Design of repo2docker¶
The repo2docker buildpacks are inspired by Heroku’s Build Packs. The philosophy for the repo2docker buildpacks includes:
using common configuration files for familiar installation and packaging tools
allowing configuration files to be combined to compose more complex setups
specifying default locations for configuration files (in the repository’s root,
repo2docker and adding to it in the future, the
developers are influenced by two primary use cases.
The use cases for
repo2docker which drive most design decisions are:
Automated image building used by projects like BinderHub
Manual image building and running the image from the command line client,
jupyter-repo2docker, by users interactively on their workstations
The core of
repo2docker can be considered a
When given an input directory which has a particular repository checked out, it
deterministically produces a Dockerfile based on the contents of the directory.
So if we run
repo2docker on the same directory multiple times, we get the
exact same Dockerfile output.
This provides a few advantages:
Reuse of cached built artifacts based on a repository’s identity increases efficiency and reliability. For example, if we had already run
repo2dockeron a git repository at a particular commit hash, we know we can just re-use the old output, since we know it is going to be the same. This provides massive performance & architectural advantages when building additional tools (like BinderHub) on top of
We produce Dockerfiles that have as much in common as possible across multiple repositories, enabling better use of the Docker build cache. This also provides massive performance advantages.
Reproducibility and version stability¶
Many ingredients go into making an image from a repository:
version of the base docker image
versions of the libraries installed by the repository
repo2docker controls the first two, the user controls the third one. The current
policy for the version of the base image is that we will use the current LTS
version Bionic Beaver (18.04) for the foreseeable future.
The version of
repo2docker used to build an image can influence which packages
are installed by default and which features are supported during the build
process. We will periodically update those packages to keep step with releases
of Jupyter Notebook, JupyterLab, etc. For packages that are installed by
default but where you want to control the version we recommend you specify them
explicitly in your dependencies.
Unix principles “do one thing well”¶
repo2docker should do one thing, and do it well. This one thing is:
Given a repository, deterministically build a docker image from it.
There’s also some convenience code (to run the built image) for users, but that’s separated out cleanly. This allows easy use by other projects (like BinderHub).
There is additional (and very useful) design advice on this in the Art of Unix Programming which is a highly recommended quick read.
Although other projects, like
s2i, exist to convert source to
repo2docker provides the additional functionality to support
composable environments. We want to easily have an image with
Python3+Julia+R-3.2 environments, rather than just one single language
environment. While generally one language environment per container works well,
in many scientific / datascience computing environments you need multiple
languages working together to get anything done. So all buildpacks are
composable, and need to be able to work well with other languages.
Pareto principle (The 80-20 Rule)¶
Roughly speaking, we want to support 80% of use cases, and provide an escape hatch (raw Dockerfiles) for the other 20%. We explicitly want to provide support only for the most common use cases - covering every possible use case never ends well.
An easy process for getting support for more languages here is to demonstrate
their value with Dockerfiles that other people can use, and then show that this
pattern is popular enough to be included inside
repo2docker. Remember that ‘yes’
is forever (very hard to remove features!), but ‘no’ is only temporary!