• Deletes the <codeSystemShortName>-UP-<newExtensionOf> Code System, which marks the upgrade complete, and the upgrade itself cannot be accessed anymore.

This describes the resources that make up the official Snow Owl® RESTful API.

Media Types

Custom media types are used in the API to let consumers choose the format of the data they wish to receive. This is done by adding one of the following types to the Accept header when you make a request. Media types are specific to resources, allowing them to change independently and support formats that other resources don’t.

The most basic media types the API supports are:

1. application/json;charset=UTF-8 (default)

2. text/plain;charset=UTF-8

3. text/csv;charset=UTF-8

4. application/octet-stream (for file downloads)

5. multipart/form-data (for file uploads)

The generic JSON media type (application/json) is available as well, but we encourage you to explicitly set the accepted content type before sending your request.

Schema

All data is sent and received as JSON. Blank fields are omitted instead of being included as null.

All non-effective time timestamps are returned in ISO 8601 format:

YYYY-MM-DDTHH:MM:SSZ

Effective Time values are sent and received in short format:

yyyyMMdd

Hypermedia

All POST requests return Location headers pointing to the created resource instead of including either the identifier or the entire created resource in the response body. These are meant to provide explicit URLs so that proper API clients don’t need to construct URLs on their own. It is highly recommended that API clients use these. Doing so will make future upgrades of the API easier for developers. All URLs are expected to be proper RFC 6570 URI templates.

Example Location Header:

http://example.com/snowowl/snomedct/MAIN/concepts/123456789

Pagination

Requests that return multiple items will be paginated to 50 items by default. You can request further pages with the searchAfter query parameter.

Link/Resource expansion

Where applicable, the expand query parameter will include nested objects in the response, to avoid having to issue multiple requests to the server.

Expanded properties should be followed by parentheses and separated by commas; any options for the expanded property should be given within the parentheses, including properties to expand. Typical values for parameters are given in the "Implementation Notes" section of each endpoint.

GET /snowowl/snomedct/MAIN/concepts?offset=0&limit=50&expand=fsn(),descriptions()

Response:

{
  "items": [
    {
      "id": "100005",
      "released": true,
      ...
      "fsn": {
        "id": "2709997016",
        "term": "SNOMED RT Concept (special concept)",
        ...
      },
      "descriptions": {
        "items": [
          {
            "id": "208187016",
            "released": true,
            ...
          },
        ],
        "offset": 0,
        "limit": 5,
        "total": 5
      }
    },
    ...
  ],
  "offset": 0,
  "limit": 50,
  "total": 421657
}

Client Errors

There are three possible types of client errors on API calls that receive request bodies:

Invalid JSON

Status: 400 Bad Request
{
  "status" : "400",
  "message" : "Invalid JSON representation",
  "developerMessage" : "detailed information about the error for developers"
}

Valid JSON but invalid representation

Status: 400 Bad Request 
{
  "status" : "400",
  "message" : "2 Validation errors",
  "developerMessage" : "Input representation syntax or validation errors. Check input values.",
  "violations" : ["violation_message_1", "violation_message_2"]
}

Conflicts

Status: 409 Conflict 
{
  "status" : "409",
  "message" : "Cannot merge source 'branch1' into target 'MAIN'."
}

Server Errors

In certain circumstances, Snow Owl might fail to process and respond to a request and responds with a 500 Internal Server Error.

Status: 500 Internal Server Error 
{
  "status" : "500",
  "message" : "Something went wrong during the processing of your request.",
  "developerMessage" : "detailed information about the error for developers"
}

To troubleshoot these please examine the log files at {SERVER_HOME}/serviceability/logs/log.log and/or raise an issue on GitHub.

Path expressions

Snow Owl is a revision-based terminology server, where each stored terminology data (concepts, descriptions, etc.) is stored in multiple revisions, across multiple branches. When requesting content from the terminology server, clients are able to specify a path value or expression to select the content they'd like to access and receive. For example, Snow Owl supports importing SNOMED CT content from different sources, allowing eg. multiple national Extensions to co-exist with the base International Edition provided by SNOMED International. Versioned editions can be consulted when non-current representations of concepts need to be accessed. Concept authoring and review can also be done in isolation. Both Java and REST API endpoints require a path parameter to select the content (or substrate) the user wishes to work with.

The following formats are accepted:

Absolute branch path

Absolute branch path parameters start with MAIN and point to a branch in the backing terminology repository. In the following example, all concepts are considered to be part of the substrate that are on branch MAIN/2021-01-31/SNOMEDCT-UK-CL or any ancestor (ie. MAIN or MAIN/2021-01-31), unless they have been modified:

GET /snomedct/MAIN/2021-01-31/SNOMEDCT-UK-CL/concepts
{
  "items": [
    {
      "id": "100000000",
      "released": true,
      "active": false,
      "effectiveTime": "20090731",
[...]

Relative branch path

Relative branch paths start with a short name identifying a SNOMED CT code system, and are relative to the code system's working branch. For example, if the working branch of code system SNOMEDCT-UK-CL is configured to MAIN/2021-01-31/SNOMEDCT-UK-CL, concepts visible on authoring task #100 can be retrieved using the following request:

GET /snomedct/SNOMEDCT-UK-CL/100/concepts

An alternative request that uses an absolute path would be the following:

GET /snomedct/MAIN/2021-01-31/SNOMEDCT-UK-CL/100/concepts

An important difference is that the relative path parameter tracks the working branch specified in the code system's settings, so requests using relative paths do not need to be adjusted when a code system is upgraded to a more recent International Edition.

Path range

The substrate represented by a path range consists of concepts that were created or modified between a starting and ending point, each identified by an absolute branch path (relative paths are not supported). The format of a path range is fromPath...toPath.

To retrieve concepts authored or edited following version 2020-08-05 of code system SNOMEDCT-UK-CL, the following path expression should be used:

GET /snomedct/MAIN/2019-07-31/SNOMEDCT-UK-CL/2020-08-05...MAIN/2021-01-31/SNOMEDCT-UK-CL/concepts

The result set includes the ones appearing or changing between versions 2019-07-31 and 2021-01-31 of the International Edition; if this is not desired, additional constraints can be added to exclude them.

Path with timestamp

To refer to a branch state at a specific point in time, use the path@timestamp format. The timestamp is an integer value expressing the number of milliseconds since the UNIX epoch, 1970-01-01 00:00:00 UTC, and corresponds to "wall clock" time, not component effective time. As an example, if the SNOMED CT International version 2021-07-31 is imported on 2021-09-01 13:50:00 UTC, the following request to retrieve concepts will not include any new or changed concepts appearing in this release:

GET /snomedct/MAIN@1630504199999/concepts

Both absolute and relative paths are supported in the path part of the expression.

Branch base point

Concept requests using a branch base point reflect the state of the branch at its beginning, before any changes on it were made. The format of a base path is path^ (only absolute paths are supported):

GET /snomedct/MAIN/2019-07-31/SNOMEDCT-UK-CL/101^/concepts

Returned concepts include all additions and modifications made on SNOMEDCT-UK-CL's working branch, up to point where task #101 starts; neither changes committed to the working branch after task #101, nor changes on task #101 itself are reflected in the result set.

Detailed API documentation is coming soon! Until then we recommend to check out the official Swagger documentation available on your Snow Owl instance at /snowowl/admin.

This describes the resources that make up the official Snow Owl® SNOMED CT Terminology API.

Current Version

SNOMED CT API endpoints currently have version v3. You have to explicitly set the version of the API via path parameter. For example:

GET /snomedct/branches

Available resources and services

• Branching

• Compare

• Commits

• Concepts

• Descriptions

• Relationships

• RefSets

• Classification

• Import

• Export

Snow Owl provides branching support for terminology repositories. In each repository there is an always existing and UP_TO_DATE branch called MAIN. The MAIN branch represents the latest working version of your terminology (similar to a master branch on GitHub).

You can create your own branches and create/edit/delete components and other resources on them. Branches are identified with their full path, which should always start with MAIN. For example the branch MAIN/a/b/c/d represents a branch under the parent MAIN/a/b/c with name d.

Later you can decide to either delete the branch or merge the branch back to its parent. To properly merge a branch back into its parent, sometimes it is required to rebase (synchronize) it first with its parent to get the latest changes. This can be decided via the state attribute of the branch, which represents the current state compared to its parent state.

Branch states

There are five different branch states available:

1. UP_TO_DATE - the branch is up-to-date with its parent there are no changes neither on the branch or on its parent

2. FORWARD - the branch has at least one commit while the parent is still unchanged. Merging a branch requires this state, otherwise it will return a HTTP 409 Conflict.

3. BEHIND - the parent of the branch has at least one commit while the branch is still unchanged. The branch can be safely rebased with its parent.

4. DIVERGED - both parent and branch have at least one commit. The branch must be rebased first before it can be safely merged back to its parent.

5. STALE - the branch is no longer in relation with its former parent, and should be deleted.

Basics

Get a branch

Response

Get all branches

Response

Create a branch

Input

Response

Delete a branch

Response

Merging

Perform a merge

Input

Response

Perform a rebase

Input

Response

Monitor progress of a merge or rebase

Response

Remove merge or rebase queue item

Response

Compare API

Comparison for current terminology changes committed to a source or target branch can be conducted by creating a compare resource.

A review identifier can be added to merge requests as an optional property. If the source or target branch state is different from the values captured when creating the review, the merge/rebase attempt will be rejected. This can happen, for example, when additional commits are added to the source or the target branch while a review is in progress; the review resource state becomes STALE in such cases.

Reviews and concept change sets have a limited lifetime. CURRENT reviews are kept for 15 minutes, while review objects in any other states are valid for 5 minutes by default. The values can be changed in the server's configuration file.

Compare two branches

Response

Read component state from comparison

Terminology components (and in fact any content) can be read from any point in time by using the special path expression: {branch}@{timestamp}. To get the state of a SNOMED CT Concept from the previous comparison on the compareBranch at the returned compareHeadTimestamp, you can use the following request:

Request

Response

To get the state of the same SNOMED CT Concept but on the base branch, you can use the following request:

Request

Response

Additionally, if required to compute what's changed on the component since the creation of the task, it is possible to get back the base version of the changed component by using another special path expression: {branch}^.

Request

Response

Coming soon!

Coming soon!

FHIR API

Fast Healthcare Interoperability Resources (FHIR) specifies resources, operations, coded data types and terminologies that are used for representing and communicating coded, structured data in the FHIR core specification within its Terminology Module.

Snow Owl's pluggable and extensible architecture allows modular development of the FHIR API both in terms of the supported functionality as well as the exposed terminologies. Additionally, Snow Owl's revision-based model allows the concurrent management of multiple versions.

Resources

The Snow Owl terminology server's FHIR API release includes support for the following resources:

• CodeSystem API

• ValueSet API

• ConceptMap API

• Bundle API

• CapabilityStatement API

Versions

Versions in Snow Owl are represented as individual FHIR Resources when accessed via the FHIR API endpoints. If there are no versions present for a given resource, only the latest development version is returned as available FHIR Resource. When accessing a terminology resource via the FHIR API, but without specifying an exact version tag, then the system will always assume and return the latest development version, including not yet published changes. It is recommended to always query a specific version of any terminology content to get consistent results, especially when the same terminology server instance is being used for both authoring and distribution.

Search

Resource representations can be filtered by the following supported official FHIR payload filters:

• _summary - to return a predefined set of properties and their values

• _elements - to return only the mandatory and the specified list of properties and nothing else

The supported search parameters:

• _id - to filter FHIR resources by their logical identifier

• name - to filter FHIR resources by their name (which in Snow Owl equals to the logical identifier)

• title - to filter FHIR resources by their title property lexically (Snow Owl by default uses exact, phrase and prefix matching during its lexical search activities)

• url - to filter FHIR resources by their assigned url value

• system - to filter FHIR resources by their assigned system value (which in Snow Owl always matches the url value)

• version - to filter FHIR resource by their version property value

• _lastUpdated - exposed but not supported yet

Sorting and paging

Sorting supported via standard FHIR sort parameters, while paging is supported with a new after parameter (using count as page size). Offset + count based traditional paging is not supported.

URIs

Globally unique logical URIs that represent a terminology resource. For code systems these are:

SNOMED CT

For SNOMED CT, Snow Owl's FHIR implementation follows the SNOMED CT URI Standard.

ICD-10

For ICD-10, Snow Owl's FHIR implementation follows the HL7 FHIR Specification.

Local Code System

Snow Owl's Local Code Systems (LCS) identified by the URI that is based on the Organization Link property stored within Snow Owl's Terminology Registry and the Short Name of the LCS e.g.: https://b2i.sg/MyLocalCodeSystem.

IDs

The logical id field of each resource is assigned by Snow Owl and is unique within it. Once it has been assigned, the id never changes. For this logical identifier, Snow Owl follows the pattern:

resourceId[/version]

For example to identify a particular SNOMED CT Edition with its version 2021-03-01:

SNOMEDCT-US/2021-03-01

For example to identify a particular LOINC code system with the version tag v2.64:

 LOINC/v2.64

REST API

Currently only JSON format is supported with UTF-8 encoding and content type of Content-Type = application/fhir+json;charset=utf-8. In case of any errors during the processing the API responds with an OperationOutCome within the response body using one of the HTTP status codes:

Snow Owl's extension API

Snow Owl exposes a comprehensive REST API to support areas such as:

• Syndication - content provisioning between servers or between the Snow Owl Authoring platform and servers

• Administration (repository and revision control management)

• Auditing

• SNOMED CT specific browsing and authoring API

This describes the resources that make up the official Snow Owl® CIS API.

ValueSet API

The endpoints /ValueSet and /ValueSet/{valueSetId} and corresponding operations expose all Value Set resources stored (or implict Value Sets if the corresponding Value Set plug-in supports it) in the server. CUD operations are not supported.

$expand

All value sets accessible via the /ValueSet endpoints can be expanded.

For SNOMED CT URIs, implicit value sets are supported:

• ?fhir_vs - all Concept IDs in the edition/version. If the base URI is http://snomed.info/sct, this means all possible SNOMED CT concepts

• ?fhir_vs=isa/[sctid] - all concept IDs that are subsumed by the specified Concept.

• ?fhir_vs=refset - all concept ids that correspond to real references sets defined in the specified SNOMED CT edition

• ?fhir_vs=refset/[sctid] - all concept IDs in the specified reference set

The following in-parameters are supported:

• activeOnly - to return only active codes in the response

• filter - to filter the results lexically

• displayLanguage - to select the language for the returned display values

• includeDesignations - whether to include all designations or not in the returned response

• count - to select the number of codes to be returned in the expansion

• after - to select codes to be returned after this last page value (cursor)

$validate-code

Codes can be validated against a given Value Set specified by the value set's logical id or canonical URL. In terms of Snow Owl terminology components, codes are validated against:

• SNOMED CT Simple Type Reference Sets with Concepts as referenced components.

• SNOMED CT Query Type Reference Sets with ECL expressions (each member is a Value Set)

• Snow Owl's generic Value Sets

Validation performs the following checks:

• The existence of the given Value Set (error if not found)

• The existence of the reference in the existing Value Set to the given code (error if not found)

• The existence of the given code in the system (error if not found)

• Potential version mismatch (_error if the reference points to a version that is different to the code's version)

• The status of the given code and reference (warning if code is inactive while reference is active)

Code Systems maintained within Snow Owl are exposed (read-only) via the endpoints /CodeSystem and /CodeSystem/{codeSystemId}. Supported concept properties are handled and returned if requested. The currently exposed code systems are:

Snow Owl OSS:

• SNOMED CT

Snow Owl:

• ATC

• ICD-10 (and extensions)

• LOINC

• OPCS

• Local Code Systems

• Any other terminology

SNOMED CT

All standard and default SNOMED CT properties are supported, including the relationship type and concrete value properties. In addition to the FHIR SNOMED CT properties, Snow Owl can return the effective time property, with the URI http://snomed.info/field/Concept.effectiveTime.

Operations

$lookup

Both GET as well as POST HTTP methods are supported. Concepts are queried based on code, version, system or Coding. Designations are included as part of the response as well as supported concept properties when requested. No date parameter is supported.

Example for looking up properties (inactive and method) of the latest version of a SNOMED CT procedure by method code:

 /CodeSystem/$lookup?system=http://snomed.info/sct&code=128927009&_format=json&property=inactive&property=http://snomed.info/id/260686004

For SNOMED CT, all common and SNOMED CT properties are supported, including all active relationship types.

$validate-code

Both GET as well as POST HTTP methods are supported for all exposed terminologies. Example for validating a SNOMED CT code:

/CodeSystem/SNOMEDCT/2021-07-31/$validate-code?code=128927009

$subsumes

Both GET as well as POST HTTP methods are supported. Subsumption testing is supported for all terminologies, including SNOMED CT.

Example for SNOMED CT (version 2021-07-31):

/CodeSystem/$subsumes?codeA=409822003&codeB=264395009&system=http://snomed.info/sct/900000000000207008/version/20210731

ConceptMap API

The endpoints /ConceptMap and /ConceptMap/{conceptMapId} and corresponding operations expose the following types of terminology resources:

• SNOMED CT Simple Map Reference Sets with Concepts as referenced components

• SNOMED CT Complex Map Reference Sets

• SNOMED CT Extended Map Reference Sets

• Snow Owl's generic Concept Maps

$translate

All concept map accessible via the /ConceptMap endpoints are considered when retrieving mappings (translations). The translate request's source that designates the source value set cannot be interpreted hence not used. With the exception of SNOMED CT where the standard URI is expected, our proprietary short name or component ids are used to designate the source/target code system.

SNOMED CT:

• Simple Map Type Reference Set mappings are considered equivalent in terms of their correlation

• The availability and format of target code systems are not guaranteed, there is an ongoing conversation at SNOMED CT International to rectify this.

Snow Owl uses a single data source, an Elasticsearch cluster (either embedded or external). To backup and restore the data, we highly recommend the official Snapshot and Restore feature from Elasticsearch. On top of that API, we highly recommend using tools, like Curator to ease the lifecycle management of your Elasticsearch cluster and your indices. See Curator here.

Below you can find a very simple guide on how to configure the backup and restore process for your Snow Owl Terminology Server using Curator.

Prerequisites

Please refer to the official Curator install guide on how to install it on various operating systems.

Configure Snapshot repository

In order to create backups for Snow Owl, you need a repository in your Elasticsearch cluster.

To create a repository (assuming shared file system repository, fs), execute the following command:

$ curl -XPUT localhost:9200/_snapshot/snowowl-snapshots -d
{
  "type": "fs",
  "settings": {
    "location": "/path/to/shared/mount",
    "compress": true
  }
}

Elasticsearch requires that the specified /path/to/shared/mount is whitelisted in the path.repo configuration setting in the elasticsearch.yml configuration file. See section Shared file system repository of the Elasticsearch reference for details.

Curator configuration file

Curator requires a single configuration file to be specified when running it. If you are using a default Elasticsearch cluster with default configurations then the default Curator recommended file should be sufficient. Any configuration changes you have made to your Elasticsearch cluster needs to be changed here as well in this config file so Curator can access your cluster without any issues.

Example curator.yml:

client:
  hosts:
    - 127.0.0.1
  port: 9200
  url_prefix:
  use_ssl: False
  certificate:
  client_cert:
  client_key:
  ssl_no_validate: False
  http_auth:
  timeout: 30
  master_only: False

logging:
  loglevel: INFO
  logfile:
  logformat: default
  blacklist: ['elasticsearch', 'urllib3']