implement FHIR Subscriptions for change notifications (R4 channel-based vs R4B/R5 topic-based differences)

Steps

1. For FHIR R4 (channel-based): POST a Subscription resource with criteria (a search query string like 'Observation?patient=123'), channel.type (rest-hook, websocket, email, or sms), and channel.endpoint pointing to your webhook URL.
2. For FHIR R4B/R5 (topic-based): POST a Subscription resource referencing a SubscriptionTopic by canonical URL in the topic field; the SubscriptionTopic defines the trigger logic server-side rather than embedding a query in the Subscription.
3. For rest-hook subscriptions, expose an HTTPS endpoint that accepts POST requests; some servers send a handshake payload on subscription activation that you must acknowledge with a 200 response.
4. Handle incoming notification payloads: R4 rest-hook sends an empty Bundle or the changed resource depending on server implementation; R4B/R5 sends a structured notification Bundle with event details.
5. To retrieve the full resource after a notification, use the ids or full URLs provided in the notification Bundle to make a standard FHIR read request.
6. Implement idempotent notification handling using the notification's event number or resource version to tolerate duplicate deliveries.