Build a transactional outbox on Symfony Messenger transports

Short summary

The dual-write problem occurs when saving data and dispatching events are separate operations that can fail independently, leaving systems inconsistent. Symfony Messenger solves this using a transactional outbox pattern: route events to a Doctrine database table within the same transaction as your data write, then drain the outbox asynchronously with a relay worker. This ensures atomicity between business logic and event delivery.

• •Dual-write problem: data saves and event dispatches can fail independently, causing lost events or ghost records
• •Symfony's Doctrine transport writes both data and events in a single database transaction, guaranteeing consistency
• •A separate relay worker drains the outbox asynchronously, decoupling request processing from downstream event delivery

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