Fundamentals of Docker

Docker is a tool that allows us to automate deployment of sofware applications inside containers running on a host machine. Docker provides an abstraction layer that seperates the application layer from infrastructure layer. Users are able to package their applications with all their dependencies into a standardized unit.

Docker Architecture

Docker vs VM

1. Basic Docker terms

• Dockerfile : simple text file that contains list of commands which docker client uses to build Docker image. It automates creation of docker image.
• Docker image : templates for docker containers. It is source code for a docker container.
• Docker container : copy of docker images. It is isolated environment that runs on host machine. Multiple containers could be created from same docker image.
• Docker engine : it is core engine of docker. dockerd is server. docker client and server can run on same host and on different hosts. Clients can communicate with dockerd with rest api calls.
• Docker registry : it is a repository that stores docker images. It could be public or private.

2. Basic Docker commands

• docker info : shows information about underlying docker engine.

docker info

Server:
 Containers: 2
  Running: 0
  Paused: 0
  Stopped: 2
 Images: 1
 Server Version: 23.0.5
.......

• docker pull : downloads docker image from docker registry.

docker pull ubuntu

Using default tag: latest
latest: Pulling from library/ubuntu
dbf6a9befcde: Pull complete
Digest: sha256:dfd64a3b4296d8c9b62aa3309984f8620b98d87e47492599ee20739e8eb54fbf
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest

• docker images : shows docker images downloaded locally.

docker images

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
postgres     latest    bf700010ce28   3 weeks ago   379MB
ubuntu       latest    3b418d7b466a   4 weeks ago   77.8MB

• docker run : runs a container from a docker image.

docker run ubuntu

• docker ps : shows running containers.

docker ps

CONTAINER ID   IMAGE      COMMAND                  CREATED       STATUS         PORTS                    NAMES
d8977c6f1260   postgres   "docker-entrypoint.s…"   3 weeks ago   Up 3 minutes   0.0.0.0:5432->5432/tcp   postgresqldb

• docker start : runs container previously created.

docker run postgresql

postgresqldb

• docker stop : stops a running container.

docker stop postgresql

postgresqldb

• docker stats : shows resource usages of containers.

docker stats

CONTAINER ID   NAME           CPU %     MEM USAGE / LIMIT    MEM %     NET I/O       BLOCK I/O   PIDS
d8977c6f1260   postgresqldb   0.01%     68.5MiB / 3.734GiB   1.79%     1.46kB / 0B   0B / 0B     6

• docker image rm / docker rmi: remove one or more containers.

docker rm 123a889b196a

123a889b196a

• docker rmi : remove docker image locally.

docker rmi hello-world

Untagged: hello-world:latest
Untagged: hello-world@sha256:fc6cf906cbfa013e80938cdf0bb199fbdbb86d6e3e013783e5a766f50f5dbce0
Deleted: sha256:9c7a54a9a43cca047013b82af109fe963fde787f63f9e016fdc3384500c2823d
Deleted: sha256:01bb4fce3eb1b56b05adf99504dafd31907a5aadac736e36b27595c8b92f07f1