Planned lessonMID-029-L01

The dual-write problem

On This Lesson 4 topics

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

explain why writing to a database and a broker separately can lose or invent events.

A dual write updates two systems (database, broker) with no shared transaction.
Commit then publish: a crash between the two loses the event. The order exists, nobody hears about it.
Publish then commit: a rollback or crash leaves an event for an order that does not exist.
Publishing inside the DB transaction does not fix it: the broker call is not part of that transaction.
Retrying makes it worse without idempotency: you may publish twice.

Draft outline details

java

@Transactional
void place(Order o) {
    repo.save(o);
    kafka.send("order-placed", o.id(), event); // not atomic with the commit
}

The service crashes after the commit but before send. What is lost?

Show answer

The OrderPlaced event; the order stays in the database.

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