Subscriptions R5 Backport, published by HL7 International / FHIR Infrastructure. This guide is not an authorized publication; it is the continuous build for version 1.2.0-ballot built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/fhir-subscription-backport-ig/ and changes regularly. See the Directory of published versions

Handling Errors

Page standards status: Informative

Errors can occur at any point in the processing or delivery of a notification. This page describes some common error scenarios and mechanisms used to detect and recover from them.

Handling Errors as a Server

Error handling as a Server is intended to be simple. A server is not expected to know the best process for every use case of every subscriber, so the focus is on allowing clients to detect there is an issue. A server SHALL:

• Increment the eventsSinceSubscriptionStart counter internally.
• Update the status of the subscription internally.
• Continue to respond to $status requests.

A server MAY:

• Continue to send heartbeat messages (with an error status set).

Discovering the error state and recovering from it are responsibilities of the subscriber. This includes resetting the Subscription to an active or requested status - a client is responsible for requesting re-activation of a subscription. Note: this is important because a subscriber must make the determination of how to recover from an error state; if a server arbitrarily resets a subscription, a subscriber may not be aware of missing notifications.

Detecting Errors as a Subscriber

There are several mechanisms available to subscribers in order to understand the current state of notification delivery. Below are some example error scenarios with details about how a subscriber can detect the problem state.

Missing Event

The diagram below shows how a subscriber can use the eventsSinceSubscriptionStart parameter on received notifications to determine that an event has been missed.

In the above sequence, the subscriber tracks the eventsSinceSubscriptionStart of each received notification (per Subscription). When the subscriber received event 23, the subscriber was aware that the last notification it received was a single notification for event 21. The subscriber then waited an amount of time to ensure that event 22 was indeed missing (and not, for example, still being processed) and started a recovery process. The recovery process itself will vary by subscriber, but should be a well-understood method for recovering in the event of errors.

Broken Communication

The diagram below show how a subscriber can use the heartbeatPeriod on a Subscription to determine errors which prevent notifications from reaching the endpoint.

In the above sequence, the subscriber is aware that the heartbeatPeriod has elapsed for a subscription without receiving any notifications. The subscriber then asks the server for the $status of the subscription, and seeing an error, begins a recovery process. As in the previous scenario, the recovery process itself will vary by subscriber, but should be a well-understood method for recovering in the event of errors.

Recovering from Errors

Clients are responsible for devising an appropriate method for recovering from errors. Often, this process will include a series or batch of requests that allow a client to know the current state. For example, an application may need to query all relevant resources for a patient in order to ensure nothing has been missed. Once an application has returned to a functional state, it should request the subscription is reactivated by updating the status to either requested or active as appropriate.

Using the $events operation

Servers MAY choose to support the $events operation, as defined in this IG. The $events operation allows clients to request events which have occurred in the past. Servers which implement the operation MAY use implementation-specific criteria to restrict availability of events (e.g., most recent 10 events, events within the past 30 days, etc.).

During a recovery process, clients MAY try to retrieve missing events via the $events operation, which should allow processing to continue as normal.