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Pattern: Transaction log tailing

Context

You have applied the Application events pattern. In order to be reliable, each step of a saga must atomically update the database and publish messages/events. It is not viable to use a distributed transaction that spans the database and the message broker.

Problem

How to publish messages/events into the outbox in the database to the message broker?

Forces

Solution

Tail the database transaction log and publish each message/event inserted into the outbox to the message broker.

The mechanism for trailing the transaction log depends on the database:

  • MySQL binlog
  • Postgres WAL
  • AWS DynamoDB table streams

Example

Resulting context

This pattern has the following benefits:

  • No 2PC
  • Guaranteed to be accurate

This pattern has the following drawbacks:

  • Relatively obscure although becoming increasing common
  • Requires database specific solutions
  • Tricky to avoid duplicate publishing

Learn more


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