Releasing a Haskell Web App on Heroku with Docker

Using Heroku to serve a Servant App

Posted by Dennis Gosnell

Releasing Haskell web applications on Heroku has become much easier with Heroku’s Docker support. This article explains how to deploy a Servant application on Heroku using Docker.

I’ve prepared an example application you can use to try deploying to Heroku. This article is divided into three sections. The first section explains how to run the example application locally. The second section explains how to run the example application locally using Docker. The third section explains how to deploy this application to Heroku.

If you want to deploy to Heroku without running locally first, feel free to skim through the first and second sections. However, if you’re new to Haskell development, I recommend going through all three sections.

Running the application locally WITHOUT Docker

The example application is a small JSON API. It provides two APIs. One is to submit simple comments. The other is to display all comments that have been submitted. The comments are saved to a PostgreSQL database.

The following will walk through how to build and run the application locally, without involving Docker or Heroku.

Build the application locally

First, clone the repository and build the application.

$ git clone
$ cd servant-on-heroku/
$ stack setup  # install the required version of ghc on your system
$ stack build  # install all dependencies and build the application

An error may occur when building the application because of missing PostgreSQL libraries.

On Arch Linux, these missing PostgreSQL libraries can be installed with the following command:

$ sudo pacman -Ss postgresql-libs

On Ubuntu, the following command can be used:

$ sudo apt-get install libpq-dev

Other platforms may use a different command to install these libraries.

Once the required PostgreSQL libraries have been installed, try running stack build again. It should succeed this time.

Now try running the application:

$ stack exec -- servant-on-heroku-api

Oops! It should fail with the following error:

servant-on-heroku-api: libpq: failed (could not connect to server: Connection refused
        Is the server running on host "localhost" (::1) and accepting
        TCP/IP connections on port 5432?
        could not connect to server: Connection refused
        Is the server running on host "localhost" ( and accepting
        TCP/IP connections on port 5432?

The example application is trying to connect to PostgreSQL. Comments are stored in PostgreSQL, so you need PostgreSQL running locally.

Setup PostgreSQL

Most OSs and distributions will have different ways of installing PostgreSQL. Check with your platform documentation on how to install PostgreSQL.

For example, here is the Arch Linux documentation for installing PostgreSQL. Here is the Ubuntu documentation.

Once PostgreSQL is installed and running, try running the application again:

$ stack exec -- servant-on-heroku-api

Another oops! It should fail with the following error:

servant-on-heroku-api: libpq: failed (FATAL:  role "mydbuser" does not exist

Looks like a PostgreSQL user and database need to be setup for our application. If you check out the application source code (src/Lib.hs), you can see that it is reading in the DATABASE_URL environment variable and using it to connect to the PostgreSQL server.

If the DATABASE_URL environment variable is not specified, the application defaults to using:

postgres://mydbuser:[email protected]:5432/mydb

It is trying to use the user mydbuser with password mydbpass to access the database named mydb. Let’s create this user and database in PostgreSQL. The following commands are specific to Arch Linux. They may differ slightly if you are on a different platform. Check your platform documentation if they don’t seem to be working.

First, create the mydbuser user with password mydbpass:


Create a database named mydb:

$ sudo -u postgres -- createdb mydb

Make sure that mydbuser can access the mydb database:

$ sudo -u postgres -- psql --command "GRANT ALL PRIVILEGES ON DATABASE mydb TO mydbuser"

PostgreSQL might need to be restarted:

$ sudo systemctl restart postgresql

It should now be possible to connect to the mydb database locally as the mydbuser user:

$ psql -U mydbuser -d mydb -h

Testing the API

Now that PostgreSQL is setup correctly, the application can be run with the following command:

$ stack exec -- servant-on-heroku-api
running servant-on-heroku on port 8080...

Let’s try sending a comment. With the application still running, try the following command:

$ curl --request POST \
    --header 'Content-Type: application/json' \
    --data '{"author": "DG", "text": "Pretty good"}' \
{ "text": "Pretty good", "author": "DG" }

Now let’s list all the comments:

$ curl --request GET \
    --header 'Content-Type: application/json' \
[{"text":"Pretty good","author":"DG"} ]

Looks like it’s working. Now let’s try with Docker!

Running the application locally WITH Docker

Docker is used to build and run the application inside a container. The following section assumes basic familiarity with Docker.

Installing Docker

Docker is installed differently on different platforms. Check your platform documentation for more advice. For instance, here are the instructions for installing on Arch Linux and Ubuntu.

After installing Docker, make sure it is running with the following command:

$ docker info

Building with Docker

We will build the application inside of Docker and create a docker image for the application.

Use docker build to build a Docker image for the application:

$ docker build -t servant-on-heroku .

This uses the Dockerfile in the current directory to build the application. The Dockerfile lists all the steps to build the application and create a reusable image.

If you take a look at the Dockerfile, you can see that it is performing the following steps:

  1. Install required packages with apt-get.
  2. Install stack.
  3. Install GHC using stack based on the application’s stack.yaml file.
  4. Install Haskell dependencies for the application using the application’s .cabal file.
  5. Building the application with stack.
  6. Create a non-root user to use to run the application.
  7. Run the application.

docker build can take up to one hour to finish creating the servant-on-heroku image.1

Testing the API with Docker

Once docker build finishes, docker images can be used to list all local images:

$ docker images
REPOSITORY           TAG       IMAGE ID       CREATED         SIZE
servant-on-heroku    latest    ff591d372461   30 seconds ago  3.92 GB

You can see the servant-on-heroku image that was just created.

Let’s try running the servant-on-heroku image. This will run the application in Docker:

$ docker run --interactive --tty --rm servant-on-heroku

Oh no! It looks like the PostgreSQL problem is back:

servant-on-heroku-api: libpq: failed (could not connect to server: Connection refused
        Is the server running on host "localhost" (::1) and accepting
        TCP/IP connections on port 5432?
could not connect to server: Connection refused
        Is the server running on host "localhost" ( and accepting
        TCP/IP connections on port 5432?

What’s happening here? Well, since the servant-on-heroku container is running as a Docker container, by default it can’t see our local network. It can’t see that PostgreSQL is running on localhost:5432.

Here’s a small trick we can use. When running the servant-on-heroku container, we can tell Docker to just let the container use our local network interface. That way, it can see PostgreSQL:

$ docker run --interactive --tty --rm --network host servant-on-heroku
running servant-on-heroku on port 8080...

With the servant-on-heroku container running, let’s try the curl commands from the previous section. Posting a comment:

$ curl --request POST \
    --header 'Content-Type: application/json' \
    --data '{"author": "EK", "text": "Not enough CT"}' \
{ "text": "Not enough CT", "author": "EK" }

Getting the comments:

$ curl --request GET \
    --header 'Content-Type: application/json' \
[{"text":"Pretty good","author": "DG"},{"text":"Not enough CT","author":"EK"}]

By the way, in order to open a shell and inspect the image by hand, the following command can be used:

$ docker run --interactive --tty --rm --network host servant-on-heroku /bin/bash

Now that we are confident our application works in Docker, it’s time for Heroku.


Once we have the application building and running successfully in Docker, it’s easy to move to Heroku. The first step is creating a Heroku account.

Creating an Account

Go here to sign up for a Heroku account. If you already have a Heroku account, you can skip this step.

We will deploy the application using Heroku’s “Free” tier, so you don’t need to worry about registering a credit card.

The majority of the instructions in this section are condensed from Heroku’s own documentation on integrating with Docker. Check out their documentation if anything is unclear.

Install the Heroku CLI Application

Heroku provides a CLI application to make it easy to work with their service. This is similar to AWS’s CLI or Digital Ocean’s CLI.

On Arch Linux, Heroku’s CLI application can be installed with the following command:

$ yaourt -S heroku-toolbelt

This installs the heroku binary to the system.

Instructions for other platforms can be found on Heroku’s site.

Once the CLI application has been downloaded, it can be used to login and authenticate with Heroku’s API:

$ heroku login

You will be asked for the username and password of the account you just created.

Create an Application on Heroku

The first step of releasing our Servant API to Heroku is to create a Heroku Application.

The following command will create a new Heroku application called servant-on-heroku. You may need to use a different name for your own application:

$ heroku apps:create servant-on-heroku

The following command lists information about the application just created (although it won’t be too interesting yet):

$ heroku apps:info servant-on-heroku
=== servant-on-heroku
Auto Cert Mgmt: false
Git URL:
Owner:          [email protected]
Region:         us
Repo Size:      0 B
Slug Size:      0 B
Stack:          cedar-14
Web URL:

Make sure to take note of the Web URL. It will come in handy later.

Install Heroku Docker Plugin

The Heroku CLI application has a plugin architecture. This allows the user to install plugins that can be used to access different parts of Heroku’s API.

There is a plugin for using Heroku’s Docker Container Registry.

The following command can be used to install the plugin:

$ heroku plugins:install heroku-container-registry

After installing the plugin, the following command can be used to make sure it works:

$ heroku container

It should return the version string for the plugin.

In order to actually use the plugin, the following command can be used to login to Heroku’s container registry.

$ heroku container:login

This adds login information for Heroku’s container registry to the file ~/.docker/config.json:

$ cat ~/.docker/config.json
  "auths": {
    "": {
      "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx="

Get the Application Running on Heroku

In order to get the application actually running on Heroku, the following command is used:

$ heroku container:push web

This builds a Docker image for the application based on the Dockerfile in the current directory. Internally, docker build is used to do this. If the image was already built in the previous step (when running docker build from the command line), then this heroku container:push command will just use the previously built image. The image is sent to Docker’s Container Registry.

Now let’s check heroku apps:info again:

$ heroku apps:info servant-on-heroku
=== servant-on-heroku
Auto Cert Mgmt: false
Git URL:
Owner:          [email protected]
Region:         us
Repo Size:      0 B
Slug Size:      0 B
Stack:          cedar-14
Web URL:

Hmm, that’s not right. See where it says Dynos:? A “dyno” is Heroku-lingo for a server that runs the web application. This line means that there aren’t any servers running the application.

In order to fix this, the heroku ps:scale command can be used to spin up one dyno to run the application:

$ heroku ps:scale web=1

This creates one “web” dyno, which will run the Servant API.2

Now run the following command to make sure the dyno is actually running:

$ heroku ps
Free dyno hours quota remaining this month: 549h 2m (99%)
For more information on dyno sleeping and how to upgrade, see:

=== web (Free): /bin/sh -c /opt/servant-on-heroku/bin/servant-on-heroku-api (1)
web.1: starting 2017/03/22 19:05:04 +0900 (~ 8s ago)

The output is somewhat noisy, but you can tell that there is now one web dyno running.

Now that the application is running, the following command can be used to access the application’s Web URL with curl. (The application Web URL can be found in the output of heroku apps:info.)

$ curl --request POST \
    --header 'Content-Type: application/json' \
    --data '{"author": "MS", "text": "Gotta make it professional"}' \

That’s strange, there appears to be another error. Let’s see how to investigate application errors on Heroku.

Debugging Application Errors

Heroku has a really nice log system. The application’s stdout and stderr logs can be inspected with the following command:

$ heroku logs
2017-03-22T10:05:49 heroku[web.1]: proc start `/opt/servant-on-heroku/bin/servant-on-heroku-api`
2017-03-22T10:05:52 app[web.1]: servant-on-heroku-api: libpq: failed (could not connect to server: Connection refused
2017-03-22T10:05:52 app[web.1]:    Is the server running on host "localhost" ( and accepting
2017-03-22T10:05:52 app[web.1]:    TCP/IP connections on port 5432?
2017-03-22T10:05:52 app[web.1]: )
2017-03-22T10:05:52 heroku[web.1]: State changed from starting to crashed

Oh no! It’s the same error that has been plaguing us this whole time. Why is it occurring again?

Well, it’s because we haven’t setup a PostgreSQL database on Heroku!

PostgreSQL on Heroku

Heroku has nice support for PostgreSQL. Heroku provides a PostgreSQL database that can be used free-of-charge.

The following command can be used to enable the PostgreSQL database add-on for the application:

$ heroku addons:create heroku-postgresql:hobby-dev

This enables the heroku-postgresql add-on in the hobby-dev tier (which is free).

After enabling it, the following command can be used to make sure the PostgreSQL database has been successfully created:

$ heroku addons:info heroku-postgresql
=== postgresql-tetrahedral-44549
Attachments:  servant-on-heroku::DATABASE
Installed at: Wed Mar 22 2017 19:22:14 GMT+0900 (JST)
Owning app:   servant-on-heroku
Plan:         heroku-postgresql:hobby-dev
Price:        free
State:        created

The database info can be checked with the pg:info command:

$ heroku pg:info
Plan:        Hobby-dev
Status:      Available
Connections: 0/20
PG Version:  9.6.1
Created:     2017-03-22 10:22 UTC
Data Size:   7.2 MB
Tables:      1
Rows:        0/10000 (In compliance)
Fork/Follow: Unsupported
Rollback:    Unsupported
Add-on:      postgresql-tetrahedral-44549

Restart the App

Now that the PostgreSQL database is up and running, let’s try restarting the application:

$ heroku ps:restart

Let’s take a look at the application logs again:

$ heroku logs
2017-03-22T10:22:15 heroku[web.1]: State changed from crashed to starting
2017-03-22T10:22:54 heroku[web.1]: proc start `/opt/servant-on-heroku/bin/servant-on-heroku-api`
2017-03-22T10:22:56 app[web.1]: Migrating: CREATe TABLE "comment"("id" SERIAL8  PRIMARY KEY UNIQUE,"author" VARCHAR NOT NULL,"text" VARCHAR NOT NULL)
2017-03-22T10:22:57 heroku[web.1]: State changed from starting to up

Looks like it worked this time! Finally!

Let’s try accessing the app using curl again:

$ curl --request POST \
    --header 'Content-Type: application/json' \
    --data '{"author": "SPJ", "text": "Avoid heroku-at-all-costs"}' \
{"text":"Avoid heroku-at-all-costs","author":"SPJ"}

And once more:

$ curl --request GET \
    --header 'Content-Type: application/json' \
[{"text":"Avoid heroku-at-all-costs","author":"SPJ"}]

Success! Looks like everything is working well!

How does the app on Heroku know how to connect to the database?

You may be wondering how the application running on Heroku knows how to connect to the database. Well, Heroku has configuration variables that it passes to the application as environment variables.

These configuration variables can be inspected with the following command:

$ heroku config
=== servant-on-heroku Config Vars
DATABASE_URL: postgres://someusername:[email protected]:5432/databasename

Setting up the PostgreSQL database creates a configuration variable called DATABASE_URL. Heroku passes this configuration variable to the application on startup as an environment variable. As discussed in a previous section, the application uses DATABASE_URL to connect to the correct database3.

Heroku’s DATABASE_URL can also be used to connect to the database on the command line:

$ psql "$(heroku config:get DATABASE_URL)"
psql (9.6.1)
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384, bits: 256, compression: off)

databasename=> select * from comment;
 id | author |           text
  1 | SPJ    | Avoid heroku-at-all-costs
(1 row)

Future (Normal) Releases

Performing future releases of the application is extremely easy. Just run the following command:

$ heroku container:push web

This rebuilds the docker image for the application and pushes it to Heroku’s container repository. It then restarts the dynos so they are running with the new code for the application.

Future Work

This application works pretty well, but there are a couple places for improvements. The lowest hanging fruit would probably be the Dockerfile. Here are a couple ideas that would make the Dockerfile a little better:

  • Use a slimmer image as the base image for the Dockerfile. Right now it is using Heroku’s images, but I don’t think there is any reason that something like Alpine Linux couldn’t be used.
  • Base the image on something with stack, GHC, and popular Haskell libraries already installed. This would greatly reduce the time it takes to do the very initial docker build.
  • At the very end of the Dockerfile, remove stack, GHC, and all Haskell libraries. This would hopefully make the docker image a little smaller. It would take less bandwidth to send the image to Heroku’s container repository.

It would also be nice to use something like docker-compose to setup the PostgreSQL database using Docker when running locally.

Alternatives to Docker for Deploying on Heroku

The only strong alternative to using Docker for deploying Haskell code to Heroku is haskellonheroku. This is a normal Heroku buildpack for Haskell. With this buildpack, you are able to use Heroku like you would with dynamic languages. All you need to do is git push your code to Heroku’s remote git repository. The new code is automatically compiled and deployed.

This sounds really good in theory, but in pracitce haskellonheroku has two big drawbacks:

  1. Heroku build times are limited to 15 minutes. haskellonheroku gets around this in a complicated way, requiring use of Amazon S3 to upload prebuilt libraries before doing a git push.
  2. haskellonheroku uses halcyon internally to accomplish most build steps. halcyon is a tool similar to stack and nix. However, it appears that development has stopped 2 years ago. halcyon does not support any of the latest GHC versions.

halcyon might have been nice a few years ago before stack existed. But now that stack is regularly used for Haskell development, moving to an alternative build tool doesn’t seem like a good decision.4


As long as you have Docker running on your local machine, it’s pretty easy to get your Haskell code on Heroku. Heroku’s free plan is nice for testing application ideas and showing them to others. It may not work for any sort of business application, but as a proof-of-concept, it’s great!

If you decide your proof-of-concept works well and you want to release it, it’s easy to add a credit card to Heroku and start running on their cheapest paid tier. Heroku has a very nice upgrade path.


  1. These seven steps are slightly complicated. Ideally, it should be possible to install GHC, install all the application dependencies, and build the application in just one command. However, I have separated it into multiple commands to take advantage of Docker’s caching ability. When re-running docker build, only commands where the input has changed will be re-run.

    For example, if you change the servant-on-heroku.cabal file and re-run docker build, it will rebuild the image from (4), starting with installing dependencies from the application’s .cabal file. docker build does not have to re-run (1), (2), or (3). It uses cached versions of the image.

    This means that if all you change is the application source code under src/ and re-run docker build, all docker build has to do is re-run (5), (6), and (7). It doesn’t have to install GHC or the application’s Haskell dependencies. This reduces a large part of the build-time. Future builds will take just a few minutes, instead of one hour.

  2. There are multiple kinds of dynos. However, it’s not something that we need to worry about for our simple web API.

  3. Heroku also makes use of the PORT environment variable for telling your application which port to listen on.

  4. Unless it is a tool that gives substantial extra power, like nix.