I’ve had my fair share of dealing with proto files in go (and to some extent JS), so I thought I’d share some stuff I’ve learnt the hard way by working with proto files.

Protoc include paths

The protoc include paths can be pretty confusing, so I’ll give a few examples of how to use it properly.

Just include the current directory

protoc requires that the files referenced are in the include path, so if you’re referencing files relative to the current directory, you’ll need to specify -I., which also means the protoc will resolve all proto import paths relative to the current directory.

$ protoc myproto/myproto.proto -I. --go_out=:.

As long as your proto file imports are all relative to the current directory, this will work.

Several include paths

If you’re using the grpc-gateway you’ll have to include the google/api/annotations.proto proto file. The way I’ve always done that is by vendoring the proto files and adding the vendor path as an include path:

$ protoc myproto/myproto.proto -I. -I./vendor/github.com/googleapis/googleapis/ --go_out=:.

Includes are specified in order of priority, so protoc will first see if ./google/api/annotations.proto exists, and if it doesn’t, it’ll check ./vendor/github.com/googleapis/googleapis/google/api/annotations.proto.

Note that vendoring is obviously a Go thing, so this might not chime well with the other Devs in the office who want to keep the proto repository language agnostic. In that case, you’ll probably define some third_party folder where you can put the external dependencies.

Use the go_package option

This isn’t something readily advertised in the introduction to protobuffers in Go or Go gRPC quick start, but I find it is essential if you ever want to import proto definitions from one proto file to another.

Raison d’être

For example, lets say we have person.proto in person/person.proto. It defines the proto package person and the message Person.

syntax = "proto3";

package person;

message Person {
  string Name = 1;
  uint32 Age = 2;
}

We also have team.proto in team/team.proto, defining the proto package team and the message Team. A Team consists of a sorted list of Persons, so team.proto will need to import that definition from person.proto.

No problem, just add an import person/person.proto to team.proto and reference it using the namespace specified by the person.proto package name:

syntax = "proto3";

package team;

import "person/person.proto"

message Team {
  repeated person.Person people = 1;
}

When we generate a go file from this definition using protoc, we’ll end up with a Go file that imports person/person.pb.go. That’s no good!

Enter the go_package option.

Using the go_package option

For a proto file defined in github.com/myuser/myprotos/myproto/myproto.proto the appropriate go_option value would be github.com/myuser/myprotos/myproto. This means that the protoc compiler can generate a go file that will include the package github.com/myuser/myprotos/myproto if you have another proto file that depends on myproto/myproto.proto.

So let’s fix the person.proto and team.proto proto files.

syntax = "proto3";

option go_package = "github.com/myuser/myprotos/person";

package person;

message Person {
  string Name = 1;
  uint32 Age = 2;
}
syntax = "proto3";

option go_package = "github.com/myuser/myprotos/team";

package team;

import "person/person.proto"

message Team {
  repeated person.Person members = 1;
}

Now, you might say we don’t strictly need to specify the go_package for team.proto, since nothing imports it at the minute. I’d still suggest adding it to all the proto files that you’ll generate go code from so that in the future when a dependency might arise, you’ve saved yourself, or even better, someone else, a whole lot of head scratching.

protoc-gen-go output paths with go_package option

One final note on the go_package option. Specifying it in your proto file means the protoc-gen-go protoc plugin outputs your generated files as if the specified output directory is at the root of the go_package path. So… you’ll probably want to slightly modify your protoc line:

protoc person/person.proto team/team.proto -I. --go_out=:$GOPATH/src

This should mean the files appear where you expect them to appear. Mind you make sure there are no typos in the go_package option as it means the files will be generated in wrong place.

protoc plugin parameters

Another thing I’ve learned through hours staring at my terminal in bewilderment is how parameters are passed to protoc plugins. For example, the protoc-gen-go plugin allows you to specify plugin=grpc as a parameter, and the protoc-gen-grpc-gateway takes a boolean parameter logtostderr=true. I also think the M parameter is a protoc-wide way to change the import path of a specific import as defined in a proto file. Parameters are comma (,) separated. Parameter specification is delimited by the colon (:) character, after which comes the desired output path. The following are all valid protoc commands illustrating this:

$ protoc myproto/myproto.proto -I. --go_out=plugin=grpc:.
$ protoc myproto/myproto.proto -I. --grpc-gateway_out=logtostderr=true,Mgoogle/api/annotations.proto=myrepo/api/annotations.proto:.

More

Feel free to reach out to me on gophers slack or on my twitter if you found this helpful or if you have any more tips I should include in this list.