What is the subsequent processing in the system if a transaction is successful but the message was not sent to RabbitMQ?

If a database transaction is successfully completed but the message was not sent to RabbitMQ, there is a consistency problem between the data state and the messaging system. Typically, the following approaches are used in such cases:

• Deferred Retry: Store information about unsent messages in a separate table or queue and periodically attempt to resend them.

• Using the "Outbox" pattern: Instead of directly sending a message to RabbitMQ within a transaction, record the event in a special table (outbox). Then, a separate process reads this table and sends messages to RabbitMQ. This guarantees atomicity of data recording and message preparation.

• Logging and Monitoring: Keep logs of message sending errors and set up alerts for quick response.

Example of the Outbox pattern:

// Within a transaction
$pdo->beginTransaction();
// 1. Update data
$pdo->prepare('UPDATE orders SET status = ? WHERE id = ?')->execute(['processed', $orderId]);
// 2. Record event in outbox
$pdo->prepare('INSERT INTO outbox (event_type, payload) VALUES (?, ?)')->execute(['order_processed', json_encode($orderData)]);
$pdo->commit();

// A separate process reads from the outbox and sends to RabbitMQ

Thus, even if sending to RabbitMQ fails, the event data is saved and can be sent later.