Protox

Protox is a native Elixir library to work with Google's Protocol Buffers (version 2 and 3).

Conformance

This library has been tested using the conformance checker provided by Google. More informations at https://github.com/EasyMile/protox-conformance.

Prerequisites

Protox uses Google's protoc (>= 3.0) to parse .proto files. It must be available in $PATH. This dependency is only required at compile-time. You can get it here.

Usage

From files:

defmodule Foo do
  @external_resource "./defs/foo.proto"
  @external_resource "./defs/bar.proto"
  @external_resource "./defs/baz/fiz.proto"

  use Protox, files: [
    "./defs/foo.proto",
    "./defs/bar.proto",
    "./defs/baz/fiz.proto",
  ]
end

From a textual description:

defmodule Bar do
  use Protox, schema: """
  syntax = "proto3";

  package fiz;

  message Baz {
  }

  message Foo {
    int32 a = 1;
    map<int32, Baz> b = 2;
  }
  """
end

The previous example will generate two modules: Fiz.Baz and Fiz.Foo.

It's possible to prepend a namespace to all generated modules:

defmodule Bar do
  use Protox, schema: """
    syntax = "proto3";

    enum Enum {
        FOO = 0;
        BAR = 1;
      }
    """,
    namespace: Namespace
end

In this case, the module Namespace.Enum will be generated.

Here's how to create a new message:

iex> %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}} |> Protox.Encode.encode()
[[[], "\b", <<3>>], <<18>>, <<4>>, "\b", <<1>>, <<18>>, <<0>>]

Note that Protox.Encode.encode/1 returns an iolist, not a binary. Such iolists can be used directly with files or sockets write operations. However, you can use :binary.list_to_bin/1 to get a binary:

iex> %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}} |> Protox.Encode.encode() |> :binary.list_to_bin()
<<8, 3, 18, 4, 8, 1, 18, 0>>

Finally, here's how to decode:

iex> <<8, 3, 18, 4, 8, 1, 18, 0>> |> Fiz.Foo.decode()
{:ok,
 %Fiz.Foo{__uf__: [], a: 3,
  b: %{1 => %Fiz.Baz{__uf__: []}}}}

The __uf__ field is explained in the section Unknown fields.

Unknown fields

If any unknown fields are encountered when decoding, they are kept in the decoded message. It's possible to access them with the function get_unknown_fields/1 defined with the message.

iex> msg = <<8, 42, 42, 4, 121, 97, 121, 101, 136, 241, 4, 83>> |> Msg.decode!()
%Msg{a: 42, b: "", z: -42, __uf__: [{5, 2, <<121, 97, 121, 101>>}]}

iex> msg |> Msg.get_unknown_fields()
[{5, 2, <<121, 97, 121, 101>>}]

You must always use get_unknown_fields/1 as the name of the struct field (e.g. __uf__) is generated at compile-time to avoid collision with the actual fields of the protobuf message.

This function returns a list of tuples {tag, wire_type, bytes}.

Unsupported features

• protobuf 3 JSON mapping
• groups
• rpc

Furthermore, all options other than packed and default are ignored.

Implementation choices

• Required fields (protobuf 2): an error is raised when decoding a message with a missing required field.

• When decoding enum aliases, the last encountered constant will be used. For instance, in the following example, :BAR will always be used if the value 1 is read on the wire.

  enum E {
    option allow_alias = true;
    FOO = 0;
    BAZ = 1;
    BAR = 1;
  }

• Unset optionals

  • For protobuf 2, unset optional fields are mapped to nil
  • For protobuf 3, unset optional fields are mapped to their default values, as mandated by the protobuf spec

Types mapping

The following table shows how Protobuf types are mapped to Elixir's ones.

Protobuf | Elixir -----------|-------------- int32 | integer() int64 | integer() uint32 | integer() uint64 | integer() sint32 | integer() sint64 | integer() fixed32 | integer() fixed64 | integer() sfixed32 | integer() sfixed64 | integer() float | float(), :infinity, :'-infinity', :nan double | float(), :infinity, :'-infinity', :nan bool | boolean() string | String.t bytes | binary() map | %{} oneof | {:field, value} enum | atom() message | struct()

Credits

Both gpb and exprotobuf were very useful in understanding how to implement Protocol Buffers.