RabbitMQ plugins in Elixir

As RabbitMQ is gradually adopting Elixir (i.e. for the next generation of its CLI tools), it’s natural that one would also want to use Elixir for writing plugins. There is a blog post from 2013 on that topic and most things there are still relevant - except the build instructions, as there was a complete revamp of build system in RabbitMQ. And I’m going to fill that gap.

RabbitMQ plugin development guide suggests that the easiest way to start a new plugin is to copy the simplest existing plugin rabbitmq_metronome. I’ve re-implemented it in Elixir at https://github.com/binarin/rabbitmq_metronome_elixir. You can just fork it and start hacking (note that it only works with what is going to be a 3.7.0 release of rabbit). Or read further down about some details that make this possible.

Suppose we’ve created an elixir scaffolding using mix new. RabbitMQ uses erlang.mk as its build system, so our first task is to integrate some mix commands into Makefile. Here is the snippet that hooks mix into build process and which needs to be added to Makefile of the original metronome plugin:

elixir_srcs := mix.exs \
               $(shell find config lib -name "*.ex" -o -name "*.exs")

app:: $(elixir_srcs) deps
     $(MIX) deps.get
     $(MIX) deps.compile
     $(MIX) compile

Running mix 3 times in a row is a bit expensive, so it’s advisable to add some aliases to mix.exs:

[
  make_all: [
    "deps.get",
    "deps.compile",
    "compile",
  ],
]

Then we can replace 3 mix calls with a single one in our Makefile:

app:: $(elixir_srcs) deps
     $(MIX) make_all

Another thing is that we can drop PROJECT_DESCRIPTION, PROJECT_MOD and PROJECT_ENV variables from Makefile, as erlang.mk uses them only to generate an .app file, and mix is already handling this task.

erlang.mk is the primary build system for RabbitMQ, so we need to tell mix that it shouldn’t fetch or build dependencies that are managed by erlang.mk. For rabbit and rabbit_common which are always the direct dependencies we add this:

[
  {
    :rabbit_common,
    # `deps` is erlang.mk directory with dependencies
    path: "deps/rabbit_common",
    compile: "true",
    override: true
  },
  {
    :rabbit,
    path: "deps/rabbit",
    compile: "true",
    override: true
  },
]

There can be an additional trouble when we use some libraries that have transient dependencies on RabbitMQ sub-projects. E.g. this is the case with amqp elixir library, which depends on amqp_client and in turn on rabbit_common. If we don’t explicitly specify that this depency is managed by erlang.mk, mix will try to fetch it - and it can fetch a version incompatible with our rabbit_common.

And that’s it. There are some thing that I haven’t figured out yet, like writing CT suites in Elixir; or that sometimes I need to delete plugins and _build folders to make my changes active. But other than that everything is fine.