Docker

Terminology

• Container: An instance of a virtualised read-write environment
• Image: A read only definition of a container

Docker containers

A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another.

Containers vs Virtual Machines

Virtual machines virtualize hardware, they emulate what a physical computer does at a low level. Containers virtualize at the operating system level. Isolation between containers that are running on the same machine is still really good. or the most part, each container feels like it has its own operating system and filesystem. In reality, a lot of resources are being shared, but they're being shared securely through namespaces.

Containers share host operating system resources, while maintaining isolation.

Images

Images are read-only definition of a container.

pull an image

docker pull 
# e.g 

docker pull docker/getting-started

View all images

docker images
``

## Basic commands

**List docker containers**

```console
docker ps

Run a container from image

docker run -d -p 8965:80 docker/getting-started:latest

-d: Run in detached mode (doesn't block your terminal) -p: Publish a container's port to the host (forwarding) hostport: The port on your local machine containerport: The port inside the container namespace/name: The name of the image (usually in the format username/repo) tag: The version of the image (often latest)

When you docker ps you'll see: 0.0.0.0:8965->80/tcp. This says that port 8965 on your computer is being forwards to port 80 on the container.

Docker container

docker stop  # stops container
docker rm  # removes container

Docker volumes

Docker volumes are like memory cards for containers. Containers are ephemeral, meaning that if you spin up a container, install a new program in that container (for example) and then stop and restart that container, that program will be lost.

This is solved by docker volumes, which are persistent storage that we can attach to docker containers.

Basic volume commands

docker volume create sick-vol
docker volume ls # will list docker volumes 
docker volume inspect sick-vol # will contain details about the new volume

Remove all unused volumes

docker volume prune

**Stop

Restart docker container

docker restart 

Networks

Networks are a way to control communication between containers.

Create a bridge network and connect containers

docker network create caddytest

List docker networks

docker network ls

Run a container without network connectivity

docker run -d --network none docker/getting-started # For example

Attach container to network

Example when containers need to communicate on the same network, e.g a proxy manager services needing to communicate on the same network as other containers

docker network connect  

Detach network from container

docker network disconnect  

Gracefully shutdown container with docker compose

docker stop 

This can be done from within the project /dir if using docker compose:

docker compose down 

If the network is attached, use network disconnection above.

Remove all containers within the docker-compose.yml file

For e.g associated services

sudo docker compose down --volumes --remove-orphans

Remove docker images

docker images --> list them
docker rmi 

Gracefully shutdown container w/out docker compose

docker stop 
docker rm 
docker network rm  --> if there's a network

Create shell session in container

sudo docker exec -it  /bin/bash

i makes exec interactive. t gives us a keyboard interface.

Execute shell command

docker exec 
docker exec  netstat -ltnp #example that lists ports in use

exit with exit command

Load balancing with Docker