What is Docker?

Docker solves the problem of operating system environment consistency (packages, libraries, environment variables, configuration state etc.) and build qualification. It allows consistent test scenarios, which can greatly add to reliable service operations.
Before container deployment schemes (like with Docker) convergent end-to-end management of build, test, release and production environments was often lacking, and could cause time-intensive diagnostics work. Since nobody liked debugging live service in production, container solutions like Docker rose to popularity alongside DevOps.

Docker uses (Linux) kernel features and puts the known chroot concept on steroids. This way we can get a universal system to build and ship applications. Docker is the popular implementation, which has informed the OCI- specification and given rise to other runtimes such as Podman.

The technical foundation

The technical foundation topics concerning Docker are:

Container images
- Image registries
Overlay file systems
Linux kernel features: namespaces, cgroups, capabilities
Network bridge interfaces
Routes and IPtables
Build environments
Execution environments
- operating system (usually a minimal Linux) and runtime dependencies (of a release build)

In addition:

Possibly virtualization (for example, when Docker is used on Microsoft Windows with Docker Desktop)
Possibly orchestration technology (such as Kubernetes)
Continuous Integration (Jenkins, GitHub Actions etc.) and Continuous Deployment schemes
- DevOps automation
…

Although Docker is one technology element within a DevOps stack, it belongs to the bigger topic of MicroService architectures. Such architectures can have organizational dependencies, related to team management and organization.

What is Cloud Native?

Cloud Native denotes a macro trend related to the standardization of Linux service management and Linux-based cloud computing. It’s when services are made for the cloud (including on-premises private cloudsas defined within NIST 800-145). Cloud Native services can be migrated between different kinds of cloud deployment scenarios, which may help with a defined onboarding, migration and exit strategy of the services in scope.

Docker under the hood

Unlike a virtual machine, a container does not need to boot the operating system kernel. Therefore containers can be created in less than a second.

The process isolation (which uses Linux namespaces, cgroups and other kernel features like capabilities happens within a libcontainer component called “runC”. Once a container gets instantiated, there is another runC instance.

Container images - layered deployable snapshots

Container images are layered deployable snapshots. They are self-contained portable environments which can instantiate a (Micro-)service. Subsequent versions of the same container images are stored as a differential overlay.

Overlay file systems - differential snapshots

Starting from a base image, overlay file systems allow maintaining a lower storage footprint because for the following container image versions it defines the differential. This is allocated onto the disk against the layered base image.

cgroups - resource quotas

cgroups is a Linux kernel feature for resource compartmentalization.

Control Groups provide a mechanism for aggregating/partitioning sets of
tasks, and all their future children, into hierarchical groups with
specialized behaviour.

The manpage is man cgroups with an s.

Handy snippets

List dockerized processes

% docker ps                                                                                                marius@sofhelk
CONTAINER ID        IMAGE                                                 COMMAND                  CREATED             STATUS                  PORTS                                                      NAMES
c0ed60585f3a        cyb3rward0g/helk-spark-worker:2.4.0                   "./spark-worker-entr…"   2 months ago        Up Less than a second                                                              helk-spark-worker

Prune Docker system

% docker system prune                                                                                      
WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all dangling images
        - all dangling build cache

If at the point of the prune action running containers are using a network, it will not get deleted.

Prune Docker containers

% docker container prune                                                                                   
WARNING! This will remove all stopped containers.

This will only remove stopped containers.

Stop all running containers of the host

% docker container stop $(docker container ls -aq)                                                         
c0ed60585f3a
0fc271e3ce26
...
78554ec24145

If you want to delete the containers, you can issue rm instead of the stop. The snapshot-images will still be on the system.

Delete Docker snapshot-images

docker stop $(docker ps -aq)
docker rm $(docker ps -aq)
docker rmi $(docker images -q)

Given that the containers are stopped:

% docker system prune -a                                                                                  
WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all images without at least one container associated to them
        - all build cache
Are you sure you want to continue? [y/N] y
Deleted Networks:
docker_helk

Deleted Images:
untagged: confluentinc/cp-ksql-server:5.1.0
untagged: confluentinc/cp-ksql-server@sha256:6bbf25d9f682a0c12c3fefbd9e380c696957675b358c39e332047ff318d6e8a8
deleted: sha256:db9d3eaf462485ea5341ee4136380abdf4b56d0c92238f2dc68ebd8ec43d1551
deleted: sha256:e6006d635444b53e795b80697d15dcd79ded0fcd2427e4a4b796ade88ea83cea

Alternatively:

% docker rmi $(docker images -a -q)

Docker