# Dead letters need a return path

> For agents: start with the [agent guide](https://ratstack.sh/llms.txt). Every page is Markdown by default; add `Accept: text/html` for HTML.

Free workshop: [Build Agent Loops That Never Stop Shipping](/tokenmaxx#interested).

Partial batch failure needs a decision about failed records and successful siblings. Parking work is a recovery boundary. It does not prove successful processing.

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

Source: [Q20](/lore/durability-bibliography#q20); [B5](/lore/durability-bibliography#b5-building-event-driven-microservices), [B9](/lore/durability-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 4.0.0 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 check is our proposal. The books do not prescribe it.

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

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

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

## Sources

1. [Bellemare, Adam. Building Event-Driven Microservices: Leveraging Organizational Data at Scale. O’Reilly, 2020.](https://www.oreilly.com/library/view/building-event-driven-microservices/9781492057888/)
   O’Reilly. Chapter 3 Communication and Data Contracts; Chapter 6 Deterministic Stream Processing; Chapter 7 Stateful Streaming; Chapter 9 Microservices Using Function-as-a-Service; Chapter 14 Supportive Tooling. Accessed 2026-10-23.

2. [Sbarski, Peter; Cui, Yan; Nair, Ajay. Serverless Architectures on AWS, Second Edition.](https://www.manning.com/books/serverless-architectures-on-aws-second-edition)
   Manning. Chapter 6, The New Search Microservice: Everything Fails, All the Time; Pay Attention to Retry Configurations. AWS-specific mechanisms are not universal runtime guarantees. Accessed 2026-10-23.

3. [Effect contributors. Effect 4.0.0. persistence/PersistedQueue.ts](https://github.com/Effect-TS/effect/blob/67ba4e46a11ccda0b6761578bfd22c04ae00167d/packages/effect/src/persistence/PersistedQueue.ts)
   Effect-TS. Pinned source for our Effect mapping, not a claim from the book. Accessed 2026-10-23.

4. [Effect contributors. Effect 4.0.0. workflow/DurableQueue.ts](https://github.com/Effect-TS/effect/blob/67ba4e46a11ccda0b6761578bfd22c04ae00167d/packages/effect/src/workflow/DurableQueue.ts)
   Effect-TS. Pinned source for our Effect mapping, not a claim from the book. Accessed 2026-10-23.

## Linked from

- add-a-feature-flag → Declare a typed feature flag, its scoped evidence ladder, approval roles, removal date, and durable ledger adapter. → [Read page](https://ratstack.sh/skills/add-a-feature-flag)

- Agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms.txt)

- Change log → What changed in the files served here, newest first. → [Read page](https://ratstack.sh/log)

- Checkpoints follow the promised commit → Persist state and progress together when one store can transact both. → [Read page](https://ratstack.sh/lore/checkpoints-follow-commits)

- durability → Keep a rat-stack app standing when a provider, pool, or dependency slows down or a worker dies. Use at any I/O boundary, connection pool, provider call, shared waiter, fan-out, flag, gate, or work that can outlive a request, or when asked "what happens when this is slow", "add a timeout", "make this durable", or "what survives a crash". → [Read page](https://ratstack.sh/skills/durability)

- Durability starts with a recovery record → A reading order for storage, replay, worker ownership, overload, and recovery tests. → [Read page](https://ratstack.sh/lore/durability)

- Feature flags → Typed feature flags with owners, removal dates, and stable subject rollouts. → [Read page](https://ratstack.sh/systems/flags)

- Flags climb on evidence → A flag ladder connects each rollout step to scoped measurements, approval, and a durable receipt. → [Read page](https://ratstack.sh/lore/flags-climb-on-evidence)

- Full agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms-full.txt)

- Golden signals expose overload → Measure latency, traffic, errors, and saturation, including failed work. → [Read page](https://ratstack.sh/lore/golden-signals-expose-overload)

- Rat Stack lore | rat-stack → Short, source-grounded notes on the ideas and decisions behind rat-stack. → [Read page](https://ratstack.sh/lore)

- Request lifetime is not durability → Worker waitUntil has a shared post-response allowance; required work needs a durable marker and redrive. → [Read page](https://ratstack.sh/lore/request-lifetime-is-not-durability)

- Source change history (log.md) → The generated change log as Markdown. → [Read page](https://ratstack.sh/log.md)

- Steady state recycles resources → Every accumulation needs a deliberate recycling policy. → [Read page](https://ratstack.sh/lore/steady-state-recycles-resources)