Docker Build Cacher
This tool is intended to speedup multi-stage Dockerfile build times by caching the results of each of the
stages separately.
Why?
Multi-stage docker file builds are great,
but they still miss a key feature: It is not possible to carry from one build to another the statically generated
cache files once the source file in your project change. Here's an example that illustrates the issue:
Imagine you create a generic Dockerfile for building node projects
FROM nodejs
RUN apt-get install nodejs yarn
WORKDIR /app
# Whenever this image is used execute these triggers
ONBUILD ADD package.json yarn.lock .
ONBUILD RUN yarn
ONBUILD RUN yarn run dist
And then you call
docker build -t nodejs-build .
So now you can use the nodejs-build image in other builds, like this:
# Automatically build yarn dependencies
FROM nodejs-build as nodedeps
# Build the final container image
FROM scratch
# Copy the generated app.js from yarn run dist
COPY --from=nodedeps /app/app.js .
...
So far so good, we have build a pretty lean docker image that discards all the node_modules
folder and only keeps the final artifact. For example a bundled reactjs application.
It's also very fast to build! Since each of the steps in the Dockerfile are cached, as long as
none of the files changed.
But that's also where the problem is: Whenever package.json or yarn.lock files change, docker
will trash all the files in node_modules and all the cached yarn packages and will start from
scratch downloading, linking and building every single dependency.
That's far from ideal. What if we could do a change in the process so that changes to those files
do not bust the yarn cache? It turns out that we can!
Enter docker-build-cacher
This utility overcomes the problem by providing a way to build the docker file and then cache the
intermediate stages. On subsequent builds, it will make sure that the static cache files generated
during previous builds will also be present.
The effect it has should be obvious: your builds will be consistently fast, at the cost of more disk space.
Installation
There are binaries provided for linux-x86_64 and MacOS, check
the releases page for downloads.
How It Works
This works by parsing the Dockerfile and extracting the COPY or ADD instructions nested inside ONBUILD for each of
the stages found in the file.
It will compare the source files present in such COPY or ADD instructions to check for changes. If it can detect changes,
it rewrites your Dockerfile on the fly so that the FROM directives in each of the stages use the locally cached images instead
of the original base image.
The effect this FROM swap has, is that disk state for the image is preserved between builds.
Usage
docker-build-cacher requires the following environment variables to be present in order to correctly build
your Dockerfile:
APP_NAME: The name for application you are trying to build. Usually this is just the folder name you are in.
GIT_BRANCH: The name of the git branch you are building. Used to "namespace" cache results
DOCKER_TAG: It will docker build -t $DOCKER_TAG . at some point. Let it know the image tag you want at the end.
This utility has two modes, Build and Cache. Both modes should be invoked for the cache to work:
# APP_NAME ispassed as argument in the build process, you can use it as an env var in your Dockerfile
export APP_NAME=fancyapp
# GIT_BRANCH is used as part of the named for the resulting cached image
export GIT_BRANCH=master
# DOCKER_TAG corresponds to the -t argument in docker build, that will be the resulting image name
export DOCKER_TAG=fancyapp:latest
docker-build-cacher build # This will build the docker file
docker-build-cacher cache # This will cache each of the stage results separately
Additionally, docker-build-cacher accepts the DOCKERFILE env variable in case the file is not present in the
current directory:
DOCKERFILE=buildfiles/Dockerfile docker-build-cacher build
At the end of the process you can call docker images and see that it has created fancyapp:latest, and if you are using
multi-stage builds, it should have created an image tag for each of the stages in your Dockerfile
Fallback Cache Keys
As mentioned before the GIT_BRANCH env variable is used as part of the name for the generated cached image, this means that
the generated cache is scope to that name. This is done so you can keep different caches where you can experiment with widly
different requirements and libraries in the dockerfile.
This has the unfortunate side effect that building other branches will require building the cache from scratch. In order to solve this
you can use the FALLBACK_BRANCH environment variable like this:
export APP_NAME=fancyapp
export GIT_BRANCH=my-feature
export FALLBACK_BRANCH=master
export DOCKER_TAG=fancyapp:latest
docker-build-cacher build
docker-build-cacher cache
The above will make the cached image for the my-feature branch to be based on the one from the master branch.
In some circumstances, you may want to execute additional instructions after
including the base builder image. For instance, building an executable or
bundle using all the dependencies already downloaded:
# Automatically build haskell stack dependencies
FROM haskell-stack as builder
COPY . .
RUN stack install
# Build the final container image
FROM scratch
COPY --from=builder /root/.local/bin/my-app
This very typical example has a shortcoming now, each time we do COPY . . we
are also invalidating the compiling artifacts created in stack install, that
is, we are losing the benefits of incremental compilation.
If you want to keep incremental compilation, or any files generated in between
the builder image and the final FROM, you can label the intermediate image so
that docker-build-cacher will include that into the cached artifacts:
# Automatically build haskell stack dependencies
FROM haskell-stack as builder
# Instructs the cacher to also copy the files generated in this stage
LABEL cache_instructions=cache
COPY . .
RUN stack install
# Build the final container image
FROM scratch
COPY --from=builder /root/.local/bin/my-app
Warning:
The files copied in COPY . . will also be cached! This not only increases the
cache size, but also has a potentially dangerous inconvenient:
Any files you delete from one build to the other will be restored again by the
cacher. For example, if you delete one file in your source tree because you
don't use it anymore or you did a refactoring, it will pop up again in the build!
This may be a problem for compilers or build tools that scan all the files in
the folder, like the Go compiler. If you are certain that keeping old files
around is not a problem, then it is safe to use this feature. The Haskell
compiler, for instance, does not care at all about extra cruft in the folder.
It is possible to pass extra arguments and flags to the docker build step by providing the environment variable DOCKER_BUILD_OPTIONS as
shown below:
DOCKER_BUILD_OPTIONS="--build-arg foo=bar --quiet" docker-build-cacher build
Building from source
Dependencies:
Install the stack tool from the link above. Then cd to the root folder of this repo and execute:
stack setup
stack install
If it is the first time, it will take a lot of time. Don't worry, it's only once you need to pay this price.
