Build a website deployer with OCurrent

This is a tutorial on how to use existing OCurrent plugins to build a continuous deployment service for your website. It will be relatively simple, but the concepts should transfer nicely.

We will learn to:

To show the flexibility of this approach the site won’t even be in OCaml, it will be a Gatsby website which uses Javascript and Node. But for good measure, we’ll use cohttp-server-lwt as the web-server to run our static site just to show more flexibility.

The first part of our pipeline is to pull content from Github. OCurrent is in now way limited to Github, but it is the only backend currently supported. So that’s what we’ll use.


In order to pull things from Github we’ll have to authenticate in order to use the Github API. You will want to go to your profile Settings > Developer Settings > Personal Access Tokens. Next you will want to generate a new token with support for access to your public repositories.

Fetching your repository

We’re almost done believe it or not! The next step is to actually use the OCurrent Github plugin (Current_github) to fetch the head commit of the main branch (ref).

module Gh = Current_github
module Git = Current_git
module Docker = Current_docker.Default

let fetch ~github ~repo () =
  let head = Gh.Api.head_commit github repo in
  let t = map head in
  Git.fetch t

The ~github parameter is our Api value, we won’t have to generate this as you will see later. And ~repo is our repository which will eventually be in the form username/repo which the user can specify from the command-line.

We first get the head of the repository using the Api’s exposed functionality and pull out the unique id using the function. Now we use the Git plugin to actually go get the specific head commit.

If we look at the plugin code we can see that it defines a new component. This is what we see in the generated dependency graph. Not only that, but it is being monitored which allows OCurrent to propagate new values when the head commit changes (like if a new commit is pushed).

A Note on Caching

One of the big benefits of OCurrent pipelines is a smart use of caching. Luckily a lot of the plugins give you this out of the box. For example, the Git.fetch function is backed by the fetch cache. In the next tutorial on writing plugins we’ll look at this more deeply, but you can see that the functor to a build a cache is relatively simple.

# #require "current.cache"
# #show Current_cache.S.BUILDER
module type BUILDER =
    type t
    val id : string
    module Key : Current_cache.S.WITH_DIGEST
    module Value : Current_cache.S.WITH_MARSHAL
    val build : t -> Current.Job.t -> Key.t -> Value.t Current.or_error Lwt.t
    val pp : Key.t Fmt.t
    val auto_cancel : bool

Building with Docker

In order to build and run are Gatsby site we’ll need two docker images:

The build process is quite simple:

  1. Pull the docker images
  2. Write a dockerfile in OCaml
  3. Build the Dockerfile using the git repository we got earlier
  4. Run the image we built

Thankfully, we can use the Docker plugin for all of this.

let build ~src () =
  let open Dockerfile in
  let dockerfile =
    let+ builder = Docker.pull ~schedule gatsby_builder
    and+ server = Docker.pull ~schedule gatsby_server in
    from ~alias:"build" (Docker.Image.hash builder)
    @@ workdir "/app"
    @@ run "yarn global add gatsby-cli"
    @@ copy ~src:[ "package.json" ] ~dst:"." ()
    @@ run "yarn"
    @@ copy ~src:[ "." ] ~dst:"." ()
    @@ run "gatsby build"
    @@ from (Docker.Image.hash server)
    @@ run "sudo apk update && sudo apk add m4"
    @@ run "opam install cohttp-lwt-unix"
    @@ copy ~from:"build" ~src:[ "/app/public" ] ~dst:"/pub" ()
    @@ workdir "/pub" @@ expose_port 8000
    @@ cmd "opam exec -- cohttp-server-lwt -p 8000 -s"
    |> fun d -> `Contents d
  in ~pull:false ~dockerfile (`Git src)

We’re using the Dockerfile eDSL for writing Dockerfiles in OCaml which almost reads like a normal file. Using the Docker plugin’s pull function we get both image and “expose” their values using the let+ syntax. With Docker.Image.hash we use the hash version of the docker image which will be more reproducible than simply the name of the image.

After that it should be mostly straightforward:

One aspect to make note of is the schedule parameter. We build this using the Current_cache module.

let schedule = Current_cache.Schedule.v ~valid_for:(Duration.of_day 7) ()

let gatsby_builder = "node:12-buster"

let gatsby_server = "ocaml/opam:alpine-ocaml-4.11"

This tells our pipeline how often we should re-pull our Docker images.

The Final Pipeline

let pipeline ~github ~repo () =
  let src = fetch ~github ~repo () in
  let image = build ~src () in image ~run_args:[ "-p=8000:8000" ] ~args:[]

Our pipeline reuses the fetch and build we have already looked at. We pull that altogether in our pipeline function and run the Docker image we produced (with the additional port mapping parameter).

The last step is to wrap this pipeline up in the Current.Engine which we can run in a thread alongside a website for viewing our pipeline in action. The website just needs a route list which can be automatically generated from our engine value.

We then run this inside the main lwt thread. You could also use this logging function if you want more information printed to the console.

let main config mode github repo =
  let engine = Current.Engine.create ~config (pipeline ~github ~repo) in
  let routes = Current_web.routes engine in
  let site =
    Current_web.Site.(v ~has_role:allow_all) ~name:"gatsby_deployer" routes
    (Lwt.choose [ Current.Engine.thread engine; ~mode site ])

Wrapping it up as a CLI tool

The final stage is to bring our pipeline together and use Cmdliner to produce a useful CLI tool.

(* Command-line parsing *)
open Cmdliner

let repo =
  @@ Arg.pos 0 (Arg.some Gh.Repo_id.cmdliner) None
  @@ ~doc:"The GitHub repository (owner/name) to monitor." ~docv:"REPO"

let cmd =
  let doc = "Monitor a GitHub repository containing a Gatsby site." in
  ( Term.(
      const main $ Current.Config.cmdliner $ Current_web.cmdliner
      $ Gh.Api.cmdliner $ repo), "gatsby_deployer" ~doc )

let () = Term.(exit @@ eval cmd)

We can use the predefined arguments that the various Current module provide which add flags like port mapping. The only one we do provide is the repo parameter to specify which repository on Github we want to pull. We will also need to provide the authentication token from the beginning by pasting it into a file (here .token).

deployer --github-token-file=./.token patricoferris/gatsby-starter-default

And that’s it! After running your pipeline go to localhost:8080 to see your pipeline in action. Once it get’s to the run component you should be able to go to localhost:8000 and see your Gatsby site built and deployed on a Cohttp server!