Adopt AEP-155: Idempotency (aep-dev#77)

* Adopt AEP-155: Idempotency

* Address Yusuke's comments.

* Update aep/general/0155/aep.md

* lint

* Qualify proto-specific guidance.

* Update to reflect new common components and live discussion.

* Minor rephrasing of list.

* Clarify invalid `first_sent`.

* Remove section about stale success responses.

Author: rofrankel

aep/general/0155/aep.md

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+# Idempotency
+
+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:
+
+- De-duplicating requests from parallel processes
+- Ensuring the safety of retries
+- Auditing
+
+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.
+
+## Guidance
+
+APIs **may** add a `aep.api.IdempotencyKey idempotency_key` parameter to
+request messages (including those of standard methods) in order to uniquely
+identify particular requests. API servers **must not** execute requests with
+the same `idempotency_key` more than once.
+
+```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"
+    }];
+
+  // The book to create.
+  Book book = 2 [(google.api.field_behavior) = REQUIRED];
+
+  // This request is only idempotent if `idempotency_key` is provided.
+  //
+  // This key will be honored for at least one hour after the first time it is
+  // seen by the server.
+  //
+  // The key is restricted to 36 ASCII characters. A random UUID is recommended.
+  aep.api.IdempotencyKey idempotency_key = 3 [
+      (aep.api.field_info).minimum_lifetime = { seconds: 3600 }
+  ];
+}
+```
+
+- [`aep.api.IdempotencyKey`][] has a `key` and a `first_sent` timestamp.
+
+  - `key` is simply a unique identifier.
+
+- Providing an idempotency key **must** guarantee idempotency.
+
+  - If a duplicate request is detected, the server **must** return one of:
+
+    - A response equivalent to the response for the previously successful
+      request, because the client most likely did not receive the previous
+      response.
+    - An error indicating that the `first_sent` field of the idempotency key is
+      invalid or cannot be honored (expired, in the future, or differs from a
+      previous `first_sent` value with the same `key`).
+    - An error, if returning an equivalent response is not possible.
+
+      For example, if a resource was created, then deleted, and then a
+      duplicate request to create the resource is received, the server **may**
+      return an error if returning the previously created resource is not
+      possible.
+
+  - APIs **should** honor idempotency keys for at least an hour.
+    - When using protocol buffers, idempotency keys that are UUIDs **must** be
+      annotated with a minimum lifetime using the extension
+      [`(aep.api.field_info).minimum_lifetime`][].
+
+- 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).
+
+  - The `first_sent` field can be used by API servers to determine if a key is
+    expired. API servers **must** reject requests with expired keys, and
+    **must** reject requests with keys that are in the future. When feasible,
+    API servers **should** reject requests that use the same `key` but have a
+    different `first_sent` timestamp.
+  - The `key` field **must** be able to be a UUID, and **may** allow UUIDs to
+    be the only valid format. The format restrictions for idempotency keys
+    **must** be documented.
+
+- Idempotency keys **should** be optional.
+
+## Further reading
+
+- 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).
+
+## Rationale
+
+### Naming the field `idempotency_key`
+
+The original content from which this AEP is derived defines a `request_id`
+field; we define `idempotency_key` instead for two reasons:
+
+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.
+
+<!-- prettier-ignore-start -->
+[idempotency-key-draft]: https://datatracker.ietf.org/doc/draft-ietf-httpapi-idempotency-key-header/
+[`aep.api.IdempotencyKey`]: https://buf.build/aep/api/file/main:aep/api/idempotency_key.proto#L21
+[`(aep.api.field_info).minimum_lifetime`]: https://buf.build/aep/api/file/main:aep/api/field_info.proto#L35
+<!-- prettier-ignore-end -->
+
+### Using UUIDs for request identification
+
+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.
+
+## Changelog
+
+- **2023-23-20**: Adopt AEP from from Google's AIP with the following changes:
+  - Rename field from `request_id` to `idempotency_key` (plus some minor
+    releated rewording).
+  - Add a common component [`aep.api.IdempotencyKey`][] and use this rather
+    than `string` for the `idempotency_key` field; add related guidance about
+    `IdempotencyKey.first_seen`.
+  - Remove guidance about annotating `idempotency_key` with
+    `(google.api.field_info).format`.
+  - Add guidance about annotating `idempotency_key` with
+    [`(aep.api.field_info).minimum_lifetime`].
+  - Update guidance about responses to be more explicit about success and error
+    cases, while allowing "equivalent" rather than identical responses for
+    subsequent requests.
+  - Temporarily removed the section about stale success responses, pending
+    further discussion.
+- **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.

aep/general/0155/aep.yaml

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+---
+id: 155
+state: approved
+slug: idempotency
+created: 2019-05-06
+placement:
+  category: requests
+  order: 60