What files or directories of a release are the bare minimum to run a release?

Let's say, I have a completely new VPS server which I've just rolled out, which I haven't installed anything on yet.

And I've compiled and build a production release of Phoenix application on my local machine which is identical to a VPS server Linux distributive- and version-wise.

In the directory _build/prod/rel/my_app123 there have been generated 4 subdirectories:

bin
erts-12.3
lib
releases

Will copying the content of rel/my_app123/, that is, these 4 subdirectories, over to a VPS will be absolutely enough in order to run an application?

Or will I have install something extra as well? Elixir and Erlang? How about production dependencies from mix.exs? Or are these have been included and compiled into into a release?

P.S. Assume that my web application has no "js", "css" and the like files, and doesn't use a database.

Solution

When you run mix release, it bundles all of your Elixir/Erlang dependencies for the MIX_ENV in question into the release directory, the erlang BEAM runtime/VM that you were using in your build, and any files that you specify in your mix project in mix.exs.

Because the BEAM runtime and code that bootstraps loading your code are included in the release, you won't need to install Elixir or Erlang on the target machine.

Things that are not included include:

any non-Elixir dependencies. For example, if you rely on openssl, you'll need to make sure you have a binary-compatible version of that installed on the machine you plan to run on (typically, the equivalent major verson release).
Portable bytecode. BEAM isn't like the Java VM. The compiled BEAM code needs to run on a substantially similar architecture. Build on an Arm64 machine for deployment on an Arm64 virtual machine, or x86 for Intel-compatible hardware, for instance. And it's probably best to use the same major OS distribution. There may be cases where "Any Linux * Same CPU architecture" is fine, but for example, building on a Windows or MacOS install of Elixir/OTP and deploying on Linux is a non-starter; you'd need to use a sufficiently similar OS.

As an example, one of my projects has its releases built on Alpine using Docker, so we only really have to worry about CPU compatibility. In our case we do need to make sure some external non-Elixir dependencies our app binds to are included on the docker image.

RUN apk add --no-cache libstdc++ openssl ncurses-libs wkhtmltopdf xvfb \
  fontconfig \
  freetype \
  ttf-dejavu

(ignore the fact that wkhtmltopdf is kind of deprecated, we're working on it. But for now it's a non-elixir dependency we rely on).

If you're building for a, say, an EC2 instance and not using Docker, you'd just need to make sure your release is built on a similar OS to what you're using for production, and make sure the production AMI (image) has those non-Elixir dependencies on it, or will at the time of deployment, perhaps using apt or another package manager. For a VPS, the solution for non-elixir dependencies will depend on whether they have the option for customizing the base machine image (maybe with Packer or Ansible)

Since you may seem to have been a bit confused about it in the comments, yes, MIX_ENV=prod mix release will build all of your production Elixir/Erlang dependencies and include them in the /_build/prod folder.

I include the whole ./prod folder in our release, but it looks like protocol consolidation binaries and the lib folder .Beam files are all in the rel folder so that's a bit unnecessary.