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. The database update and sending of the message must be atomic in order to avoid data inconsistencies and bugs. However, it is not viable to use a distributed transaction that spans the database and the message broker to atomically update the database and publish messages/events.
How to reliably/atomically update the database and publish messages/events?
A service that uses a relational database inserts messages/events into an outbox table (e.g.
MESSAGE) as part of the local transaction.
An service that uses a NoSQL database appends the messages/events to attribute of the record (e.g. document or item) being updated.
A separate Message Relay process publishes the events inserted into database to a message broker.
This pattern has the following benefits:
This pattern has the following drawbacks:
This pattern also has the following issues:
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