Using server sent events

You can use Server-Sent Events (SSEs) to get notified when the state of digital twins changes.

The client sends a request and server holds a connection until a new message is ready, then it sends the message back to the client while still keeping the connection open so that it can be used for another message once it becomes available. Once a new message is ready, it is sent back to the client on the same initial connection.

The SSE API for receiving change notifications is the /things endpoint:

http://localhost/things

When the endpoint is invoked with an HTTP header Accept with value text/event-stream, a Server-Sent Event stream of change notifications is created.

With SSE API /things you can only get notifications about specific changes. You specify the IDs of the Things and you can do:

• Fields projection
• Filtering by namespaces
• Filtering by RQL expression

SSE use cases

The use cases below show how to:

• Create things and subscribe for all events connected with them
• Delete things and subscribe for all events connected with them
• Change property on Change property level and subscribe for all events for this specific property

  Use Case	URI Subscription	Result
  Create two Things with the following Ids: test.server.sent.events:testThing_1 test.server.sent.events:testThing_2   Subscribe for all events for specific namespace	/thigs?namespaces=<namespace>   For example: /things?namespaces=test.server.sent.events	Data: {"thingId":"test.server.sent.events:testThing_1","policyId":"test.server.sent.events:testThing_1","features":{"switch":{"properties":{"on":true}}}} Data: {"thingId":"test.server.sent.events:testThing_2","policyId":"test.server.sent.events:testThing_2","features":{"switch":{"properties":{"on":true}}}}
  Delete two Things with the following Ids: test.server.sent.events:testThing_1 test.server.sent.events:testThing_2   Subscribe for all events for specific namespace	/thigs?namespaces=<namespace>   For example: /things?namespaces=test.server.sent.events	Data: {"thingId":"test.server.sent.events:testThing_1"} Data: {"thingId":"test.server.sent.events:testThing_2"}
  Change property on Change property level   Subscribe for all events for specific property	/things?ids=<thingId>&fields=thingId,features/<second_feature_name>/properties/on	Data: {"thingId":"test.server.sent.events:testThing_23","features":{"secondSwitch":{"properties":{"on":true}}}} Data: {"thingId":"test.server.sent.events:testThing_23"}