Enum Idempotency (2.32.0-rc)

When a RPC fails with a retryable error, the google-cloud-cpp client libraries automatically retry the RPC if the RPC is idempotent. For each service, the library define a policy that determines whether a given request is idempotent. In many cases this can be determined statically, for example, read-only operations are always idempotent. In some cases, the contents of the request may need to be examined to determine if the operation is idempotent. For example, performing operations with pre-conditions, such that the pre-conditions change when the operation succeed, is typically idempotent.

Applications may override the default idempotency policy, though we anticipate that this would be needed only in very rare circumstances. A few examples include:

• In some services deleting "the most recent" entry may be idempotent if the system has been configured to keep no history or versions, as the deletion may succeed only once. In contrast, deleting "the most recent entry" is not idempotent if the system keeps multiple versions. Google Cloud Storage or Bigtable can be configured either way.
• In some applications, creating a duplicate entry may be acceptable as the system will deduplicate them later. In such systems it may be preferable to retry the operation even though it is not idempotent.