# rpc-request-response-unique

Source: https://apigenie.pl/docs/linter/protobuf/rules/rpc-request-response-unique

This rule checks whether every message is used as the request or the response of a single RPC.

## Rule Details

A message shared by several RPCs, or used both as a request and as a response, ties these RPCs
together: a field added for one of them appears in all the others, and cannot be removed again
without breaking them.

The `rpc-request-response-unique` rule goes through the RPCs of all the services of the file in
order, first the request and then the response type of each. When a type was already used by an
earlier RPC, or as the request of the same RPC, it reports, at the RPC that uses it again:

`Message '<type>' is used more than once: as <first use> and as <this use>`

where each use reads `the request of '<rpc>'` or `the response of '<rpc>'`; in files with
several services, RPCs are named with their service, e.g. `'PetService.GetPet'`. An RPC whose
request and response are the same message is reported too. In the message, `<type>` is the
type name as written in the RPC. Types are compared by their fully-qualified names, so
`GetPetResponse` and `acme.pets.v1.GetPetResponse` are the same message.

Only the RPCs of the linted file are compared.

## Configuration

The configuration block is an object with boolean properties:

- `allow_google_protobuf_empty_requests`: whether `google.protobuf.Empty` may be the request of
  several RPCs (default `false`).
- `allow_google_protobuf_empty_responses`: whether `google.protobuf.Empty` may be the response
  of several RPCs (default `false`).

**Configuration Example**

```yaml
version: "1.0"
commands:
  lint:
    protobuf:
      rules:
        rpc-request-response-unique:
          - warning
          - allow_google_protobuf_empty_requests: true
            allow_google_protobuf_empty_responses: true
```

## Examples

**Good**. With the default configuration.

```proto
syntax = "proto3";

package acme.pets.v1;

service PetService {
  rpc GetPet(GetPetRequest) returns (GetPetResponse);
  rpc UpdatePet(UpdatePetRequest) returns (UpdatePetResponse);
}

message Pet {
  string name = 1;
}

message GetPetRequest {
  string name = 1;
}

message GetPetResponse {
  Pet pet = 1;
}

message UpdatePetRequest {
  Pet pet = 1;
}

message UpdatePetResponse {
  Pet pet = 1;
}
```

**Bad**. With the default configuration, the linter reports
`Message 'Pet' is used more than once: as the response of 'GetPet' and as the response of 'UpdatePet'`.

```proto
syntax = "proto3";

package acme.pets.v1;

service PetService {
  rpc GetPet(GetPetRequest) returns (Pet);
  rpc UpdatePet(UpdatePetRequest) returns (Pet);
}

message Pet {
  string name = 1;
}

message GetPetRequest {
  string name = 1;
}

message UpdatePetRequest {
  Pet pet = 1;
}
```

## When Not to Use It

You may disable this rule if the RPCs are part of an existing API that you cannot change. To
allow a single RPC, add an `// apigenie-lint-disable rpc-request-response-unique` comment to it.

## Compatibility

This rule is compatible with proto3 files. proto2 and editions files are rejected by the reader
and are not linted. The rule resolves the request and response types among the declarations of
the file and of the files it imports; a type that cannot be resolved is compared by its name as
written, without a leading dot.