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s2i-ocaml-builder

This is a first attempt at dockerizing OCaml application using Openshift's source-to-image tool.

Disclaimer

This tool is far from being usable at all. However, I hope to make it better and better by improving my understanding of Docker and of OCaml packaging and building processes.

Requirements

Building the s2i-ocaml-builder requires Docker.

Using s2i-ocaml-builder to build application images requires s2i. Refer to s2i installation instructions.

You will also need the s2i-ocaml-runner image.

Installation

Building s2i-ocaml-builder

Clone this repository and cd into it.

$ git clone https://github.com/Richard-Degenne/s2i-ocaml-builder
$ cd s2i-ocaml-builder

Then, build the s2i-ocaml-builder image with

$ docker build -t s2i-ocaml-builder .

Building an application image

This exemple uses a very simple echo server. Feel free to follow along by cloning it.

The image uses the OPAM metadata to build the application. Make sure your project has a valid OPAM file.

Make sure you have the s2i-ocaml-runner image at hand, since we are going to tell s2i to use it as the runtime image. See this page for further details about s2i runtime images.

To build the application image, run the following

$ s2i build . s2i-ocaml-builder echo-server:test \
  --runtime-image s2i-ocaml-runner \
  --runtime-artifact /home/opam/.opam/4.04.1/bin/run \
  --env PACKAGE_NAME=echo-server
  • echo-server:test is the name/tag of the image that will be produced.
  • PACKAGE_NAME is an environment variable sent to the builder image. It is necessary for it to locate the OPAM file.

The only condition for the container to start properly is to have OPAM install an executable named run that will start your application.

Running an application image

To run the built application, spawn a container using the target image and publish needed port(s).

$ docker run -d -p5000:5000 echo-server:test

Further documentation

S2I's default documentation is available in S2I_README.md.

Todo list

Make the running process more generic

Imposing an executable name on the developer's side is a hassle. Find a way to make this more generic, maybe using a Procfile of some sort.

Besides, I believe it makes it impossible to give the run command parameters.

Reduce logging

For the time being, all commands outputs the default flow of information to stdout, which isn't very relevant in an non-interactive context such as here.

On the other hand, concise logs will help troubleshooting and will make the whole thing look better.

Improve build workflow

I'm very unfamiliar with OCaml build toolchains. I think there might a far better way of handling the build of the application. See s2i/bin/assemble.

Wrap the build command in a script

The build command is pretty obscure and subject to evolutions. A good thing would be to hide all the details in a script with a simpler interface.