Projections and commands

Writing a projection

A service that implements Tilttek\Activity\Projection\Projection is tagged tilttek_activity.projection by autoconfiguration and joins Projections. Its checkpoint is kept in tilttek_activity_checkpoint under its name. The projection keeps its own state where it wants — its own table, a cache —, and must tolerate seeing an entry twice after a crash (see the library's documentation).

Live projections

With projections: live, the default, each new entry catches up, in the same request, every projection whose filter reads it. A failing projection lets its exception through after the entry is written, and its checkpoint stays before the entry.

Projections through Messenger

With projections: messenger, each new entry sends, for every projection whose filter reads it, a CatchUpProjection message on the projection_bus; CatchUpProjectionHandler catches that projection up. The request only writes the entry and sends the message: route the message to an asynchronous transport, consumed by one worker so that a projection never runs twice at the same time.

# config/packages/messenger.yaml
framework:
    messenger:
        transports:
            projections: '%env(MESSENGER_TRANSPORT_DSN)%'
        routing:
            Tilttek\ActivityBundle\Infrastructure\Messenger\CatchUpProjection: projections

bin/console messenger:consume projections --limit=1000

The bundle routes nothing itself: the transport, its retries and the number of its consumers are the application's choice, written in its own framework.messenger.routing. Without a route, Messenger handles the message on the bus when it is sent, in the request, as live projections do.

The message carries identifiers only: the projection's name and the entry's position. The activity stays in the journal, which the handler reads.

• The journal is the outbox. The entry is written before the message is sent: a message lost on the way is made up by the projection's next message, which catches it up past every entry it has not seen, or by tilttek:activity:catch-up.
• The checkpoints are the inbox. A message delivered twice, or after a later one, finds the projection already past its entry and applies nothing; a message for a projection the application no longer has is dropped.

Without symfony/messenger, projections: messenger fails when the container is compiled.

Manual projections

With projections: manual, nothing follows the journal as it is written: run tilttek:activity:catch-up from a scheduled task.

Commands

An unknown projection or a date that does not parse ends the command with an error and changes nothing. Run one catch-up or rebuild of a projection at a time.