# Dead letters need a return path

Principle `dead-letters-need-a-return-path`. Exhausted work needs durable evidence, an owner, and deliberate redrive.

Also searched as: dead-letter queue, redrive, poison work.

Partial batch failure needs a decision about failed records and successful siblings. Parking work is a recovery boundary, not evidence of successful processing.

> Do you put the failed event into a DLQ and process it later?

Source: [Q20](/bibliography#q20); [B5](/bibliography#b5-building-event-driven-microservices), [B9](/bibliography#b9-serverless-architectures-on-aws).

## Failure it prevents

A failed item disappears, blocks useful work, or causes successful siblings to run repeatedly.

## Our Effect 4.0.5 reading

PersistedQueue retains failed records. DurableQueue exhaustion can leave its DurableDeferred unresolved and the workflow parked. Provide out-of-band repair and requeue.

Source: `packages/effect/src/persistence/PersistedQueue.ts`, `packages/effect/src/workflow/DurableQueue.ts`.

## How to check

Exhaust an item, find its failure record, repair it, and prove redrive resumes the intended waiter without repeating successful siblings. This is our proposed verification, not a quoted procedure.

Continue with [Retry needs idempotent effects](/principles/retry-needs-idempotency), [Golden signals expose overload](/principles/golden-signals-expose-overload).

Connect this to [the existing rat-stack guidance](https://ratstack.sh/lore/concurrent-work-needs-an-owner).

Use [the durability skill](/skills/durability) for the full review. Return to [the durability index](/principles/durability).

## Skills
- [Durability starts with a recovery record](/principles/durability)

## Sources
- Bellemare, Adam. *Building Event-Driven Microservices: Leveraging Organizational Data at Scale*. O’Reilly, 2020. [Link](https://www.oreilly.com/library/view/building-event-driven-microservices/9781492057888/). ([b5-building-event-driven-microservices](/bibliography#b5-building-event-driven-microservices))
- Sbarski, Peter; Cui, Yan; Nair, Ajay. *Serverless Architectures on AWS*, Second Edition. [Link](https://www.manning.com/books/serverless-architectures-on-aws-second-edition). ([b9-serverless-architectures-on-aws](/bibliography#b9-serverless-architectures-on-aws))

## References
- [Effect contributors. Effect 4.0.5. persistence/PersistedQueue.ts](https://github.com/Effect-TS/effect/blob/7f524e25cfb49a71ac39b0f44278d08d40dda5de/packages/effect/src/persistence/PersistedQueue.ts). Pinned source for our Effect mapping, not a claim from the book.
- [Effect contributors. Effect 4.0.5. workflow/DurableQueue.ts](https://github.com/Effect-TS/effect/blob/7f524e25cfb49a71ac39b0f44278d08d40dda5de/packages/effect/src/workflow/DurableQueue.ts). Pinned source for our Effect mapping, not a claim from the book.