TL;DR: if you just want to get started use
Docker support, see the Docker
page on the stack wiki. The rest of this post gives background
on the benefits, implementation, and reasons for our choices.
Using LXC for containerization is an integral component of the
FP Complete Haskell Center and School of Haskell, so
lightweight virtualization was not new to us. We started tentative
experiments using Docker for command-line development about a year
ago and it quickly became an indispensable part of our development
tool chain. We soon wrote a wrapper script that did user ID mapping
and volume mounting so that developers could just prefix their
cabal or build script commands with the wrapper
and have them automagically run in a container without developers
needing to adjust their usual workflow for Docker. The wrapper's
functionality was integrated into an internal build tool and formed
the core of its sandboxing approach. Then that internal build tool
became stack which got
its own non-Docker based sandboxing approach. But the basic core of
that original wrapper script is still available, and there are
significant benefits to using stack's Docker integration for
The primary pain point we are solving with our use of Docker is ensuring that all developers are using a consistent environment for building and testing code.
Before Docker, our approach involved having developers all run the same Linux distribution version, install the same additional OS packages, and use hsenv sandboxes (and, as they stabilized, Cabal sandboxes) for Haskell package sandboxing. However, this proved deficient in several ways:
In the process of solving the main problems, there were some additional goals:
stackcommands should work as close to normally as possible when Docker is enabled.
When Docker is
enabled in stack.yaml, every invocation of
(with the exception of certain sub-commands) transparently
re-invokes itself in an ephemeral Docker container which has the
project root directory and the stack home (
bind-mounted. The container exists only to provide the environment
in which the build runs, nothing is actually written to the
container's file-system (any writes happen in the bind-mounted
directories) and it the container is destroyed immediately after
stack exits (using
docker run --rm). This
means upgrading to a new image is easy, since it's just a matter of
creating ephemeral containers from the new image. The directories
are bind-mounted to the same file-system location in the container,
which makes it possible to switch between using docker and not and
still have everything work.
Docker runs processes in containers as root by default, which
would result in files all over our project and stack home being
owned by root when they should be owned by the host OS user. There
docker run --user option to specify a different
user ID to run the process as, but it works best if that user
already exists in the Docker image. In this case, we don't know the
user ID of the developer at image creation. We work around that by
docker run --env to pass in the host user's UID
and GID, and adding an ENTRYPOINT which, inside the container,
creates the user and then uses
sudo -u to run the
build command as that user.
stack and the entrypoint:
stack.yaml resolver setting to construct
the Docker image tag (which can be overridden).
Copies the Stackage LTS snapshot's build plan and the Hackage
index from the image into
~/.stack, if they are newer.
This way, they do not need to be downloaded which enables
Internet-connectionless operation once you have the Docker
Determines whether the stdin/stdout/stderr file handles are
connected to a terminal device and, if so, runs in interactive
docker run --interactive --tty).
Unfortunately there doesn't seem to be a way to get a Dockerized
process to behave just like a normal process when it comes to
stdin/stdout/stderr on the host, but this is close enough that it
behaves as expected most of the time.
Volume-mounts special project-and-image-specific
~/.ghc directories into the
image, which means that if you use
directly it will end up with an automatic "sandbox" (before
stack added its own, this was the sandboxing approach
of our internal build tool, but it is no longer necessary since
stack doesn't touch
Checks that the version of Docker installed on the host is recent enough.
options to adjust Docker behaviour for common development use
cases, plus the ability to pass arbitrary arguments to
run for the less common cases.
For each GHC version + Stackage LTS snapshot combination, we tag several images (which layer on top of each other):
stackdescribed in the previous section.
Most of a developer's work is done using a build or full image, and they can test using a run image. The actual production environment for a server can be built on run.
We create and push these images using a Shake script on the host, and Propellor in the container to control what is in the image. This provides far more flexibility than basic Dockerfiles, and is why we can easily mix-and-match and patch images. Our image build process also allows us to provide custom images for clients. These might include additional tools or proprietary libraries specific to a customer's requirements. We intend to open the image build tool, but it currently contains proprietary information and needs to be refactored before we can extract that and open the rest.
Nothing is perfect, and we have run into some challenges with Docker:
It is Linux-only. While Docker does have some support for other host operating systems using boot2docker this has not been reliable enough in practice. In particular, since it uses VirtualBox under the surface, it relies on VirtualBox's extremely slow "shared folders" for bind-mounting directories from the host into the container, which makes it nearly unusable for Haskell builds.
The V1 private Docker registry is not very reliable, so we use a variant of this approach to run a static registry hosted directly from S3. We're pleased with this static S3 registry since it means we don't need to set up high availability for our various registries, so we haven't tried the new V2 registry yet.
Some corporate customers have extremely restrictive firewalls, which pose difficulties for downloading Docker images from the registry. The static registry helps with this as well.
Docker images, especially when they include a complete set of
pre-built packages from Stackage, use a lot of disk space. To help
stack keeps track of which images it uses
and makes it easy to
clean up old images.
Since the images are large, we have hit limitations in the
default device-mapper storage driver configuration which require
some tuning. In particular, we've hit default maximum container
file system size with the device-mapper driver and and must use the
option to increase it. When using btrfs, it is not uncommon to
No space left on device errors even though there
is plenty of disk space. This is a
well known issue with btrfs due to it running our of space for
metadata, which requires a re-balance. We have found the aufs and
overlay drivers to work out-of-the-box.
There are many other ways to use Docker, but we didn't find that the "obvious" ones met our goals.
The Official Haskell
image (which didn't exist when we started using Docker)
approach of iteratively developing using
and Dockerfile has some disadvantages:
The Vagrant-style approach of having a persistent container with the project directory bind-mounted into it, while much better, has other disadvantages:
There are plenty of directions to take Docker support in
stack as the container ecosystem evolves. There is
work-in-progress to have
stack create new Docker
images containing executables automatically, and this works even if
you perform the builds without Docker. Moving toward more general
support is another direction we are considering. A better solution
to using containers on non-Linux operating system is desirable. As
stack's support for editor integration via ide-backend improves,
this will apply equally well to Docker use.
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