This page is part of the Subscriptions R5 Backport (v0.1.0: STU 1 Ballot 1) based on FHIR R4. The current version which supercedes this version is 1.0.0. For a full list of available versions, see the Directory of published versions

Components

The subscription mechanism is composed of three parts:

• Subscription Topics
  • Define the data and change used to trigger notifications
  • Define the filters allowed to clients
  • Always referenced by canonical URL
• Subscriptions
  • Describe a client’s request to be notified about events defined by a SubscriptionTopic
  • Set filters on events (as defined in the referenced SubscriptionTopic)
  • Describe the channel and endpoint used to send notifications
  • Describe the payload included in notifications (MIME type, content level, etc.)
• Notification Bundles
  • Describe the contents of a notification
  • Contain zero or more notification payloads

Definitionally, a Subscription requires the a SubscriptionTopic - without a resource describing the change of interest, a Subscription has no meaning and will not trigger any client notifications.

While active, a Subscription relies on both Bundle and Parameters for sending notifications.

When using the Subscription resource, the FHIR server combines the roles of publisher and information distributer. Some arrangements of the ‘publish and subscribe’ pattern describe separate agents for the two roles. This specification makes no recommendations towards the internal architecture of server implementations.

Subscription Topics

In FHIR R5, the SubscriptionTopic resource is used to define conceptual or computable events for Subscription resources. Conceptually, subscription topics specify: a type of data (e.g., Observation, Condition, etc.), a type of change (e.g., create, delete, update, etc.), and a set of allowed filters.

In order to limit the scope of this Implementation Guide, SubscriptionTopic definitions are not defined here. Since topics are always referenced by their canonical URL, servers using FHIR R4 have no need to implement any of the functionality around topics themselves.

In order to support discovery of which topics a server supports (a key feature of R5 subscriptions), the Subscription/$topic-list operation has been defined.

Subscriptions

The Subscription resource is used to request notifications for a specific client about a specific topic. Conceptually, a subscription specifies: a topic (SubscriptionTopic, by canonical URL), the notification channel (e.g., REST, websockets, email, etc.), and the notification payload (e.g., MIME type, amount of detail, etc.).

When a FHIR Server accepts a request to create a Subscription, the server is indicating to the client that the server:

• is capable of detecting when events covered by the requestion SubscriptionTopic occur, and
• is willing to make a simple best effort attempt at delivering a notification for each occurance of a matching event.

In order to add the functionality of R5 subscriptions to the R4 Subscription resource, this guide defines several extensions. A list of extensions defined by this guide can be found on the Artifacts page.

Subscription Notifications

In FHIR R5, a new type of Bundle has been introduced, which uses the new SubscriptionStatus resource to convey status information in notifications. For FHIR R4, notifications are instead based on a history Bundle, and a Parameters resource is used to convey meta-information.

Since notifications use history type Bundles, a few additional elements are required. Specifically, in order to meet the requirements of bdl-3 and bdl-4, Bundle.entry.request must exist for each entry. For the status resource (entry[0]), the request SHALL filled out to match a request to the $status operation.

For additional entries, the request SHOULD be filled out in a way that makes sense given the subscription (e.g., a POST or PUT operation on the resource, etc.). However, a server MAY choose to simply include a GET to the relevant resource instead.

Unless otherwise specified by a server implementation and channel, the Subscriptions Framework does not involve guaranteed delivery of notifications. While the Subscriptions Framework is able to support such mechanisms, defining them are beyond the scope of the standard.

Clients should be aware of some limitations regarding delivery. In particular:

• Some notifications might not be delivered.
• Some notifications might be delivered multiple times.
• In order to mitigate the impact from the above issues, the Subscriptions Framework includes mechanisms to detect both scenarios.