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Originally LXC-based, widespread. Created for shipping and running applications. Encourages running one process per container.

Basics

term meaning
image Packaged application along with environment. Product of docker build command. Snapshot of a container. Analogous to the concept from Object Oriented Programming: class
container Running instance of image. Analogous to the concept from Object Oriented Programming: class instance
tag Extra information appended to image name. Helpful for enclosing image version information, if omitted: latest is assumed
registry Repository that stores Docker Images, can be public or private
Dockerfile Text file, contains commands that assemble image

Each image is identified by ID, can contain different names and tags.
Image name contains of slash delimited name components, with optionally repository name prefixed. If the name doesn't contain registry name, then the public docker.io is assumed.

The process specified as ENTRYPOINT in Dockerfile becomes PID 1 process, it means that it will not handle SIGKILL signal.

The container lifetime is equal to the encapsulating (ENTRYPOINTs) process lifetime. The process dies, the container dies too.

Usage

Common commands:

operation command
Build image from Dockerfile. Invoke passing directory containing Dockerfile or pass the dockerfile with -f. The mandatory argument is called build context - all dockerfile instructions are relative to that path and all non-ignored files from that directory are send to Docker engine docker build -t my-tag . Read more
Create and start container (simplest form) docker run --name <some_name> <image tag or name> [args]
Stop container docker stop <container name or id>
Remove container docker rm <container name or id>
Remove image docker rmi <image name or id>
Push image to registry
  1. Tag image with repository URL first. Mind that repo may support the v1 and/or v2 'naming' format. Briefly speaking: v1 format doesn't support multiple path segments in image name, thus image: rootName/subName is invalid. For v2 it is fine: docker tag <image> <repourl>/<name>:<tag>
  2. Login to desired repository, docker.io is the default docker login <repourl>
  3. docker push <repourl>/<name>:<tag>
Show running containers docker ps
Show container/image details docker inspect container_or_image_name

Building images

Dockerfile contains all of the needed instructions to build the image with given application.
Each instruction corresponds to filesystem layer. Image is built within build context (the directory passed to docker build command - simply speaking). All COPY-kind instructions are relative to that build context.
The bigger the build context (directory contents and size) the longer it takes to send them to Docker engine.

Design goals when building images

There is a lot of things to keep in mind when building images. Many of them will be containerized-application specific: different set of guidelies for JVM application, python application, etc.

However there are some fundamental, containerized-application agnostic set of rules:

image size

The image is never downloaded once and there is a difference when downloading over 1GB image vs 100MB.

Storage is also limited especially when images doesn't re-use the layers.

Huge images impact the downtime of your application, the bigger the image, the longer it takes to download it, the longer the downtime. When the Kubernetes reschedules the POD to another Node which doesn't contain the image (or the image has changed), the time user waits for application is also longer

Security the fewer the libraries in container the better,

The main goal when building the image should be: - minimize the number of layers, the new layer is created only with: COPY, RUN and ADD commands. - use cache as much as possible. When the Dockerfile processed, each instruction is examined if its outcome is present in the cache already. The ADD and COPY will check if the underlying file has changed, the RUN commands will check only if the command string has not changed. Thus building the image with RUN apt-get update second time won't update the latest packages. - once the cache is invalidated (instruction that misses cache occurred), all subsequent instructions won't check the cache

The verify layer re-use: docker system df -v

one application in container
  • management of the image that ships one application is easier: one log source, one application to monitor. Usage is easier too, as there is no need for supervisord-like tools that govern multiple applications inside one container.
  • decoupled dependencies, easier to re-deploy
process reaping

When the (main) application spawns processes inside of container it's the application's responsibility to cleanup process after it completes. Usually it was done by PID 1 init process, but the docker lacks one (by default at least)
Since container uses its own pid namespace, as long as the container lives, the host's init process cannot clean orphaned processes.

Image types

There are two types of images in docker terminology: - parent image - the image that is specified in FROM clause. Your image is based on parent image. Your image after build becomes parent image.
- base image - the image that has no FROM or FROM scratch

Most Dockerfiles use parent images in their FROM clause.

Building base images

In order to build base image: - use debootstrap for Debian-based distributions (tool that installs Debian-based distributions into given filesystem) - archive it and docker import image.tar name

It is also possible to build base image from Dockerfile: 

FROM scratch
ADD some_binary /
CMD ["/some_binary"]

Such image will be able to execute the specified binary only.

Debugging

Failed container can be pretty easily debugged 1. Grab exited container ID
docker ps -a 2. Create image
docker commit <container id> 3. Run image corresponding to failed container, overriding its potential ENTRYPOINT
docker run -it --entrypoint /bin/bash <image id>

Live container can be debugged as well: 1. Grab the container name or ID: docker ps 2. Attach to the container with: docker exec -it <container id> /bin/bash

Networking

Networking system in Docker is configurable. It is possible to set different networking driver for containers using: docker run --network=host ...

bridge

Default driver, creates the bridge that connects all containers withing Docker host. Docker host manages IP addresses and iptables rules governing inbound/outbound traffic. From Docker host it should be possible to reach any port on the running container (host has routing configured for containers subnet: ip r s) However by default containers will be unreachable from outside (outside of Docker host). In order to tell Docker host, that it should add iptables rules allows access to containers the port publish option must be used: docker run... -p 8080:80 ....
The -p hostPort:containerPort maps (TCP by default) port 8080 on the host to the container port 80.

host

Uses the host networking interfaces. No network isolation. The ip a output from within the container will print same entries as ip a on the Docker host

overlay

Use when multiple containers need to communicate with each other and containers are scattered among different Docker hosts

macvlan

TODO

none

Disables networking

custom

TODO

Volumes

TODO

Configuration

dockerd uses following configuration files:

/etc/docker/daemon.json

/etc/default/docker (for Debian based OSes)

Change storage-driver:

> cat /etc/docker/daemon.json
{
  "storage-driver": "btrfs"
}

For full list of supported drivers refer to storage drivers

Change dockerd sockets:

> cat /etc/docker/daemon.json
{
        "hosts": ["unix:///var/run/docker.sock", "tcp://0.0.0.0:2376"]
}

It is possible that run init/upstart/service script specifies -H flag for startup command.
This must be removed from script itself, otherwise dockerd will fail to start

Underlying technology

Several linux kernel features are used by Docker.

Namespaces

5 Linux namespaces are used in Docker implementation

  1. pid
  2. net
  3. ipc
  4. mnt
  5. uts

The cgoup namespace is not used.

Control groups (cgroups)

Allows limiting available resources (e.g. hardware resources) for containers. For instance allows to specify available amount of memory

Union file systems

UnionFS

Container format

The mix of aforementioned three (namespaces, cgroups and union fs) is called the container format. The current default: libcontainer. This abstractions will allow other formats like BSD Jalis to be used interchangeably

References

  1. https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#label
  2. https://lwn.net/Articles/621006/
  3. https://www.slideshare.net/kerneltlv/namespaces-and-cgroups-the-basis-of-linux-containers
  4. https://codefresh.io/docker-tutorial/java_docker_pipeline/
  5. https://docs.docker.com/get-started/overview/
  6. https://stackoverflow.com/questions/49162358/docker-init-zombies-why-does-it-matter