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Pattern: Application publishes events

Context

You have applied the Saga pattern. In order to be reliable, services must atomically publish events whenever their state changes. It is not viable to use a distributed transaction that spans the database and the message broker.

Problem

How to reliably/atomically publish events whenever state changes?

Forces

  • 2PC is not an option

Solution

The application insert events into an EVENTS table as part of the local transaction. A separate process polls the EVENTS table and publishes the events to a message broker.

Result context

This pattern has the following benefits:

  • High level domain events
  • No 2PC

This pattern has the following drawbacks:

  • Requires changes to the application, which are potentially error prone.
  • Tricky to avoid duplicate publishing
  • Tricky to publish events in order
  • Only works for SQL and some NoSQL databases

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