Docker Terminology

let’s clarify some terminology that is used frequently in the Docker ecosystem.

• Images - The file system and configuration of our application which are used to create containers. To find out more about a Docker image, run docker image inspect alpine. 

• Containers - Running instances of Docker images — containers run the actual applications. A container includes an application and all of its dependencies. It shares the kernel with other containers, and runs as an isolated process in user space on the host OS. You created a container using docker run which you did using the alpine image that you downloaded. A list of running containers can be seen using the docker container ls command.

• Docker daemon - The background service running on the host that manages building, running and distributing Docker containers.

• Docker client - The command line tool that allows the user to interact with the Docker daemon.

• Docker Store - Store is, among other things, a registry of Docker images. You can think of the registry as a directory of all available Docker images. 

• Layers - A Docker image is built up from a series of layers. Each layer represents an instruction in the image’s Dockerfile. Each layer except the last one is read-only.

• Dockerfile - A text file that contains all the commands, in order, needed to build a given image. The Dockerfile referencepage lists the various commands and format details for Dockerfiles.

• Volumes - A special Docker container layer that allows data to persist and be shared separately from the container itself. Think of volumes as a way to abstract and manage your persistent data separately from the application itself.

For real applications IT users and app teams need more sophisticated tools. Docker supplies two such tools: Docker Compose and Docker Swarm Mode. The two tools have some similarities but some important differences:

• Compose is used to control multiple containers on a single system. Much like the Dockerfile we looked at to build an image, there is a text file that describes the application: which images to use, how many instances, the network connections, etc. But Compose only runs on a single system so while it is useful, we are going to skip Compose1 and go straight to Docker Swarm Mode.

• Swarm Mode tells Docker that you will be running many Docker engines and you want to coordinate operations across all of them. Swarm mode combines the ability to not only define the application architecture, like Compose, but to define and maintain high availability levels, scaling, load balancing, and more. With all this functionality, Swarm mode is used more often in production environments than it’s more simplistic cousin, Compose.

References:

1-  https://training.play-with-docker.com/ops-s1-hello/
2- https://training.play-with-docker.com/ops-s1-images/
3- https://training.play-with-docker.com/ops-s1-swarm-intro/