The retry likely repeated a task-creation request whose first outcome was unknown to the caller. The server may have created the landing task, then the worker crashed or lost the response before recording success. Retrying the POST could therefore create a second task. Prevent this by assigning the logical task one durable identity and having the receiver treat every request with that identity as the same operation.
Why a retry can create a duplicate
A timeout, lost response, or worker crash tells the caller only that it did not record confirmation; it does not prove the server failed to act. If the server completed the POST before communication broke, a retry can repeat the side effect. AWS describes this broader distributed-systems problem as the tension between at-most-once and at-least-once actions: retry until confirmation and duplicate delivery becomes possible. AWS Well-Architected guidance recommends idempotency to make repeated requests safe.
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The same concern appears in message processing: Amazon SQS standard queues can deliver a message more than once in rare circumstances, and AWS advises designing consumers to handle duplicates. That is an illustration of the general failure mode, not evidence that a queue was involved in this landing-task incident. Amazon SQS standard queue delivery documentation.
What idempotency means for task creation
Give one logical landing task a stable idempotency key, and send that same key with every attempt to create it. At the receiver, the key must be persisted with the task or operation state. When the receiver sees the key again, it should return or recognize the original task and result instead of creating another task.
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The receiver must also handle concurrent retries safely. If two requests with the same key arrive together, a uniqueness constraint, transaction, or equivalent concurrency control should ensure that only one can claim the key and create the task. AWS guidance calls for tracking token state and managing atomicity and consistency; the concrete mechanism depends on the service and its data store. AWS Well-Architected guidance.
How to make the retry safe
- Create and persist the operation identity before retryable work. Mint or deterministically derive one key for the logical landing task. If a workflow engine can replay a step, make the identity durable across that replay; generating a new key during each replay defeats deduplication. AWS Durable Execution programming model.
- Attach the same key to every attempt. Use the target API’s documented idempotency header or client-token field, including its required format, scope, parameter-matching rules, and retention period. Do not make up a key convention for an endpoint whose contract is unknown.
- Claim the key and create or retrieve the task atomically. Store enough state to distinguish an in-progress operation from a completed one. Use a transaction, uniqueness constraint, or equivalent control so simultaneous requests cannot both create a task.
- Define the duplicate response. On a repeated key, return the original task or result, or provide a duplicate outcome the caller handles as success for that logical operation. A duplicate response is not, by itself, proof that the original operation failed.
- Propagate identity across side-effecting boundaries. If creating the landing task triggers another service or external action, pass through a stable identity or use that downstream service’s own deduplication contract. Idempotency at the first API does not automatically prevent duplicated downstream effects.
Why a workflow retry setting is not enough
Retry policies decide when an attempt is repeated; they do not necessarily make the operation safe to repeat. AWS Durable Execution distinguishes at-least-once behavior from at-most-once semantics for an individual retry attempt and warns that “Neither guarantees the step runs exactly once across the entire workflow.” When retries are enabled, a replay may still issue the external request again. Use a stable key with an idempotency-enabled external service rather than relying on the workflow mode alone. AWS Durable Execution programming model.
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Choose an API-supported key or manage deduplication yourself
| Approach | What it gives you | What to verify |
|---|---|---|
| Receiver-supported idempotency key | The API recognizes repeated requests and can return the first task or saved result. | Key format and scope, parameter matching, retention/expiry, behavior for in-progress requests, and duplicate response. Follow the endpoint’s current documentation. |
| Application-managed deduplication | Your task service stores operation keys and maps them to task state, even if the API has no built-in key facility. | Persistence lifetime, atomic key claiming under concurrent requests, recovery of pending operations, and propagation to downstream side effects. |
Stripe illustrates a vendor-specific POST contract: it documents saving the first status code and response body for a key, checking that later parameters match, and pruning keys once they are at least 24 hours old. Those are Stripe’s rules, not universal defaults for a landing-task API. Stripe idempotent requests.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsAWS ECS RunTask is another product-specific example: it accepts a client token for idempotent task creation. The phrase “landing task” does not establish that the endpoint is ECS, so use this only as an example of an API contract to check, not as a diagnosis. Amazon ECS RunTask API.
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Test the failure window, not just the happy path
Exercise the precise ambiguity that caused the duplicate:
- Submit a task-creation request with a chosen operation key.
- Allow the server to create the task, but simulate losing the response or crashing the caller before it records completion.
- Replay the request with the exact same key.
- Confirm that the retry resolves to the original task and that only one task exists.
- Send two same-key requests concurrently and verify that only one task is created.
- Check a later replay against the documented key-retention period so an expired key cannot silently be treated as new while the logical operation is still being retried.
These checks test both the lost-confirmation window and the concurrency race. Without the endpoint’s identity, API contract, and storage details, it is not possible to name the exact setting or retention period for this particular landing-task service.
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