Chorex - Choreographic Programming in Elixir

Synopsis

Note: this documentation is current as of 2024-05-30. The project is evolving rapidly, so this README may occasionally get out-of-sync with what the project can do.

Describe the choreography in a module with the defchor macro:

defmodule TestChor do
  defchor [Buyer, Seller] do
    Buyer.get_book_title() ~> Seller.(b)
    Seller.get_price(b) ~> Buyer.(p)
    Buyer.(p)
  end
end

Implement the actors:

defmodule MyBuyer do
  use TestChor.Chorex, :buyer

  def get_book_title(), do: "Das Glasperlenspiel"
end

defmodule MySeller do
  use TestChor.Chorex, :seller

  def get_price(_b), do: 42
end

Elsewhere in your program:

ps = spawn(MySeller, :init, [])
pb = spawn(MyBuyer, :init, [])

config = %{Seller => ps, Buyer => pb, :super => self()}

send(ps, {:config, config})
send(pb, {:config, config})

receive do
  {:choreography_return, Buyer, val} -> IO.puts("Got #{val}")
end

The program should print Got 42 to the terminal.

Description

Chorex is a library for choreographic programming in Elixir. Choreographic programming is a programming paradigm where you specify the interactions between different entities in a concurrent system in one global view, and then extract implementations for each of those actors. See § Bibliography for references on choreographic programming in general.

Installation

Chorex is available on Hex.pm. Install by including the following in your mix.exs file under the deps list:

def deps do
  [
    ...,
    {:chorex, "~> 0.1.0"},
    ...
  ]
end

You can install development versions of Chorex directly from GitHub like so:

def deps do
  [
    ...,
    {:chorex, github: "utahplt/chorex"},
    ...
  ]
end

Note that this is experimental software and stuff will break. Please don’t rely on this for anything production-grade. Not yet at least.

What is a choreography?

A choreography is a birds-eye view of an interaction between nodes in a distributed system. You have some set of actors/—in Elixir parlance processes—that exchange /messages while also running some /local computation/—i.e. functions that don’t rely on talking to other nodes in the system.

Choreography syntax

Chorex introduces some new syntax for choreographies. Here’s a breakdown of how it works:

defchor [Actor1, Actor2, ...] do
  ...choreography body...
end

The defchor macro wraps a choreography and translates it into core Elixir code. You give defchor a list of actors, specified as if they were module names, and then a do block wraps the choreography body.

Actor1.(var1) ~> Actor2.(var2_a)
Actor1.func_1() ~> Actor2.(var2_b)
Actor1.func_2(var1_a, var1_b) ~> Actor2.(var2_c)
Actor1.(var1_a + var1_b) ~> Actor2.(var2_c)

The ~> indicates sending a message between actors. The left-hand-side must be Actor1.<something>, where that <something> bit can be one of three things:

1. A variable local to Actor1
2. A function local to Actor1 (with or without arguments, also all local to Actor1)
3. An expression local to Actor1

The right-and-side must be Actor2.<var_name>. This means that the left-hand-side will be computed on Actor1 and send to Actor2 where it will be stored in variable <var_name>.

ACHTUNG!! mix format will rewrite Actor1.var1 to Actor1.var1() which is a function call instead of a variable! Wrap variables in parens like Actor1.(var1) if you want to use mix format! This is an unfortunate drawback—suggestions on fixing this would be welcome.

Local functions are not defined as part of the choreography; instead, you implement these in a separate Elixir module. More on that later.

if Actor1.make_decision() do
  Actor1[L] ~> Actor2
  ...
else
  Actor1[R] ~> Actor2
  ...
end

if expressions are supported. Some actor makes a choice of which branch to go down. It is then crucial (and, at this point, entirely up to the user) that that deciding actor inform all other actors about the choice of branch with the special ActorName[L] ~> OtherActorName syntax. Note the lack of . and variable names. Furthermore, the true branch is always L (left) and the false branch is always R (right).

def higher_order_chor(other_chor) do
  ... other_chor.(...) ...
end

Chorex supports higher-order choreographies. These are choreographies that take another choreography as an argument where it can be applied like a function.

def some_local_chor(Actor.(var_name)) do
  Actor.(var_name) ~> OtherActor.(other_var)
  OtherActor.(other_var)
end

This creates a choreography that can be passed as an argument to the higher_order_chor function. This takes as an argument a variable living at a particular actor, and returns another value on a potentially different node.

You would combine the choreographies like so:

defchor [Actor, OtherActor] do
  def higher_order_chor(other_chor) do
    ... other_chor.(...) ...
  end

  def some_local_chor(Actor.(var_name)) do
    Actor.(var_name) ~> OtherActor.(other_var)
    OtherActor.(other_var)
  end

  higher_order_chor(&some_local_chor/1)
end

Right now these functions are limited to a single argument.

with OtherActor.(other_var) <- other_chor.(Actor.(var)) do
  ...
end

You can use with to bind a variable to the result of calling a higher-order choreography. Note that right now you can only have one <- in the expression.

Creating a choreography

To create a choreography, start by making a module, and writing the choreography with the defchor macro.

defmodule Bookstore do
  defchor [Actor1, Actor2] do
    Actor1.(... some expr ...) ~> Actor2.(some_var)
    Actor2.some_computation(some_var) ~> Actor1.(the_result)
    ...
  end
end

You will need to make a module for every actor you specify at the beginning of defchor and mark which actor you’re implementing like so:

defmodule MyFirstActor do
  use Bookstore.Chorex, :actor1

  ...
end

defmodule MySecondActor do
  use Bookstore.Chorex, :actor2

  def some_computation(val), do: ...
end

These modules will need to implement all of the local functions specified in the choreography. Chorex will use Elixir’s behaviour mechanism to warn you if you don’t implement every function needed. In the above example, the MySecondActor implements the role of Actor2 in the choreography, and therefore needs to implement the some_computation function.

Note: Actor names do not need to be the same as the modules implementing them! It is useful to do that, but there exist instances where you might want to write one choreography and implement it in different ways.

Running a choreography

To fire off the choreography, you need to spin up a process for each actor and then tell each actor where to find the other actors in the system. For the above example, you could do this:

first_actor = spawn(MyFirstActor, :init, [])
second_actor = spawn(MySecondActor, :init, [])

config = %{Actor1 => first_actor, Actor2 => second_actor, :super => self()}
send(first_actor, config)
send(second_actor, config)

Once the actors are done, they will send the last value they computed to :super tagged with the actor they were implementing. So, for this example, you could see what Actor1 computed by awaiting:

receive do
  {:choreography_return, Actor1, val} -> IO.inspect(val, label: "Actor1's return: ")
end

Using a choreography with the rest of your project

The local functions are free to call any other code you have—they’re just normal Elixir. If that code sends and receives messages not managed by the choreography library, there is no guarantee that this will be deadlock-free.

Development

Changelog

We will collect change descriptions here until we come up with a more stable format when changes get bigger.

• v0.1.0; 2024-05-30

  Initial release. Lots of rough edges so please, be patient. :)

High-level internals

The defchor macro is implemented in the Chorex module.

• The defchor macro gathers a list of actors.
• For each actor, call project on the body of the choreography. The project function keeps track of the current actor as the “label” variable. (This vernacular borrowed from the academic literature.)
• The functions project and project_sequence are mutually recursive: project_sequence gets invoked whenever project encounters a block with multiple instructions.
• The project function walks the AST, it gathers a list of functions that will need to be implemented by each actor’s implementing module, as well as a list of top-level functions for each projection.
  • This gathering is handled by the WriterMonad module, which provides the monadic do ... end form as well as return and mzero.
• Finally the macro generates modules for each actor under the Chorex module it generates.

So, for example, if you have a simple Choreography like this:

defchor [Alice, Bob] do
  Alice.pick_modulus() ~> Bob.(m)
  Bob.gen_key(m) ~> Alice.(bob_key)
  Alice.encrypt(message, bob_key)
end

This will get transformed into (roughly) this code:

defmodule Chorex do
  (
    def alice do
      quote do
        import Alice
        @behaviour Alice
        def init() do
          Alice.init(__MODULE__)
        end
      end
    end

    defmodule Alice do
      @callback encrypt(any(), any()) :: any()
      @callback pick_modulus() :: any()
      def init(impl) do
        receive do
          {:config, config} ->
            ret = run_choreography(impl, config)
            send(config[:super], {:choreography_return, Alice, ret})
        end
      end

      def run_choreography(impl, config) do
        if function_exported?(impl, :run_choreography, 2) do
          impl.run_choreography(impl, config)
        else
          send(config[Bob], impl.pick_modulus())

          (
            bob_key =
              receive do
                msg -> msg
              end

            impl.encrypt(message, bob_key)
          )
        end
      end
    end
  )

  (
    def bob do
      quote do
        import Bob
        @behaviour Bob
        def init() do
          Bob.init(__MODULE__)
        end
      end
    end

    defmodule Bob do
      @callback gen_key(any()) :: any()
      def init(impl) do
        receive do
          {:config, config} ->
            ret = run_choreography(impl, config)
            send(config[:super], {:choreography_return, Bob, ret})
        end
      end

      def run_choreography(impl, config) do
        if function_exported?(impl, :run_choreography, 2) do
          impl.run_choreography(impl, config)
        else
          m =
            receive do
              msg -> msg
            end

          send(config[Alice], impl.gen_key(m))
        end
      end
    end
  )

  defmacro __using__(which) do
    apply(__MODULE__, which, [])
  end
end

You can see there’s a Chorex.Alice module and a Chorex.Bob module.

Testing

Simply clone the repository and run mix test.

Bibliography

• Hirsch & Garg (2022-01-16) Pirouette: Higher-Order Typed Functional Choreographies, Proceedings of the ACM on Programming Languages. https://doi.org/10.1145/3498684

• Lugović & Montesi (2023-10-15) Real-World Choreographic Programming: Full-Duplex Asynchrony and Interoperability, The Art, Science, and Engineering of Programming. https://doi.org/10.22152/programming-journal.org/2024/8/8

Authors

This is a project by the Utah PLT group. Primary development by Ashton Wiersdorf.