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Adopt AEP-155: Idempotency
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Address Yusuke's comments.
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Update aep/general/0155/aep.md
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Qualify proto-specific guidance.
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# Idempotency | ||
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It is sometimes useful for an API to have a unique, customer-provided | ||
identifier for particular requests. This can be useful for several purposes, | ||
such as: | ||
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- De-duplicating requests from parallel processes | ||
- Ensuring the safety of retries | ||
- Auditing | ||
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The purpose of idempotency keys is to provide idempotency guarantees: allowing | ||
the same request to be issued more than once without subsequent calls having | ||
any effect. In the event of a network failure, the client can retry the | ||
request, and the server can detect duplication and ensure that the request is | ||
only processed once. | ||
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## Guidance | ||
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APIs **may** add a `string idempotency_key` parameter to request messages | ||
(including those of standard methods) in order to uniquely identify particular | ||
requests. | ||
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```proto | ||
message CreateBookRequest { | ||
// The parent resource where this book will be created. | ||
// Format: publishers/{publisher} | ||
string parent = 1 [ | ||
(google.api.field_behavior) = REQUIRED, | ||
(google.api.resource_reference) = { | ||
child_type: "library.example.com/Book" | ||
}]; | ||
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// The book to create. | ||
Book book = 2 [(google.api.field_behavior) = REQUIRED]; | ||
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// A unique identifier for this request. Restricted to 36 ASCII characters. | ||
// A random UUID is recommended. | ||
// This request is only idempotent if a `idempotency_key` is provided. | ||
string idempotency_key = 3 [(google.api.field_info).format = UUID4]; | ||
} | ||
``` | ||
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- Providing an idempotency key **must** guarantee idempotency. | ||
- If a duplicate request is detected, the server **should** return the | ||
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response for the previously successful request, because the client most | ||
likely did not receive the previous response. | ||
- APIs **should** honor idempotency keys for at least an hour. | ||
- The `idempotency_key` field **must** be provided on the request message to | ||
which it applies (and it **must not** be a field on resources themselves). | ||
- Idempotency keys **should** be optional. | ||
- Idempotency keys **must** be able to be UUIDs, and **may** allow UUIDs to be | ||
the only valid format. The format restrictions for idempotency keys **must** | ||
be documented. | ||
- When using protocol buffers, idempotency keys that are UUIDs **must** be | ||
annotated with the `google.api.FieldInfo.Format` value `UUID4` using the | ||
extension `(google.api.field_info).format = UUID4`. See | ||
[AEP-202](../0202/aep.md) for more. | ||
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### Stale success responses | ||
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In some unusual situations, it may not be possible to return an identical | ||
success response. For example, a duplicate request to create a resource may | ||
arrive after the resource has not only been created, but subsequently updated; | ||
because the service has no other need to retain the historical data, it is no | ||
longer feasible to return an identical success response. | ||
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In this situation, the method **may** return the current state of the resource | ||
instead. In other words, it is permissible to substitute the historical success | ||
response with a similar response that reflects more current data. | ||
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## Further reading | ||
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- For which codes to retry, see [AEP-194](https://aep.dev/194). | ||
- For how to retry errors in client libraries, see | ||
[AEP-4221](https://aep.dev/4221). | ||
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## Rationale | ||
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### Naming the field `idempotency_key` | ||
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The original content from which this AEP is derived defines a `request_id` | ||
field; we define `idempotency_key` instead for two reasons: | ||
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1. There is an [active Internet-Draft][idempotency-key-draft] to standardize an | ||
HTTP header named `Idempotency-Key`. | ||
1. There may be edge cases in which separately identifying idempotent requests | ||
is useful; `request_id` would be more appropriate for such use cases. For | ||
example, an API producer might be testing the idempotency behavior of the | ||
API server, and might want to issue multiple requests with the same | ||
`idempotency_key` and trace the behavior of each request separately. | ||
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<!-- prettier-ignore-start --> | ||
[idempotency-key-draft]: https://datatracker.ietf.org/doc/draft-ietf-httpapi-idempotency-key-header/ | ||
<!-- prettier-ignore-end --> | ||
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### Using UUIDs for request identification | ||
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When a value is required to be unique, leaving the format open-ended can lead | ||
to API consumers incorrectly providing a duplicate identifier. As such, | ||
standardizing on a universally unique identifier drastically reduces the chance | ||
for collisions when done correctly. | ||
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## Changelog | ||
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- **2023-11-21**: Adopt AEP from from Google's AIP; rename field from | ||
`request_id` to `idempotency_key`. | ||
- **2023-10-02**: Add UUID format extension guidance. | ||
- **2019-08-01**: Changed the examples from "shelves" to "publishers", to | ||
present a better example of resource ownership. |
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--- | ||
id: 155 | ||
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state: approved | ||
slug: idempotency | ||
created: 2019-05-06 | ||
placement: | ||
category: requests | ||
order: 60 |
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