Build reliable asynchronous workflows
Use queues to absorb bursts while respecting retries, ordering, and duplicate delivery.
- Explain backpressure, at-least-once delivery, and the transactional outbox pattern.
A queue separates producers from workers and can absorb short bursts. If work arrives faster than workers can process it, backlog grows; backpressure limits or slows producers before the queue exhausts resources. Many brokers provide at-least-once delivery, so a message may be delivered again after a worker fails. Make handlers idempotent, track retries and dead letters, and use an outbox when a database update and event publication must not get out of sync.
1queued = 120
2processed_per_minute = 30
3minutes = 2
4remaining = queued - processed_per_minute * minutes
5print(f"backlog after 2 min: {remaining}")backlog after 2 min: 60
The outbox writes the business update and an event record in one database transaction. A separate publisher sends pending events and marks them delivered; consumers still need idempotency because publication can repeat.
Key takeaways
Queues absorb bursts but do not create processing capacity.
Plan for duplicate delivery and make consumers idempotent.
Use backpressure and monitor queue age, not just queue depth.
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