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docker.md

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Terms

  • Docker Engine: consists of two parts: a daemon, a server process that manages all the containers, and a client, which acts as a remote control for the daemon.
  • Container: Process in a box. The box contains everything the process needs to run, including the filesystem, shell, and libraries. They are not enabled by default.
  • ** Virtual machine**: The hardware virtualises a VM at the hardware level. This is unlike containers, which are virtualised by the operating system.
  • Commit: Save changes made to the container.

Fun facts

  • Containers are simply isolated processes (rather than isolated "machines") that only see what they need to see. Containers are powered by two features in the linux kernel: namespaces, which allow container processes to appear as if they are running on their own operating system, and cgroups, which limit the amount of resource a container can access.

When to use Docker

  • The fact that it is good enough for everyone, and that people tend to know what they are looking at when they read a Dockerfile, means that you can probably use it whenever you want, even when other tools are better.

When not to use Docker

  • You application runs only on Windows or macOS.
  • Your application is tightly coupled to the OS (like needing direct access to CPU or memory).
  • Your application is a GUI.
  • Docker Swarm is simpler than Kubernetes, but it is not recommended anymore. It is easy to use, but it won't scale, at least not as well as Kubernetes clusters.

Get started

  • Docker has pre-built images; run docker search (name) to find them, and docker pull (user)/(name) get them.
  • To run something in a container, run docker run (user)/(name) (command).
  • Run ubuntu with sudo docker run -it ubuntu bash. See that it is running using sudo docker ps. But once you exit from the shell, the container dies; every time you run sudo docker run -it ubuntu bash, you get a brand new user in a brand new shell. It would appear that -i is interactive, and -t is terminal.
  • To install something in a container, run docker run (user)/(name) apt-get install -y .... The -y is required because docker commands cannot be interactive.
  • To view a list of commands run in the container, run docker ps -l. It shows you IDs of states after the commands are run.
  • To commit a container, run docker commit (id from above) (new container name e.g. foo/bar). Now you can docker run foo/bar.
  • To inspect a container, run docker inspect (id from above).
  • If you sign up for a Docker account, you can push your own images onto the repository using docker push (container name e.g. foo/bar).
  • The advantage Vagrant has over Docker is: full isolation. Docker cannot guarantee the virtual hardware that the environment gets.
  • Minikube can be started using minikube start. Minikube runs a docker cluster inside a VM, so it may take a while.

Dockerfile

Dockerfiles are essentially scripts that define what you will install, what the container will run, and what ports the container will expose. Read the damn guide. There is also that example file you can look at. It does not include required files to run the example Flask app.

  • In the same directory with Dockerfile, running sudo docker build -t (your container name in all lowercase) . will build a new container. Check if you actually built one with docker images. Note that python:2.7-slim is a base container that does not imply ubuntu... in fact they are mostly Debian and Windows ones.
  • Run sudo docker run -p 4000:80 (your container name) to run it. You access the container's port 80 from your own port 4000.
  • Alternatively, with -d, sudo docker run -p 4000:80 (your container name) runs the container in detached mode. With detached mode, you can run your container and close the terminal afterwards, and your container will continue to run.
  • Container IDs come in short and long forms. The long form is LOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOONG, and the short form is only LOOOOOOOOOO.
  • There is no difference between docker ps and docker container ls. Both list your containers.
  • VOLUME /foo creates some kind of mount at... some place's /foo. The container can access /foo and see files there. Then /foo is never deleted, and the same container can see the volume again when it restarts. Where is /foo actually? /var/lib/docker/volumes/. You can find them with docker volume ls.
  • RUN is a step inside the dockerfile that runs commands inside a docker image. CMD defines a default thing to run when the container starts (usually as the command that runs your application server), but doesn't run the CMD when the image is being built.
  • Each RUN adds a layer of IO diffs that may increase the final image size. So geniuses at stack overflow recommend running everything with a single RUN statement where it makes sense.
  • It is perhaps best to COPY your files into the container last, to minimise the size of the diff you get from every build.

Docker compose