Protox

Protox is an Elixir library to work with Google's Protocol Buffers (aka protobuf), versions 2 and 3.

Generally speaking, a lot of efforts have been put into making sure that the library is reliable (for instance using property based testing and by having a 100% code coverage). Therefore, this library passes all the tests of the conformance checker provided by Google. See Conformance section for more information.

This library is easy to use: you just point to the *.proto files or give the schema to the Protox macro, no need to generate any file! However, should you need to generate files, a mix task is available (see Files generation).

This library also provides a full-blown Elixir experience with protobuf messages. For instance, given the following protobuf msg.proto file:

syntax = "proto3";

message Msg{
  int32 a = 1;
  map<int32, string> b = 2;
}

You can interact with Msg as if it were a native Elixir structure. For example, note how the map b is translated into an Elixir Map:

iex> %Msg{a: 42, b: %{1 => "a map entry"}}

Prerequisites

Protox uses Google's protoc (>= 3.0) to parse .proto files. It must be available in $PATH. You can download it here or you can install it with your favorite package manager (brew install protobuf, apt install protobuf-compiler, etc.).

This dependency is only required at compile-time.

Installation

Add :protox to your list of dependencies in mix.exs:

def deps do
  [{:protox, "~> 1.1.0"}]
end

Usage with a textual description

Here's how to generate the modules 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

This example will generate two modules: Fiz.Baz and Fiz.Foo. Note that the module in which the Protox macro is called is completely ignored and therefore does not appear in the names of the generated modules.

Usage with files

Here's how to generate the modules from a set of files:

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

Again, the module in which the Protox macro is called is completely ignored.

Encode

Here's how to create and encode a new message:

iex> msg = %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}}
iex> {:ok, iodata} = Protox.Encode.encode(msg)

Or, with throwing style:

iex> iodata = Protox.Encode.encode!(msg)

Note that Protox.Encode.encode!/1 returns an IO data, not a binary, for efficiency reasons. Such IO data can be used directly with files or sockets write operations, and therefore you don't need to transform them:

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

iex> {:ok, file} = File.open("msg.bin", [:write])
{:ok, #PID<0.1023.0>}

iex> IO.binwrite(file, iodata)
:ok

However, you can use :binary.list_to_bin/1 or IO.iodata_to_binary to get a binary should the need arises:

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

Decode

Here's how to decode a message from a binary:

iex> {:ok, msg} = Fiz.Foo.decode(<<8, 3, 18, 4, 8, 1, 18, 0>>)

Or, with throwing style:

iex> msg = Fiz.Foo.decode!(<<8, 3, 18, 4, 8, 1, 18, 0>>)

Prepend namespaces

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

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

    message Msg {
        int32 a = 1;
      }
    """,
    namespace: Namespace
end

In this example, the module Namespace.Msg is generated:

iex> msg = %Namespace.Msg{a: 42}

Specify import path

An import path can be specified using the path: option that specifies the directory in which to search for import:

defmodule Baz do
  use Protox,
    files: [
      "./defs/prefix/foo.proto",
      "./defs/prefix/bar/bar.proto",
    ],
    path: "./defs"
end

It corresponds to the -I option of protoc.

Unknown fields

Unknown fields are fields that are present on the wire but which do not correspond to an entry in the protobuf definition. Typically, it occurs when the sender has a newer version of the protobuf definition. It makes possible to have backward compatibility as the receiver with an old version of the protobuf definition will still be able to decode old fields.

When unknown fields are encountered at decoding time, they are kept in the decoded message. It's possible to access them with the function unknown_fields/1 defined with the message.

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

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

You must always use unknown_fields/1 as the name of the 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}. For more information, please see protobuf encoding guide.

When you encode a message that contains unknown fields, they will be reencoded in the serialized output.

Unsupported features

• Protobuf 3 JSON mapping
• Groups (deprecated in protobuf)
• All options other than packed and default are ignored as they concern other languages implementation details.

Implementation choices

• This library enforces the presence of required fields (Protobuf 2). Therefore an error is raised when encoding or decoding a message with a missing required field:

  defmodule Bar do
    use Protox, schema: """
      syntax = "proto2";
  
      message Required {
        required int32 a = 1;
      }
    """
  end
  
  iex> Protox.Encode.encode!(%Required{})
  ** (Protox.RequiredFieldsError) Some required fields are not set: [:a]
  
  iex> Required.decode!(<<>>)
  ** (Protox.RequiredFieldsError) Some required fields are not set: [:a]

• When decoding enum aliases, the last encountered constant is used. For instance, in the following example, :BAR is always 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. You can use the generated default/1 function to get the default value of a field:

    defmodule Bar do
      use Protox,
      schema: """
        syntax = "proto2";
    
        message Foo {
          optional int32 a = 1 [default = 42];
        }
      """
    end
    
    iex> Foo.default(:a)
    {:ok, 42}
    
    iex> %Foo{}.a
    nil

    It means that if you need to know if a field has been set by the sender, you just have to test if its value is nil or not.

  • For Protobuf 3, unset optional fields are mapped to their default values, as mandated by the Protobuf spec:

    defmodule Bar do
      use Protox,
      schema: """
        syntax = "proto3";
    
        message Foo {
          int32 a = 1;
        }
      """
    end
    
    iex> Foo.default(:a)
    {:ok, 0}
    
    iex> %Foo{}.a
    0

• Messages and enums names: names are converted using the Macro.camelize/1 function. Thus, in the following example, non_camel_message becomes NonCamelMessage, but the field non_camel_field is left unchanged:

  defmodule Bar do
    use Protox,
    schema: """
      syntax = "proto3";
  
      message non_camel_message {
      }
  
      message CamelMessage {
        int32 non_camel_field = 1;
      }
    """
  end
  
  
  iex> msg = %NonCamelMessage{}
  %NonCamelMessage{__uf__: []}
  
  iex> msg = %CamelMessage{}
  %CamelMessage{__uf__: [], non_camel_field: 0}

Generated code reference

The detailed reference of the generated code is available here.

Files generation

It's also possible to generate a file that will contain all code corresponding to the protobuf messages:

MIX_ENV=prod mix protox.generate --output-path=/path/to/message.ex --include-path=. test/messages.proto test/samples/proto2.proto

The --include-path option is the same as the option described in section Specify import path.

The generated file will be usable in any project as long as protox is declared in the dependancies (the generated file is not a standalone, it still needs functions from the protox runtime).

If you have large protobuf files, you can use the --multiple-files option to generate one file per module.

mkdir generated
MIX_ENV=prod mix protox.generate --multiple-files --output-path=generated --include-path=. test/messages.proto test/samples/proto2.proto

Doing so, Elixir will be able to parallelize the compilation of generated modules.

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() | integer()
message	struct()

Conformance

The protox library has been thoroughly tested using the conformance checker provided by Google. Note that only the binary part is tested as protox supports only this format. For instance, JSON tests are skipped.

Here's how to launch the conformance test:

• Get conformance-test-runner sources.

• Compile conformance-test-runner (macOS and Linux only): tar xf v3.13.0.tar.gz && cd protobuf-3.13.0 && ./autogen.sh && ./configure && make -j && cd conformance && make -j.

• Run mix protox.conformance --runner=/path/to/protobuf-3.13.0/conformance/conformance-test-runner. A report will be generated in a directory conformance_report. If everything's fine, the following text should be displayed:

  CONFORMANCE TEST BEGIN ====================================
  
  CONFORMANCE SUITE PASSED: 1302 successes, 708 skipped, 0 expected failures, 0 unexpected failures.
  
  
  CONFORMANCE TEST BEGIN ====================================
  
  CONFORMANCE SUITE PASSED: 0 successes, 69 skipped, 0 expected failures, 0 unexpected failures.
  

You can alternatively launch these conformance tests with mix test by setting the PROTOBUF_CONFORMANCE_RUNNER environment variable and including the conformance tag:

PROTOBUF_CONFORMANCE_RUNNER=./protobuf-3.13.0/conformance/conformance-test-runner MIX_ENV=test mix test --include conformance

Benchmarks

You can launch benchmarks to see how Protox perform:

mix run ./benchmarks/generate_payloads.exs # first time only, generates random payloads
mix run ./benchmarks/run.exs --lib=./benchmarks/protox.exs
mix run ./benchmarks/load.exs

Credits

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