João Freitas

The author describes common antipatterns when building Docker container images, and provides tips on how to configure them for a better solution.

This is a list of recurring anti-patterns that I see when I help folks with their container build pipelines, and suggestions to avoid them or refactor them into something better.

And since only a Sith deals in absolutes, keep in mind that these anti-patterns aren’t always bad.

Many of them are harmless when used separately. But when combined, they can easily compromise your productivity and waste time and resources, as we will see.

Big images

It’s better to have smaller images, because they will generally be faster to build, push and pull, use less disk space and network.

But how big is big?

For microservices with relatively few dependencies, I don’t worry about images below 100 MB. For more complex workloads (monoliths or, say, data science apps), it’s fine to have images up to 1 GB. Above that, I would start to investigate.

I wrote a series of blog posts about optimizing the size of your images (part 1, part 2, part 3), so I’m not going to repeat that here; instead, let’s focus on some exceptions to the rule.

All-in-one mega images

Sometimes you need Node, PHP, Python, Ruby, and a few database engines in your image, as well as hundreds of libraries, because your image will be used as a base for a PAAS or CI platform. This is the case on platforms that have just one available image to run all apps and all jobs; then the image needs to have everything installed, of course.

I don’t have magic solutions for this. Keep in mind that you will probably need to support multiple images anyway eventually, so when you introduce support for, say, version selection, you might want to allow selection of smaller images with a tighter focus. Just an idea!

Data sets

Some code (especially in data science) needs a data set to function. It could be a reference genome, a machine learning model, a huge graph on which we’ll do some computation…

It’s tempting to put the dataset in the image, so that the container can “just work” no matter where and how we run it. And if the dataset is small, that’s generally fine.

But if the data set is big (let’s say, more than 1 GB) it will start becoming a problem. Sure, if your Dockerfile is well organized, the model will be added before the code; but if you add the model after the code, it will be a catastrophe. Builds will be slow, use up a lot of disk space, and if code must be tested on remote machines (as opposed to locally), the model will be pushed/pulled every time and use a lot of disk space on the remote machines too. That’s very bad.

Instead, consider mounting the data set from a volume. Assume that your code can access the data it needs on, say, /data.

When you run locally with a tool like Compose, you can use a bind-mount from a local directory (which will act as a cache) and a separate container to load the data. The Compose file would look like this:

    - ./data:/data
    command: |
      if ! [ -f /data/dataset ]; then
        curl ... -o /data/dataset
        touch /data/ready
    build: worker
    - ./data:/data
    command: |
      while ! [ -f /data/ready ]; do sleep 1; done
      exec worker     

The data-worker will wait for the data to be available before starting, and data-loader will download the data to the local directory data. It will download it only once. If you need to download the data again, just delete that directory and run again.

Now, when running e.g. on Kubernetes, we can leverage an initContainer to download the data, with a Pod spec similar to this:

  - name: data
  - name: data-loader
    - name: data
      mountPath: /data
    - curl
    - ...
    - -o
    - /data/dataset
  - name: data-worker
    image: .../worker
    - name: data
      mountPath: /data

Note that the worker container doesn’t need to wait for the data to be loaded, since Kubernetes will start it only after the initContainer is done.

If we run multiple workers per node, we can also use a hostPath volume (instead of an ephemeral emptyDir volume) so that the data only gets loaded once.

Another option is to leverage a DaemonSet to automatically populate that data directory on every node of the cluster ahead of time.

The best option depends on your particular use case. Do you have a single, big data set? Multiple ones? How often do they change?

The big upside is that your images will be much smaller, and they will still behave identically in local environments and in remote clusters, without requiring you to add special code to download or manage the model in your app logic. Big win!

Small images

It’s also possible to have images that are too small. Wait, what’s wrong with an image that would just be 5 MB?

Nothing wrong with the size of the image, but if it’s so small, it might be missing some useful tools, and that might cost you and your colleagues a lot of time when troubleshooting the image.

Images built with distroless or with FROM scratch might be small, but if your team is regularly stumped because they can’t even get a shell in the image to e.g. check which version of a particular file is there, see running processes with ps, or network connections with netstat or ss, what’s the point?

⚠️ This is extremely context-dependent. Some teams never need to get a shell in an image. Or, if you use Docker, you can use docker cp to copy some static tools (e.g. busybox) to a running container and check what’s going on. Or, if you’re working with local images, you can easily rebuild your image and add the tools that you need. Or, if you’re running on Kubernetes, you can enable the ephemeral containers alpha feature. But on most production Kubernetes clusters, you won’t have access to the underlying container engine and you may not be able to enable alpha features, so…

Here is one way to add a very basic toolkit to an existing image. This example shows a distroless image but it should work with other images as well:

COPY --from=busybox /bin/busybox /busybox
SHELL ["/busybox", "sh", "-c"]
RUN /busybox --install

If you want more tools, there is a very elegant way to leverage Nixery and install your tools without clobbering the existing image. For code deployed on Kubernetes, it’s even possible to add the tools in a volume, so that you don’t need to rebuild and redeploy a new image. If you’re interested, let me know, and I’ll write a follow-up post about that!

Overall, I personally like to build on top of Alpine images, because they’re tiny (Alpine is 5 MB) and once you have Alpine you can apk add whatever you want when you need it. Network traffic acting up? Install tcpdump and ngrep. Need to JSON stuff in and out? curl and jq to the rescue!

Bottom line: small images are generally good, and distroless is honestly some pretty awesome sauce in the right circumstances. If your circumstances are “I can’t get in my container and I’m resorting to adding print() statements to my code and pushing it all the way through CI to staging because I can’t kubectl exec ls”, you might want to reconsider. Just saying!

Zip, tar, and other archives

(Added December 15th, 2021.)

It is generally a bad idea to add an archive (zip, tar.gz or otherwise) to a container image. It is certainly a bad idea if the container unpacks that archive when it starts, because it will waste time and disk space, without providing any gain whatsoever!

It turns out that Docker images are already compressed when they are stored on a registry and when they are pushed to, or pulled from, a registry. This means two things:

If we include an archive (e.g. a tarball) and decompress it when the container starts:

If you notice that a Dockerfile is copying an archive, it is almost always better to uncompress the archive (e.g. using a multi-stage build) and copy the uncompressed files.

Rebuilding common bases

It’s pretty common to have a common base image shared between multiple apps, or multiple components within the same app. Especially when you have a bunch of non-trivial dependencies and they take a while to build; it sounds like a good idea to shove them in a base image, and reference that image from our other images.

If that image takes a long time to build (say, more than a few minutes), I recommend that you store that base image in a registry, and instead of building it locally, pull it from that registry.


Reason #1: pulling an image is almost always faster than building it. (Yes, there are exceptions, but trust me, they’re pretty rare.)

Reason #2: since this is the base on top of which everything else gets build, you probably want to make sure that you have a very specific set of versions in that image; otherwise we’re back to problems like “works on my machine” - exactly what we were trying to avoid by using containers! If everyone rebuilds the base image locally, we need to be extra careful about making that build process deterministic and reproducible: pinning all versions; checking the hashes of all downloads; using && or set -e in all the appropriate places to abort immediately if something fails within a list of commands in the build process. Or, we can simply store the base image in a registry, and now we’re sure that everyone is using the same one. Done.

What if we need to tweak that base image, though? Is there an easy way to do that without pushing a new version of the base image (which shouldn’t be necessary if we only need it locally), or without editing Dockerfiles?

If you’re using Compose, here is an example of a foundation image pattern. It’s a very simple pattern (I don’t think it’ll blow your mind!) but I often see it reimplemented with shell scripts, Makefiles, and other tools, so I thought it could be useful to show that it’s possible to do it with just Compose. If you build one of your apps, it will pull the base image; but if you need a custom base image, you can rebuild that specific image separately with docker-compose build.

Building from the root of a giant monorepo

I don’t have strong opinions for or against monorepos, but if your code lives in a monorepo, you probably have different subdirectories corresponding to different services and containers.

For instance:

├── app1
│   └── source...
└── app2
    └── source...

One possibility is to put the Dockerfiles at the root of the repository (or in their own, separate subdirectory), for instance like this:

├── app1
│   └── source...
├── app2
│   └── source...
├── Dockerfile.app1
└── Dockerfile.app2

We can then build each service with e.g. docker build . -f Dockerfile.app1. The problem with this approach is that if we use the “old” Docker builder (not BuildKit), the first thing that it does is upload the entire repo to the Docker Engine. If you have a giant 5 GB repo, Docker will copy 5 GB at the beginning of each build, even if your Dockerfile is otherwise well-designed and leverages caching perfectly.

I prefer to have Dockerfiles in each subdirectory, so that they can be built independently, in a small and isolated context:

├── app1
│   ├── Dockerfile
│   └── source...
└── app2
    ├── Dockerfile
    └── source...

We can then go to directories app1 or app2 and run docker build ., and it will only need the content of that subdirectory.

However, sometimes, the build process needs dependencies that live outside of the application directory; for instance some shared code in the lib subdirectory below:

├── app1
│   └── source...
├── app2
│   └── source...
└── lib
    └── source...

What should we do in this situation?

Solution #1: package the dependencies in their own images. When building the images for app1 and app2, instead of copying that lib directory from the repository, copy it from a lib image or a common base image. Of course, this may or may not be relevant in your situation, because one of the main selling points of monorepos is that a particular commit can describe exactly which version of the code and its dependencies we are using; and this solution can break that.

Solution #2: use BuildKit. BuildKit doesn’t need to copy the entire build context, so it will be much more efficient in that scenario.

Let’s talk more about BuildKit in that context!

Not using BuildKit

BuildKit is a new backend for docker build. It’s a complete rehaul with a ton of new features, including parallel builds, cross-arch builds (e.g. building ARM images on Intel and vice versa), building images in Kubernetes Pods, and much more; while remaining fully compatible with the existing Dockerfile syntax. It’s like switching to a fully electric car: we still drive it with a wheel and two pedals, but internally it is completely different from the old thing.

If you are using a recent version of Docker Desktop, you are probably already using BuildKit, so that’s great. Otherwise (in particular, if you’re on Linux), set the environment variable DOCKER_BUILDKIT=1 and run your docker build or docker-compose command; for instance:

DOCKER_BUILDKIT=1 docker build . --tag test

If you end up liking the result (and I’m pretty confident that you will), you can set that variable in your shell profile.

“How do I know if I’m using BuildKit?”

Build output without BuildKit:

Sending build context to Docker daemon  529.9kB
Step 1/92 : FROM golang:alpine AS builder
 ---> cfd0f4793b46
Step 90/92 : RUN (     ab -V ...
 ---> Running in 645af9563c4d
Removing intermediate container 645af9563c4d
 ---> 0972a40bd5bb
Step 91/92 : CMD   if tty >/dev/null; then ...
 ---> Running in 50226973af9f
Removing intermediate container 50226973af9f
 ---> 2e963346566b
Step 92/92 : EXPOSE 22/tcp
 ---> Running in e06a628465b3
Removing intermediate container e06a628465b3
 ---> 37d860630477
Successfully built 37d860630477

Build output for the same Dockerfile, with BuildKit:

 => [internal] load build definition from Dockerfile                                           0.0s
 => => transferring dockerfile: 8.91kB                                                         0.0s
 => [internal] load .dockerignore                                                              0.0s
 => => transferring context: 2B                                                                0.0s
 => [internal] load metadata for                               0.0s
 => [stage-19 27/28] COPY /usr/local/bin                                     0.0s
 => [stage-19 28/28] RUN (     ab -V | head -n1 ;    bash --version | head -n1 ;    curl --ve  0.7s
 => exporting to image                                                                         2.0s
 => => exporting layers                                                                        2.0s
 => => writing image sha256:9bd0149e04b9828f9e0ab2b09222376464ee3ca00a2de0564f973e2f90e0cfdb   0.0s

So make sure that you’re using BuildKit: I can’t think of any downside. It should never be slower, and in many cases, it will make your builds much faster.

Requiring rebuilds for every single change

That’s another anti-pattern. Granted, if you use a compiled language, and want to run the code in containers, you might have to rebuild each time you make a code change.

But if you’re using an interpreted language, or if you’re working on static files or templates, it shouldn’t be necessary to rebuild images (and recreate containers) after each change.

Most of the development workflows that I see are using correctly volumes, or live update with tools like Tilt; but once in a while, I see someone with e.g. generated Python code, or re-running webpack completely after each change (instead of using the webpack dev server), for instance.

(By the way, if you try to deploy your changes to a development Kubernetes cluster really fast, you should absolutely check Ellen KörbesQuest for the Fastest Deployment Time (video and slides). Spoilers, I have enough fingers on one hand to count the seconds between “Save my Go code in my editor” and “that code is now running on my remote Kubernetes clusters”. 💯)

Again, that anti-pattern is not always a big deal. If your build only takes a couple of seconds and the new layers are just a few megabytes, it’s probably alright if you rebuild and recreate containers all the time.

Using custom scripts instead of existing tools

We’ve all done it: the good old ./ (or build.bat). More than two decades ago, when I was doing my bachelor’s degree in computer science, most of my C homework assignments were built with a crappy shell script instead of a Makefile. Not because I didn’t know about Makefiles, but because we worked on both Linux and HP/UX and I kept finding creative ways to shoot myself in the foot with subtle differences between their respective implementations of make. (This might be why I tend to stay away from bashisms today, when I can.)

There are many tools out there providing outstanding developer experience. Compose, Skaffold, Tilt, just to name a few. They have excellent documentations and tutorials, and are used by thousands of developers out there. Some of your developers already know them and know how to maintain Compose files or Tiltfiles.

If our homemade deployment script is just about 10 lines, it’s not doing anything complicated, and can be replaced by a Compose file or Tiltfile. (Keep in mind that if it’s using any external tool like Terraform or a cloud CLI, we need to make sure that this is installed, which will always be at least as much work as “git clone ; docker-compose up”.)

If our homemade deployment script is about 100 lines, it might be doing something more complex. Building an image and then pushing it and then kicking a CI job and then provisioning a staging cluster to test that image, obtaining the address of the cluster to inject it in a local client; that kind of thing; handling many variations and special cases. If it’s 100 lines, there can’t be that many variations, and we’re exactly at the point where everyone will start adding their own particular special case to the script, slowly taking us to the next stage.

If our homemade deployment script has a thousand lines or more, it probably has a lot of custom logic in it, and handles a lot of situations; that’s great! It also means that it now requires you to write documentation, tests, and maybe even run internal training for new hires. Unfortunately, in my experience, these scripts are at least 10x bigger (often more like 100x) than an equivalent Compose file or Tiltfile. They have more bugs, less features, and nobody outside your team or organization knows how to use them.

If you work with one of these bigger deployment scripts, my suggestion is to try to remove rather than add code to it. Move the really custom parts to independent, standalone scripts that can run equally well locally or in containers. Replace the non-custom parts with standard tooling. It’s easier to maintain many small scripts rather than a big one.

“But we want to hide the complexity of containers / Docker / Kubernetes from our developers!”

You do you; but I think the best way to empower developers is to hide that complexity behind standard tools, because when they need to dive into the tooling, they can tap into a rich ecosystem instead of having to rely on your internal tooling or platform team.

Forcing things to run in containers

I like running all my stuff in containers, but I think it’s a very bad idea to force folks to run things in containers.

Let’s say that we have a script that uses the gcloud CLI, Terraform, and a few other tools like crane and jq.

On most platforms, these tools are easy to install with your preferred package manager. The script should therefore be able to run locally.

But to make things easier for our developers (and make sure that we use up-to-date versions of these tools), we build a container image with all these tools. Instead of running the script directly, we tell our devs to use that image.

At first, it looks like this just means replacing with docker run yadda-image. In practice, we will need to expose some env vars, bind-mount some volumes for credentials and code. We might end up writing a new script (that will do the docker run behind the scenes). And that’s where we can hit trouble.

Compare these two options:

Method #1: to do this task, run the script This script requires tools X, Y, and Z to be installed. If you don’t want to install these tools locally, you can run that script in a container by using image yadda/deploy (built using the Dockerfile in this subdirectory) and the following docker or docker-compose command: …

Method #2: to do this task, run the script This script requires Docker to be installed.

At first, method #2 seems better, and that’s why so many teams go this route. Look, it’s shorter, and there are less requirements! Except it’s missing a lot of details. Method #1 manages to tell you a lot of details about the requirements, in just a few lines. In method #2 you need to open the script to see what it’s doing. Probably an easy task if it’s a small 10-line script; harder if it’s one of these giant scripts that we were discussing in the previous section.

Before shipping this new workflow to our users, a good litmus test is to check how hard it is it to make changes to the script and run it. Can we still run the script locally, or is there something that prevents us from doing so?

And this gets worse when we run the script in a remote environment, for instance in CI or on Kubernetes!

Indeed, if our script must call Docker (or Compose), what happens if we try to run that script in an environment that is already containerized? Sometimes we can use Docker-in-Docker in CI, but it’s not always an option; so if our script relies on invoking Docker or Compose, we’re in trouble.

On the other hand, if we’re sticking to “run in an environment that has packages X, Y, and Z” it’s way easier to do because we already know which packages we need and which image has them.

Using overly complex tools

After recommending that you use tools rather than shell scripts, here is the opposite advice. Don’t add a complex dependency if the problem can be solved with a few lines of script (or with a tool that is already used in the stack).

Example: let’s say that we need to generate a file (configuration or otherwise) from a template and environment variables. In many cases, a here document is sufficient.

If the template has many $, rather than escaping them, we could use [envsubst] from the gettext package.

If the variables come from a JSON file instead of the environment, we might prepare them with a tool like jq.

If some variables need to be transformed, e.g. lowercase, remove special characters, spaces, encode or decode base64, compute hashes… We can install extra tools to do all these transformations before calling envsubst.

Perhaps we also need to support loops? At that point, we might decide to invest in a proper templating engine. That’s where things get really interesting!

If our stack includes a language like Node, Python, or Ruby, there is a good chance that we can find a small package that does what we need. (For instance, in Python, the Jinja2 package provides the j2 CLI tool.) On the other hand, if our stack doesn’t include Python, adding Python just so that we can install Jinja2 feels excessive.

If we are already using Terraform, it has a powerful templating engine that can generate local or remote files. Great! But adding Terraform just for its templating engine might also be a tad much.

(To be honest, if I’m in a very minimal environment and I need to generate fancy templates, I would probably write a script that outputs the whole file that I need, and redirect the output to the file to be generated. But each situation is different!)

We also need to be careful about using tools that are difficult to learn, and/or that very few folks know how to use. Bazel is probably one of the most efficient ways to produce artifacts and run CI on huge codebases, but how many of your colleagues are sufficiently familiar with Bazel to maintain build rules? And when that one person leaves, what will you do? 😬

Conflicting names for scripts and images

Another memory from my early days in computer science: during my first year using UNIX, I kept shooting myself in the foot by calling my test scripts and programs test.

So what?

This is not a big problem in itself; but I was using DOS before. On DOS, if you want to run a program named HELLO.COM or HELLO.EXE located in your current directory, you can run hello directly; you don’t have to do ./hello like on UNIX. So I had customized my login scripts so that . was in my $PATH.

Maybe you see where this is going: instead of running ./test I was running test and ended up calling /usr/bin/test (also known as /usr/bin/[) and wondering why nothing happened (because without arguments, /usr/bin/test doesn’t display anything and just exits).

My advice: avoid to name your scripts in a way that could conflict with other popular programs. Some folks will see it and they will be careful, others might not notice and accidentally run the wrong thing.

This is particularly true with 2-letter commands, because UNIX has so many of them! For instance:

Building with Dockerfiles

Finally, sometimes, using a Dockerfile to build your image isn’t the best solution. In Moving and Building Container Images, The Right Way, Jason Hall explains in particular how to build and push images containing Go programs efficiently and securely. Spoilers: it’s specific to Go (because Go has an outstanding toolchain), but even if you want to containerize other languages, it’s a good read, I promise.

Jason also mentions Buildpacks. I’m not a huge fan of Buildpacks; perhaps because they remind me of my time at dotCloud, and that after working for half a decade with similar build systems, it felt like a huge relief to work with Dockerfiles. 🤷🏻 But they definitely have merits so if you feel like Dockerfiles are too much (or, depending on the perspective, not enough) you should definitely check Buildpacks.

And more

As I said in the introduction of this series of tips: don’t treat these recommendations as absolute rules. What I’m saying is “hey, careful, if you do this, it can have unexpected consequences; look, here is what I suggest to improve the situation”.

When I deliver container training, I have a whole section about tips & tricks to build “better images” and write “better Dockerfiles”. I wrap it up with the following conclusion:

The point of containers isn’t to get smaller images. The point of containers is to help us ship code faster, more reliably, with less bugs, and/or at a bigger scale. Let’s say that you implement multi-stage builds, and you realize that now your tests run slower or are breaking randomly. Roll back, and try to address the main pain point instead! If you spend half of the day waiting for your code to get to staging or production because images take forever to push and pull, then, yes, maybe it’s a great idea to optimize image size. But if it’s not helping you to meet your goals, don’t do it.

Thanks for reading!

Acknowledgements: I would like to thank Dana Engebretson for our conversations, as well as her suggestions and feedback while writing this blog post. If you’re looking for a consultant to support machine learning workflows, you should absolutely reach out to her!

#reads #jérôme petazzoni #docker #containers #anti patterns