Microservice Architecture

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

Related patterns

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