My application example

When making Erlang or Elixir releases, usually the application run inside an embedded Erlang Run Time System (ERTS). The ERTS will be copied from Yocto build environment to the target application.

The layer meta-erlang provides additional bitbake classes to handle this process when using rebar3 or mix tool. However the rebar.config and mix.exs configuration files need to be prepared in advanced to detected when special environment configuration are available in order to make the proper release.

Erlang

preparing the rebar.config file

A basic rebar3 config looks like this:

{erl_opts, [debug_info]}.
{deps, []}.

{relx, [{release, {'axon', "0.1.0"},
         [axon,
          sasl]},

        {sys_config, "./config/sys.config"},
        {vm_args, "./config/vm.args"},

        {dev_mode, true},
        {include_erts, true},
        {system_libs, true},

        {extended_start_script, true},
        {generate_start_script, true}]
}.

{profiles, [{prod, [{relx, [{dev_mode, false}
                            {include_erts, true},
                            {system_libs, true}
                           ]}]
            }]
}.

In the above config, the relx must have the include_erts and system_libs setted to true, like this:

{relx, ...
        {include_erts, true},
        {system_libs, true},
...
}.

inheriting rebar3 class

The meta-erlang class rebar3 provides everything needed in order to generate a cross compiled Erlang release. The rebar3 class uses the rebar3 tar command passing the following arguments:

• rebar as ${REBAR_PROFILE} tar
• --system_libs ${REBAR3_TARGET_SYSTEM_LIBS}
• --include-erts ${REBAR3_TARGET_INCLUDE_ERTS}
• -n ${REBAR3_RELEASE_NAME}

While REBAR3_TARGET_SYSTEM_LIBS and REBAR3_TARGET_INCLUDE_ERTS are detected automatically pointing to the correct target paths; the variables REBAR_PROFILE and REBAR3_RELEASE_NAME must be defined in the application recipe like this:

inherit rebar3

REBAR_PRODULE = "prod"
REBAR3_RELEASE_NAME = "my-application"

Calling rebar3 tar command is necessary to avoid less changes in the application rebar.config file. So, rebar3 (which delegates to relx) will use the correct target paths.

!> Remember: rebar3 supports independent release configurations.

Elixir

release with distillery

This approach uses distillery to handle the release generation. The usual config is documented here. An extra step is necessary to copy the crosscompile elixir and erlang libraries to the release package. The following code show the additional configuration:

environment :prod do
  # MIX_TARGET_INCLUDE_ERTS is set by meta-erlang/classes/mix.bbclass
  set include_erts: System.get_env("MIX_TARGET_INCLUDE_ERTS")
  set include_src: false
  set cookie: :"d.K@rG|YpY`|?i&]Fsdljji)JMhk)%j)}f|Y6cquU!He<]u=sHp`.o2p[I3ee]gD"
  set vm_args: "rel/vm.args"
end

The include_erts distillery config will use the value from MIX_TARGET_INCLUDE_ERTS variable.

release with Elixir

The mix.bbclass is prepare to handle Elixir releases by default. So, following the procedures about how to configure an Elixir application with mix release should be enough for most of the cases.

Be aware that you need to configure the Elixir release parameter :include_erts to get the value of MIX_TARGET_INCLUDE_ERTS environment variable. Like this:

 releases: [
          epad: [
            applications: [
              epad: :permanent
            ],
            steps: [
              :assemble,
              :tar
            ],
            include_erts: System.get_env("MIX_TARGET_INCLUDE_ERTS")
          ]
        ]