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: `GET /v1/messages/123456` - gRPC:
`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: `GET
/v1/messages/123456?revision=2&sub.subfield=foo` - gRPC: `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¶m=B`. In the case of a message type, each field of the message is mapped to a
separate parameter, such as `...?foo.a=A&foo.b=B&foo.c=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:
`PATCH /v1/messages/123456 { "text": "Hi!" }` - gRPC: `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: `PATCH /v1/messages/123456 { "text": "Hi!" }` - gRPC:
`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: `GET /v1/messages/123456` - gRPC: `GetMessage(message_id: "123456")` - HTTP:
`GET /v1/users/me/messages/123456` - gRPC: `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. The following example selects a
gRPC method and applies an `HttpRule` to it: http: rules: - 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 Networking API. For a detailed explanation
see:
https://developers.google.com/api-client-library/java/google-http-java-client/json