Protox

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

Prerequisites

Protox uses Google's protoc (>= 3.0) to parse .proto files. It must be available in $PATH. 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{a: 3, b: %{1 => %Fiz.Baz{}}}}

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, __unknown_fields__: [{5, 2, <<121, 97, 121, 101>>}]}

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

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

It 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

Protobuf	Elixir
int32	integer()
int64	integer()
uint32	integer()
uint64	integer()
sint32	integer()
sint64	integer()
fixed32	integer()
fixed64	integer()
sfixed32	integer()
sfixed64	integer()
float	float()
double	float()
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.