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Pattern: Publish events using database triggers

Context

A service command typically needs to update the database and send messages/events. For example, a service that participates in a saga needs to atomically update the database and sends messages/events. Similarly, a service that publishes a domain event must atomically update an aggregate and publish an event.

A service must atomically update the database and send messages in order to avoid data inconsistencies and bugs. However, it is not viable to use a traditional distributed transaction (2PC) that spans the database and the message broker to atomically update the database and publish messages/events. The message broker might not support 2PC. And even if does, it’s often undesirable to couple the service to both the database and the message.

But without using 2PC, sending a message in the middle of a transaction is not reliable. There’s no guarantee that the transaction will commit. Similarly, if a service sends a message after committing the transaction there’s no guarantee that it won’t crash before sending the message.

In addition, messages must be sent to the message broker in the order they were sent by the service. They must usually be delivered to each consumer in the same order although that’s outside the scope of this pattern. For example, let’s suppose that an aggregate is updated by a series of transactions T1, T2, etc. This transactions might be performed by the same service instance or by different service instances. Each transaction publishes a corresponding event: T1 -> E1, T2 -> E2, etc. Since T1 precedes T2, event E1 must be published before E2.

Problem

How to reliably/atomically update the database and send messages/events?

Forces

• 2PC is not an option
• If the database transaction commits messages must be sent. Conversely, if the database rolls back, the messages must not be sent
• Messages must be sent to the message broker in the order they were sent by the service. This ordering must be preserved across multiple service instances that update the same aggregate.

Solution

One or more database triggers insert events into an EVENTS table, which is polled by a separate process that publishes the events.

Related patterns

• The Saga pattern creates the need for this pattern.
• The Event sourcing is an alternative pattern

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