Change Notifications

Change notifications deliver events to your application whenever a digital twin or device state changes.

How to receive change notifications

Ditto publishes events through three channels. Each delivers events to authenticated subjects that have the required authorization:

Filtering

You can filter events on the Ditto backend before they reach your application. Each API provides its own mechanism for specifying filters, but all support the same filter types.

Filter by namespace

Provide a comma-separated list of namespaces to receive events only from Things in those namespaces:

namespaces=org.eclipse.ditto.one,org.eclipse.ditto.two

Filter by RQL expression

For more granular control, use an RQL expression to filter based on:

• Thing data – filter on the modified values in the event payload
• Ditto Protocol fields – filter on message metadata using placeholders:
  • topic:action – filter for lifecycle events (created, deleted)
  • resource:path – filter by the affected resource path

Examples

Only emit events when count changes to a value greater than 42:

filter=gt(attributes/count,42)

Only emit events for Things starting with “myThing” when the “lamp” feature changes:

filter=and(like(thingId,"org.eclipse.ditto:myThing*"),exists(features/lamp))

Only emit events when manufacturer starts with “ACME & Sons” (note the encoded &):

filter=like(attributes/manufacturer,"ACME %26 Sons*")

Only emit events for Thing creation and deletion:

filter=and(in(topic:action,'created','deleted'),eq(resource:path,'/'))

See the full RQL expression reference for the complete query language.

Further reading

• Signal Enrichment – add extra context (like attributes) to events
• Signals & Communication Pattern – understand the signal types
• WebSocket Protocol Binding – subscribe via WebSocket
• Server Sent Events – subscribe via SSE
• Connections – subscribe via managed connections