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+---
+id: 4234
+state: reviewing
+created: 2021-06-07
+---
+
+# Common service client mixins
+
+Often, APIs can have common features for administering resources that are
+separated into utility services. These common, utility services are shared among
+the discrete product services. These common services each have a
+centrally-defined surface, but individual instances are hosted alongside each
+product service. Thus, the surfaces of each common service hosted by the product
+service are "mixed in" at runtime.
+
+
+## Guidance
+
+Client libraries **may** provide client methods for the common services that its
+API declares to be mixed-in. Such methods improve the user experience by
+presenting the mixin methods from a client configured to communicate with the
+product service that hosts the mixin service.
+
+If client libraries support mixin services, they **must** support the following
+common services (and **may** support others):
+
+- `google.cloud.location.Locations`
+- `google.iam.v1.IAMPolicy`
+- `google.longrunning.Operations`
+
+**Note:** The list of supported mixin services and the mixin services
+themselves both change infrequently. Any new common service to be supported
+**must** be added to the list here and generators **must** be updated.
+
+To be generated into a client library, a mixin service **must** be declared
+under [apis] in a [google.api.Service]. Furthermore, only the RPCs with
+[google.api.http] bindings declared in the [http] configuration of the same
+[google.api.Service] can be generated (more details on these bindings in
+AIP-127). If a mixin service RPC does not have a [google.api.http] rule declared
+in the [google.api.Service], it **must not** be generated.
+
+
+## Implementing mixin support
+
+### Generator configuration
+
+Client library generators **must** accept the file path of a
+[google.api.Service] in YAML form. This file is specified via a flag that
+**must** be optional. In other words, the flag **must not** be required for
+basic client generation that omits the mixin methods.
+
+### Mixin API client configuration
+
+If a mixin API utilizes client library configuration (i.e. annotations, default
+retry settings, etc.) such as `google.api.method_signature` or request header
+injection, generators **may** support it in the generated mixin methods if the
+configuration is accessible.
+
+Where client library configuration calls for a fully-qualified name, the
+_mixin's_ fully-qualified name **must** be used. That is to say, the host
+service **must not** refer to a mixin element as if it was in its own package.
+
+For example, configuring `google.iam.v1.IAMPolicy.GetIAMPolicy` with default
+retry and timeout settings would be as follows:
+
+```json
+{
+  "name": [{ "service": "google.iam.v1.IAMPolicy", "method": "GetIAMPolicy" }],
+  "timeout": "60s",
+  "retryPolicy": {
+    "initialBackoff": "0.1s",
+    "maxBackoff": "5s",
+    "backoffMultiplier": 1.3,
+    "retryableStatusCodes": ["UNKNOWN"]
+  }
+}
+```
+
+### Generating mixin methods
+
+The mixin API RPCs **should** be generated as methods on the surface of the
+host API's service client library, alongside the host service's RPCs. This
+presents them as top-level methods on the client, but under-the-hood, the
+appropriate mixin API stub or request should be invoked. However, generators
+**may** choose to present the mixin API RPCs in a different manner if it is more
+language idiomatic.
+
+**Note:** For gRPC clients, the mixin API gRPC stub **must** be used in order
+to properly construct the gRPC request. For example, the generated, Go gRPC stub
+[LocationsClient] must be used to invoke RPCs for the
+`google.cloud.location.Locations` mixin, as opposed to that of the host
+service's gRPC stub.
+
+### Multiple host service clients
+
+If there are multiple services defined by the host API that would result in
+multiple clients being generated, generators **must** include the mixin
+methods on all eligible, generated service clients.
+
+### Overriding a duplicate RPC
+
+Client library generators **must not** generate a mixin method on any host
+service client if a host service already defines an RPC with the same name. For
+example, take the following service definition:
+
+```proto
+service LibraryService {
+  rpc ListBooks(ListBookRequest) returns (ListBooksResponse);
+
+  // Other host API RPCs...
+
+  // Redefinitions of the google.iam.v1.IAMPolicy mixin service.
+  rpc GetIamPolicy(google.iam.v1.GetIamPolicyRequest)
+      returns (google.iam.v1.Policy);
+
+  rpc SetIamPolicy(google.iam.v1.SetIamPolicyRequest)
+      returns (google.iam.v1.Policy);
+
+  rpc TestIamPermissions(google.iam.v1.TestIamPermissionsRequest)
+      returns (google.iam.v1.TestIamPermissionsResponse);
+}
+```
+
+Should the host API declare the `google.iam.v1.IAMPolicy` as a mixin service,
+client library generators **must not** generate the resulting mixin methods that
+match the names explicitly declared above for _any_ service client in the host
+proto package, but they **must** generate the other methods for this mixin. In
+this case, the generator **may** log a warning indicating that a collision was
+avoided, or silently skip the mixin methods in question.
+
+[apis]: https://github.com/googleapis/googleapis/blob/master/google/api/service.proto#L96
+[google.api.Service]: https://github.com/googleapis/googleapis/blob/master/google/api/service.proto
+[google.api.Http]: https://github.com/googleapis/googleapis/blob/master/google/api/http.proto
+[http]: https://github.com/googleapis/googleapis/blob/master/google/api/service.proto#L124
+[LocationsClient]: https://pkg.go.dev/google.golang.org/genproto@v0.0.0-20210325141258-5636347f2b14/googleapis/cloud/location#LocationsClient