/// Defines the HTTP configuration for an API service. It contains a list of
/// \[HttpRule][google.api.HttpRule\], each specifying the mapping of an RPC method
/// to one or more HTTP REST API methods.
/// # 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&sub.subfield=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¶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 | 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 \[HttpRule.body][google.api.HttpRule.body\]. 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 \[HttpRule.body][google.api.HttpRule.body\] is "*", there is no URL query parameter, all fields
/// are passed via URL path and HTTP request body.
/// 3. If \[HttpRule.body][google.api.HttpRule.body\] 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.
/// Nested message and enum types in `HttpRule`.
/// A custom pattern is used for defining custom HTTP verb.