Events: State

For any services with a remote client (UI), it is desirable to publish current service state and sensor readings. This state tends to be a superset of the published history data, and may include non-numerical values.

We reserved topic space for these messages under brewcast/state. Because detailed state data requires support from consuming clients, we do not define a global payload format for messages.

The payload schema for Spark state events can be found here.

Topic

Service state events should be published to a topic matching brewcast/state/<Service ID>. You should NOT publish directly to the brewcast/state topic.

GOOD:

• brewcast/state/my-service-one
• brewcast/state/my-service-one/subtype
• brewcast/state/my-service-one/my-service/subtype

BAD:

• brewcast/state
• my-service
• my-service/my-service-one

Flags

You can use any valid value for the qos flag in MQTT.

It is common to set the retain flag for state events. This allows quick synchronization with newly connected clients.

Payload

Empty messages are used to clear retained messages set for a topic.

If a payload is set, it must be a JSON-serialized object. There is no pre-defined schema for message payloads. Each service is free to define its own.