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https://github.com/phadej/heroku-docker-haskell-test

Heroku + docker + haskell = ?
https://github.com/phadej/heroku-docker-haskell-test

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Heroku + docker + haskell = ?

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# Heroku + Docker + Haskell = ?



This experiment is triggered by many things:

- I like Haskell

- I like docker too

- And I like how easy is Heroku

- [Haskell Web Server in a 5MB Docker image](https://www.fpcomplete.com/blog/2015/05/haskell-web-server-in-5mb)

- [Introducing 'heroku docker:release': Build & Deploy Heroku Apps with Docker](https://blog.heroku.com/archives/2015/5/5/introducing_heroku_docker_release_build_deploy_heroku_apps_with_docker)



[evening-earth-1057.herokuapp.com](https://evening-earth-1057.herokuapp.com/) - Working example.



## Starting point



Deploying Haskell to heroku isn't simple, there are different buildpacks:

- [Recommended "Haskell on Heroku"](https://github.com/mietek/haskell-on-heroku)

- and [heroku-buildpack-ghc](https://github.com/begriffs/heroku-buildpack-ghc)



With both buildpacks, and Heroku overall, I don't like that building happens in the cloud.

A bit like in the *AWS ELB* blog post, I'd rather build app locally (or on CI machine, or in *build* cloud), and deploy artifacts to Heroku.



There are two approaches!



## inline-build-pack



Using [inline-build-pack](https://github.com/kr/heroku-buildpack-inline), we can build our app locally, bundle artifacts, and push to Heroku.

All three scripts: [`detect`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/bin/detect), [`release`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/bin/release), and [`compile`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/bin/compile) turn out to be no-op.

Looks like that on [cedar14](https://devcenter.heroku.com/articles/cedar-14-migration)

stack, there is `libgmp.so.10`, so we don't need to bundle it anymore, as `heroku-buildpack-ghc` does.

Turns out that [null-buildpack](https://github.com/ryandotsmith/null-buildpack) could be enough too.



Still one problem still exist: how to build a heroku-runnable binary executable on e.g. OSX? Here the docker comes to help!

Using two simple scripts: [`prebuild`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/bin/prebuild) and [`compilewithdocker`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/bin/compilewithdocker) we produce needed executable.



But committing artifacts to the repository is highly unelegant!



## Using heroku docker



We can build an own [`Dockerfile`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/Dockerfile) to build the Haskell binaries. It's actually quite easy.

Unfortunately (maybe for good) Dockerfiles doesn't support multi-inheritance, so we need to install GHC manually.

Fortunately I have previously done [docker ghc image](https://github.com/phadej/docker-ghc), so we can just copy paste GHC installing spells.

After that we install few dependencies† so we don't need to rebuild everything when deploying.

And finally we build our application and leave it in `/app` directory which is then picked up by heroku deployment.



The first build takes time as then the build image is built, it took about 15 minutes on my machine.

The `heroku docker:release` takes under one minute. After update to use

[`stackage-update`](https://github.com/fpco/stackage-update) in place of `cabal update` the `heroku docker:release`

takes about half a minute.



## Conclusion



Using docker deployment you can tweak the build process, which is a huge win.

Heroku infrastructure makes the other parts of deployment process and devops easy too.

The only negative aspect, *with greater power comes greater responsibility*, you can *accidentally* deploy uncommited changes.



---



## Remarks



- † we could add cabal file to docker and install exact dependencies

- there is [`.dockerignore`](https://github.com/phadej/heroku-docker-haskell-test/blob/master/.dockerignore) to ignore host cabal sandbox one uses for development

- This approach could be used to deploy scala (which takes enormous time to build e.g. play2 app), or to make Clojure deploy using uberjar.