The bug between DB commit and queue enqueue

Aslı Kök — Mon, 23 Mar 2026 15:41:32 +0000

Hey everyone,

While working with FastAPI and Celery, I ran into a subtle reliability issue that I think is easy to overlook.

A very common flow looks like this:

db.commit() celery_task.delay(...)

At first glance, this seems perfectly fine.
But there’s a problem: this is not atomic.

The hidden bug
If your process crashes between these two lines:

the database transaction is already committed
but the task is never enqueued

The job is silently lost. No retry. No error. No visibility.

This makes it especially dangerous in production systems where reliability matters.

A possible solution: Transactional Outbox

One approach to solve this is the Transactional Outbox pattern:

Write both the state change and an "event" into the database in the same transaction
Use a separate worker to read and publish those events to the queue

This ensures that nothing is lost, but it comes with trade-offs:

I put together a more detailed write-up here:

Curious about real-world approaches

I’m really interested in how others handle this in production:

Would love to hear your experience.

DEV Community: Aslı Kök