# gRPC Transcoding gRPC Transcoding is a feature for mapping between a gRPC method and one or
more HTTP REST endpoints. It allows developers to build a single API service that supports both
gRPC APIs and REST APIs. Many systems, including [Google
APIs](https://github.com/googleapis/googleapis), [Cloud
Endpoints](https://cloud.google.com/endpoints), [gRPC Gateway](https://github.com/grpc-ecosystem
/grpc-gateway), and [Envoy](https://github.com/envoyproxy/envoy) proxy support this feature and
use it for large scale production services. `HttpRule` defines the schema of the gRPC/REST
mapping. The mapping specifies how different portions of the gRPC request message are mapped to
the URL path, URL query parameters, and HTTP request body. It also controls how the gRPC response
message is mapped to the HTTP response body. `HttpRule` is typically specified as an
`google.api.http` annotation on the gRPC method. Each mapping specifies a URL path template and
an HTTP method. The path template may refer to one or more fields in the gRPC request message, as
long as each field is a non-repeated field with a primitive (non-message) type. The path template
controls how fields of the request message are mapped to the URL path. Example: service Messaging
{ rpc GetMessage(GetMessageRequest) returns (Message) { option (google.api.http) = { get:
"/v1/{name=messages}" }; } } message GetMessageRequest { string name = 1; // Mapped to URL path.
} message Message { string text = 1; // The resource content. } This enables an HTTP REST to gRPC
mapping as below: HTTP | gRPC -----|----- `GET /v1/messages/123456` | `GetMessage(name:
"messages/123456")` Any fields in the request message which are not bound by the path template
automatically become HTTP query parameters if there is no HTTP request body. For example: service
Messaging { rpc GetMessage(GetMessageRequest) returns (Message) { option (google.api.http) = {
get:"/v1/messages/{message_id}" }; } } message GetMessageRequest { message SubMessage { string
subfield = 1; } string message_id = 1; // Mapped to URL path. int64 revision = 2; // Mapped to
URL query parameter `revision`. SubMessage sub = 3; // Mapped to URL query parameter
`sub.subfield`. } This enables a HTTP JSON to RPC mapping as below: HTTP | gRPC -----|----- `GET
/v1/messages/123456?revision=2=foo` | `GetMessage(message_id: "123456" revision: 2 sub:
SubMessage(subfield: "foo"))` Note that fields which are mapped to URL query parameters must have
a primitive type or a repeated primitive type or a non-repeated message type. In the case of a
repeated type, the parameter can be repeated in the URL as `...?param=A=B`. In the case of a
message type, each field of the message is mapped to a separate parameter, such as
`...?foo.a=A=B=C`. For HTTP methods that allow a request body, the `body` field specifies the
mapping. Consider a REST update method on the message resource collection: service Messaging {
rpc UpdateMessage(UpdateMessageRequest) returns (Message) { option (google.api.http) = { patch:
"/v1/messages/{message_id}" body: "message" }; } } message UpdateMessageRequest { string
message_id = 1; // mapped to the URL Message message = 2; // mapped to the body } The following
HTTP JSON to RPC mapping is enabled, where the representation of the JSON in the request body is
determined by protos JSON encoding: HTTP | gRPC -----|----- `PATCH /v1/messages/123456 { "text":
"Hi!" }` | `UpdateMessage(message_id: "123456" message { text: "Hi!" })` The special name `*` can
be used in the body mapping to define that every field not bound by the path template should be
mapped to the request body. This enables the following alternative definition of the update
method: service Messaging { rpc UpdateMessage(Message) returns (Message) { option
(google.api.http) = { patch: "/v1/messages/{message_id}" body: "*" }; } } message Message {
string message_id = 1; string text = 2; } The following HTTP JSON to RPC mapping is enabled: HTTP
| gRPC -----|----- `PATCH /v1/messages/123456 { "text": "Hi!" }` | `UpdateMessage(message_id:
"123456" text: "Hi!")` Note that when using `*` in the body mapping, it is not possible to have
HTTP parameters, as all fields not bound by the path end in the body. This makes this option more
rarely used in practice when defining REST APIs. The common usage of `*` is in custom methods
which don't use the URL at all for transferring data. It is possible to define multiple HTTP
methods for one RPC by using the `additional_bindings` option. Example: service Messaging { rpc
GetMessage(GetMessageRequest) returns (Message) { option (google.api.http) = { get:
"/v1/messages/{message_id}" additional_bindings { get:
"/v1/users/{user_id}/messages/{message_id}" } }; } } message GetMessageRequest { string
message_id = 1; string user_id = 2; } This enables the following two alternative HTTP JSON to RPC
mappings: HTTP | gRPC -----|----- `GET /v1/messages/123456` | `GetMessage(message_id: "123456")`
`GET /v1/users/me/messages/123456` | `GetMessage(user_id: "me" message_id: "123456")` ## Rules
for HTTP mapping 1. Leaf request fields (recursive expansion nested messages in the request
message) are classified into three categories: - Fields referred by the path template. They are
passed via the URL path. - Fields referred by the HttpRule.body. They are passed via the HTTP
request body. - All other fields are passed via the URL query parameters, and the parameter name
is the field path in the request message. A repeated field can be represented as multiple query
parameters under the same name. 2. If HttpRule.body is "*", there is no URL query parameter, all
fields are passed via URL path and HTTP request body. 3. If HttpRule.body is omitted, there is no
HTTP request body, all fields are passed via URL path and URL query parameters. ### Path template
syntax Template = "/" Segments [ Verb ] ; Segments = Segment { "/" Segment } ; Segment = "*" |
"**" | LITERAL | Variable ; Variable = "{" FieldPath [ "=" Segments ] "}" ; FieldPath = IDENT {
"." IDENT } ; Verb = ":" LITERAL ; The syntax `*` matches a single URL path segment. The syntax
`**` matches zero or more URL path segments, which must be the last part of the URL path except
the `Verb`. The syntax `Variable` matches part of the URL path as specified by its template. A
variable template must not contain other variables. If a variable matches a single path segment,
its template may be omitted, e.g. `{var}` is equivalent to `{var=*}`. The syntax `LITERAL`
matches literal text in the URL path. If the `LITERAL` contains any reserved character, such
characters should be percent-encoded before the matching. If a variable contains exactly one path
segment, such as `"{var}"` or `"{var=*}"`, when such a variable is expanded into a URL path on
the client side, all characters except `[-_.~0-9a-zA-Z]` are percent-encoded. The server side
does the reverse decoding. Such variables show up in the [Discovery
Document](https://developers.google.com/discovery/v1/reference/apis) as `{var}`. If a variable
contains multiple path segments, such as `"{var=foo}"` or `"{var=**}"`, when such a variable is
expanded into a URL path on the client side, all characters except `[-_.~/0-9a-zA-Z]` are
percent-encoded. The server side does the reverse decoding, except "%2F" and "%2f" are left
unchanged. Such variables show up in the [Discovery
Document](https://developers.google.com/discovery/v1/reference/apis) as `{+var}`. ## Using gRPC
API Service Configuration gRPC API Service Configuration (service config) is a configuration
language for configuring a gRPC service to become a user-facing product. The service config is
simply the YAML representation of the `google.api.Service` proto message. As an alternative to
annotating your proto file, you can configure gRPC transcoding in your service config YAML files.
You do this by specifying a `HttpRule` that maps the gRPC method to a REST endpoint, achieving
the same effect as the proto annotation. This can be particularly useful if you have a proto that
is reused in multiple services. Note that any transcoding specified in the service config will
override any matching transcoding configuration in the proto. Example: http: rules: # Selects a
gRPC method and applies HttpRule to it. - selector: example.v1.Messaging.GetMessage get:
/v1/messages/{message_id}/{sub.subfield} ## Special notes When gRPC Transcoding is used to map a
gRPC to JSON REST endpoints, the proto to JSON conversion must follow the [proto3
specification](https://developers.google.com/protocol-buffers/docs/proto3#json). While the single
segment variable follows the semantics of [RFC 6570](https://tools.ietf.org/html/rfc6570) Section
3.2.2 Simple String Expansion, the multi segment variable **does not** follow RFC 6570 Section
3.2.3 Reserved Expansion. The reason is that the Reserved Expansion does not expand special
characters like `?` and `#`, which would lead to invalid URLs. As the result, gRPC Transcoding
uses a custom encoding for multi segment variables. The path variables **must not** refer to any
repeated or mapped field, because client libraries are not capable of handling such variable
expansion. The path variables **must not** capture the leading "/" character. The reason is that
the most common use case "{var}" does not capture the leading "/" character. For consistency, all
path variables must share the same behavior. Repeated message fields must not be mapped to URL
query parameters, because no client library can support such complicated mapping. If an API needs
to use a JSON array for request or response body, it can map the request or response body to a
repeated field. However, some gRPC Transcoding implementations may not support this feature.
This is the Java data model class that specifies how to parse/serialize into the JSON that is
transmitted over HTTP when working with the Service Management API. For a detailed explanation
see:
https://developers.google.com/api-client-library/java/google-http-java-client/json