Listener Examples

Match scoring

# listeners/scoring.listeners.causet
listeners:
  - on: POINTS_AWARDED
    where: event.payload.apply_to_match
    resolve:
      match: lookup(match, event.payload.match_id)
    mutate:
      - op: add
        path: /stats/home_score
        value: event.payload.delta
        on: match
      - op: set
        path: /last_scored_at
        value: event.ts
        on: match
 
  - on: MATCH_COMPLETED
    resolve:
      venue: lookup(venue, event.payload.venue_id)
    mutate:
      - op: add
        path: /completed_match_count
        value: 1
        on: venue
      - op: push
        path: /recent_match_ids
        value: event.entity_id
        on: venue

Concert app — follower counter

Sync artist.follower_count in the same commit as FOLLOW_ARTIST:

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

Trade-off: listeners give synchronous counters; Projection Examples use async increment: — often better under high concurrency.

More examples

Example	Pattern
Concert App	Full domain with projections as primary read path
Intent Examples	Events that trigger listeners
Projection Examples	Async alternative for counters

Next steps

Defining Listeners — where, resolve, supported ops
Best Practices — when not to use listeners

Best Practices Overview