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XDR

XDR is an open data format for serializing and de-serializing structured data based on shared definitions, defined in RFC 4506. This library aims to provide an idiomatic interface for working with XDR data in Elixir.

Documentation

Detailed documentation and examples can be found at https://hexdocs.pm/exdr.

Installation

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

def deps do
  [
    {:exdr, "~> 0.1.1"}
  ]
end

Basic Usage

To work with XDR, an application needs to be able to do five things:

Define the types and structure of the data
Create instances of the data types with specific values
Encode that structured data into a binary representation
Decode a binary representation into structured, typed data
Extract the raw values from the data structures

The main XDR module provides functions for each of these operations. These examples illustrate their usage:

Building data types

Fully-determined types can be built by configuring and combining the predefined XDR types.

int_type = XDR.build_type(XDR.Type.Int)
name_type = XDR.build_type(XDR.Type.VariableOpaque)
five_ints_type = XDR.build_type(XDR.Type.Array, type: int_type, length: 5)
student_type = XDR.build_type(XDR.Type.Struct,
  name: name_type,
  quiz_scores: five_ints_type,
  homework_scores: five_ints_type
)

The configuration required by each built-in type varies. See the documentation for more details.

Adding specific values

Once the data types are defined, values can be built. Again, the applicable values depend on the data types.

{:ok, single_score} = XDR.build_value(int_type, 92)
{:ok, single_name} = XDR.build_value(name_type, "Student A")

Complex data types can be built up all at once, rather than having to initialize each subsidiary value:

{:ok, student_a} = XDR.build_value(student_type,
  name: "Student A",
  quiz_scores: [100, 93, 60, 88, 100],
  homework_scores: [66, 80, 100, 99, 0]
)

Encoding data

Once data structures have been defined and specific values created, they can be encoded into a binary XDR representation:

{:ok, single_score_encoding} = XDR.encode(single_score)
{:ok, single_name_encoding} = XDR.encode(single_name)
{:ok, student_a_encoding} = XDR.encode(student_a)

Decoding data

Any XDR implementation with access to the same type definitions will be able to decode the binary representations into structured data.

{:ok, single_score_decoded} = XDR.decode(int_type, single_score_encoding)
{:ok, single_name_decoded} = XDR.decode(name_type, single_name_encoding)
{:ok, student_a_decoded} = XDR.decode(student_type, student_a_encoding)

# Upon decoding, these values are fully-fledged XDR types
%XDR.Type.Struct{fields: fields} = student_a_decoded

Extracting underlying values

To use the data, we probably want to extract the raw values from the XDR metadata:

{:ok, student_a_data} = XDR.extract_value(student_a_decoded)

# this should match what we put in originally
[
  name: "Student A",
  quiz_scores: [100, 93, 60, 88, 100],
  homework_scores: [66, 80, 100, 99, 0]
] = student_a_data

Custom XDR Type Definitions

When building complex apps, it's not convenient to have to build up the types manually every time time they're needed, so an application can predefine its XDR types and compile them into the application. The XDR.Base module provides the define_type macro for registering and accessing application-specific types.

defmodule MyXDR
  @moduledoc """
  We can define custom types in our own module and then work with them through that module.

  All of the functions demonstrated above on XDR are available on MyXDR,
  and we can use simple string names to reference our predefined XDR types.
  """

  use XDR.Base

  # `define_type` works just like `build_type` with an extra `name` parameter at the front
  define_type("Student", Struct,
    name: "Name",
    quiz_scores: "Scores",
    homework_scores: "Scores",
  )

  define_type("Scores", Array,
    type: "Score",
    length: 5
  )

  define_type("Score", Int)

  define_type("Name", VariableOpaque)
end

# using the predefined types in our code
{:ok, student_b} = MyXDR.build_value("Student",
  name: "Student B",
  quiz_scores: [93, 60, 88, 100, 84],
  homework_scores: [80, 100, 99, 0, 90]
)
{:ok, encoded_student_b} = MyXDR.encode(student_b)
{:ok, decoded_student_b} = MyXDR.decode("Student", encoded_student_b)

For a real-life example, see the Stellar.XDR module, which was generated from the Stellar type definitions using the xdrgen tool.

Contributing and Development

See CONTRIBUTING.md for guidance on how to develop for this library.

Bug reports and pull requests are welcome on GitHub at https://github.com/revelrylabs/exdr. Check out CONTRIBUTING.md for more info.

Everyone is welcome to participate in the project. We expect contributors to adhere to the Contributor Covenant Code of Conduct (see CODE_OF_CONDUCT.md).

Releases

See RELEASES.md for details about the release process.

License

See LICENSE for details.

revelrylabs / exdr

XDR