Tell us about the Transactional Outbox pattern: what is it and how does it work?

The Transactional Outbox pattern is used for reliable integration between microservices when it is necessary to guarantee that an event will be published only if the transaction with the data has succeeded.

The idea is that when data is changed in the database within a single transaction, a message (outbox) is simultaneously written to a special table. After the transaction commits, a separate process reads these messages from the table and sends them to a queue or message broker.

This solves the "two-phase commit" problem between the database and the messaging system, ensuring atomicity and consistency.

Example in Go (simplified):

// Within a transaction
trx, _ := db.Begin()
// Update data
_, err := trx.Exec("UPDATE orders SET status = ? WHERE id = ?", "processed", orderID)
// Write event to outbox
_, err = trx.Exec("INSERT INTO outbox (event_type, payload) VALUES (?, ?)", "OrderProcessed", eventPayload)
// Commit the transaction
err = trx.Commit()

// A separate worker reads from the outbox and publishes events