Listeners Overview

A listener runs when a matching event is emitted — before the ledger append completes. It can resolve related entities and mutate their state in-process, with no Kafka lag.

Use listeners for atomic cross-entity consistency. Use projections for queryable read models. Use side_effects for async fan-out.

How listeners fit into the write path

Listeners sit between emit and ledger commit. Projections consume after Kafka.

Declaring listeners

# app.causet
includes:
  listeners: [./listeners/**/*.listeners.causet]

listeners:
  - on: ARTIST_FOLLOWED
    resolve:
      artist: lookup(artist, event.payload.artist_id)
    mutate:
      - op: add
        path: /follower_count
        value: 1
        on: artist

In this section

Next steps

• Intents — where events are emitted
• Projections — async read models
• Workflows — submit and schedule in side_effects