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Transactional Event Queues ​

Transactional Event Queues allow to schedule events and background tasks for asynchronous exactly once processing and withultimate resilience.

Event Queues: Concept ​

The Outbox Pattern is a reliable strategy used in distributed systems to ensure that messages or events are consistently recorded and delivered, even in the face of failures. Event Queues not only apply this pattern to outbound messages, but also to inbound messages and to internal background tasks. So, event queues can be used for four different use cases:

  • Outbox → for outbound calls to remote services
  • Inbox → for asynchronously handling inbound requests
  • Background tasks → e.g., scheduled periodically
  • Remote Callbacks → implementing SAGA patterns

The core principle remains the same:

Instead of being sent directy to receivers, event messages are persisted in an Event Queue table in the database -- within the same transaction as the triggering action, if applicable.

Later on, these event messages are read from the database and actually sent to the receiving services, hence processed asynchronously -- with retries, if necessary, so guaranteeing ultimate resilience.

Outbox ​

Regarding the outbox, please see the following existing documentation:

Inbox Beta ​

Through the inbox, inbound messages can be accepted as asynchronous tasks. That is, the messaging service persists the message to the database, acknowledges it to the message broker, and schedules its processing.

Simply configure your messaging service for Node.js as cds.requires.messaging.inboxed = true and for CAP Java as cds.messaging.services

Inboxing moves the dead letter queue into your CAP app❗️

With the inbox, all messages are acknowledged towards the message broker regardless of whether they can be processed or not. Hence, failures need to be managed via the dead letter queue built on cds.outbox.Messages.

Learn more about the dead letter queue in Node.js.Learn more about the dead letter queue in Java.

Inboxing is especially beneficial in case the message broker does not allow to trigger redelivery and/ or "fix" the message payload.

More to Come ​

This documentation is not complete yet, or the APIs are not released for general availability. Stay tuned to upcoming releases for further updates.