+++
date = "2013-05-28T00:00:00+01:00"
title = "Build Erlang releases with Erlang.mk and Relx"

+++

Building OTP releases has always been a difficult task. Tools like
Reltool or Rebar have made this simpler, but
it's no panacea. This article will show you an alternative and
hopefully much simpler solution.

There is two steps to building a release. First you need to build
the various OTP applications you want to include in the release. Once
done, you need to create the release itself, by including the Erlang
runtime system alongside the applications, a boot script to start the
node and all its applications, and some configuration files.

https://github.com/extend/erlang.mk[Erlang.mk] solves
the first step. It is an include file for GNU Make. Just
including it in a Makefile is enough to allow building your project,
fetching and building dependencies, building documentation, performing
static analysis and more.

https://github.com/erlware/relx[Relx] solves the second
step. It is a release creation tool, wrapped into a single executable
file. It doesn't require a configuration file. And if you do need one,
it will be a pretty small one.

Let's take a look at the smallest Erlang.mk powered
Makefile. There is only one thing required: defining the project
name.

[source,make]
----
PROJECT = my_project

include erlang.mk
----

Simply doing this allows you to build your application by typing
`make`, running tests using `make tests`, and
more. It will even compile your '.dtl' files found in the
'templates/' directory if you are using ErlyDTL!

Let's now take a look at a simplified version of the Makefile for
this website. I only removed a few targets that were off-topic.

[source,make]
----
PROJECT = ninenines

DEPS = cowboy erlydtl
dep_cowboy_commit = 0.8.5
dep_erlydtl_commit = 4d0dc8fb

.PHONY: release clean-release

release: clean-release all projects
	relx -o rel/$(PROJECT)

clean-release: clean-projects
	rm -rf rel/$(PROJECT)

include erlang.mk
----

You can see here how to define dependencies. First you list all
the dependency names, then you have one line per dependency, giving
the repository URL and the commit number, tag or branch you want.

Then you can see two targets defined, with `release`
becoming the default target, because it was defined first. You can
override the default target `all`, which builds the
application and its dependencies, this way.

And as you can see, the `release` target uses
Relx to build a release into the 'rel/ninenines/'
directory. Let's take a look at the configuration file for this release.

[source,erlang]
----
{release, {ninenines, "1"}, [ninenines]}.

{extended_start_script, true}.
{sys_config, "rel/sys.config"}.

{overlay, [
	{mkdir, "log"},
	{copy, "rel/vm.args",
		"releases/\{\{release_name\}\}-\{\{release_version\}\}/vm.args"}
]}.
----

The first line defines a release named `ninenines`, which
has a version number `"1"` and includes one application, also
named `ninenines`, although it doesn't have to.

We then use the `extended_start_script` option to tell
Relx that we would like to have a start script that allows
us to not only start the release, but do so with the node in the
background, or also to allow us to connect to a running node, and so on.
This start script has the same features as the one tools like
Rebar generates.

The rest of the file just makes sure our configuration files are
where we expect them. Relx will automatically take care
of your 'sys.config' file as long as you tell it where to
find it. The 'vm.args' file used by the extended start script
needs to be handled more explicitly by using an overlay however.

How does Relx find what applications to include?
By looking at the application dependencies in the '.app'
file of each OTP application. Make sure you put all dependencies in
there, _including_ library applications, and Relx
will find everything for you.

For example, this release includes the following applications.
Only what's strictly required.

----
compiler-4.9.1	crypto-2.3     kernel-2.16.1	ranch-0.8.3    syntax_tools-1.6.11
cowboy-0.8.5	erlydtl-0.7.0  ninenines-0.2.0	stdlib-1.19.1
----

The 'sys.config' file is standard and
http://www.erlang.org/doc/man/config.html[well documented].
The 'vm.args' file is just an optionally multiline file
containing all the flags to pass to the Erlang VM, for example
`-name ninenines@127.0.0.1 -heart`.

Building OTP releases has always been a difficult task. Until now.