> ## Documentation Index
> Fetch the complete documentation index at: https://arklowdocs.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Delivery guarantees

Here's what Arklow can guarantee about delivery.

## At-least-once delivery

Arklow uses at-least-once delivery. The same action can reach a destination more than once when an acknowledgement is lost, a lease expires, or a retry is requested.

Each delivery is an attempt. Attempts begin at `0` and increase when Arklow redelivers the action. The action ID remains the same across attempts.

Build consumers to handle repeated delivery safely. Where the work has an external side effect, use the action ID or a stable business identifier as an idempotency key. The attempt number identifies only one delivery of that work; use action-level identity for idempotency.

## Attempt-aware settlement

An acknowledgement, rejection, or deadline extension names both the action and its attempt number. It applies only while that attempt is authoritative.

This prevents a delayed acknowledgement from attempt `0` from completing an action that has already moved on to attempt `1`. Arklow refuses stale settlements, protecting the newer delivery.

The destination can settle an attempt in three ways:

| Signal | Result                                                          |
| ------ | --------------------------------------------------------------- |
| Ack    | The action succeeds                                             |
| Nack   | The action is retried, or fails when the rejection is permanent |
| Modack | The current attempt receives more time to settle                |

## Source deduplication

Queue sources identify work using the source and the provider's message ID. If SQS or Pub/Sub redelivers the same source message after acceptance, the redelivery resolves to the original action.

[HTTP ingress](/resources/sources/arklow/ingress) has no equivalent idempotency key. Every accepted POST creates a new action, even when two requests carry identical payloads.

Source deduplication and destination delivery answer different questions:

* Source deduplication prevents one queue message from creating multiple actions.
* At-least-once delivery allows one accepted action to have multiple delivery attempts.

An action created once may still be delivered more than once. Consumers must therefore remain idempotent even when work arrived through a deduplicated source.