Elixir Protocol Buffer

Warning: only protocol buffers 3 is supported. Use protobuf-elixir if you need support for version 2 (protobuf-elixir was a major inspiration for this project).

This is a protocol buffer encoder and decoder. Its goal is to be fast at the cost of larger generated files. This is achieved by generating a significant part of the encoding and decoding logic at generation time with the protoc plugin.

Encoding and decoding performance is ~3-4x times faster than protobuf-elixir. For example, if we take the %Everything structure used in our tests, which has all field types, including all array types (with 2 values per array) and a few maps, pbuf takes ~14µs to encode and ~24µs to decode, versus 66µs and 67µs. However, the .beam file is quite a bit larger: 19K vs 7K.

(Note that there is limited support for version 2 syntax, but only enough to allow the protoc plugin to bootstrap itself. This may may or may not provide all the version 2 support you need).

Installation

Assuming you already have protoc installed, you'll want to run:

$ mix escript.install hex pbuf

to install the pbuf elixir generator. This will place protoc-gen-fast-elixir in your ~/.mix/escript/ folder. This must be on your $PATH.

You can then generate elixir files using the protoc command with the -fast-elixir_out=PATH flag:

protoc --fast-elixir_out=generated/ myschema.proto 

Note the name fast-elixir_out. This allows you to also have protobuf installed in order to support proto2 syntax.

Encoding

The generated code is normal Elixir modules with a defstruct. Use new/1 to create new instances:

user = Models.User.new(name: "leto", age: 2000)

And Pbuf.encode!/1 and Pbuf.encode_to_iodata!/1 to encode them:

data = Pbuf.encode!(user)

Only structures generated by protoc can be passed to encode!/1 and encode_to_iodata!/1; you cannot pass maps or other structures.

These functions will raise a Pbuf.Encoder.Error on invalid data (such as assigning a float to a bool field). There are currently no non-raising functions.

Decoding

Decoding is done via Pbuf.decode!/2 or Pbuf.decode/2:

user = Pbuf.decode!(Models.User, data)

As an alternative, you can also use: Models.User.decode!(data) or decode/1 to avoid raising on invalid data.

Enumerations

A field declared as an enum should be set to the atom representation of the protocol buffer name, or the integer value. For example, a message defined as:

message User {
  UserType type = 1;
}

enum UserType {
  USER_TYPE_UNKNOWN = 0;
  USER_TYPE_PENDING = 1;
  USER_TYPE_NORMAL = 2;
  USER_TYPE_DELETED = 3;
}

Should be used as:

user = User.new(type: :USER_TYPE_PENDING)
# OR
user = User.new(type: 1)

(casing is preserved from the proto file)

Advanced Enums

You'll likely want to map your protocol buffer enums to specific atoms. With a bit of work, the generator can do this for you.

First, you'll need to specify a custom option, say in options.proto:

syntax = "proto2";

import 'google/protobuf/descriptor.proto';

extend google.protobuf.EnumValueOptions {
  optional string elixir_atom = 78832;
}

You can them import this .proto file like any other and use the option:

import 'options.proto';

enum HTTPMethod {
  HTTP_METHOD_GET = 0 [(elixir_atom) = 'get'];
  HTTP_METHOD_POST = 1 [(elixir_atom) = 'post'];
}

The value will now be :get and :post rather than :HTTP_METHOD_GET and :HTTP_METHOD_POST.

For this to work, Google's proto definitions must be available when you run protoc:

protoc -I=/usr/local/include/proto/ -I=. ...

They are available from the protocol buffer source: https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-osx-x86_64.zip.

Oneofs

By default, a oneof field must be set to a tuple where the first element is the name of the field and the second is the value. Given:

message Event {
  oneof event_oneof {
    Commit commit = 1;
    Wiki wiki = 2;
  }
}

Then valid values for event_oneof are: nil, {:commit, Commit.t} or {:wiki, Wiki.t}.

Jason and Oneofs

Generated structures have a @derive Jason.Encoder. For simple messages, this means you can use Jason.encode(struct) to generate a json representation of your messages.

This fails for oneofs, since Jason can't encode tuples ({:type, value}). You can configure a different oneof format by using the special elixir_oneof_format option:

extend google.protobuf.EnumValueOptions {
  optional string elixir_atom = 78832;
}

And then, at the file level, specifying either:

option (elixir_oneof_format) = 1

and using:

  %{__type: :commit, value: Commit.t}

OR specifying

option (elixir_oneof_format) = 2

and using:

  %{commit: Commit.t}

Json Message Encoding

It's possible to not generate @derive Jason.Encoder on a per-message basis by using a custom option, say in options.proto:

syntax = "proto2";

extend google.protobuf.MessageOptions {
  int32 json_message = 78832;
}

And then using it in your message:

message Something {
  option (json_message) = 0;
  ...
}

Json Field Encoding

It's possible to automatically encode and decode a bytes field to and from Json. First, define a FieldOptions:

syntax = "proto2";

extend google.protobuf.MessageOptions {
  int32 json_field = 78832;
}

And then using it in your message:

message Something {
  bytes data = 1 [(json_field) = 1];
}

Which results in:

  something = [data: %{over: 9000}]
  |> Something.new()
  |> something.encode!()
  |> Somethihg.decode!()

  something.data == %{"over" => 9000}

Note that if you assign the json_field a value of 2, keys will be atomified.

Json Skip Field

It's possible to NOT encoding individual fields using json_field = -1

message Something {
  string id = 1;
  bytes secret = 2 [(json_field) = -1];
}