Docker/Docker Compose

October 26, 2022

Docker:
- Docker is a set of platform-as-a-service products that deliver software in isolated packages. Each package bundles its own software, libraries, and configuration files.
Why not a virtual machine?
- Unlike virtual machines, containers do not have a high overhead and hence enable more efficient usage of the underlying system and resources.
Container:
- Containers offer a logical packaging mechanism in which applications can be abstracted from the environment in which they actually run. This decoupling allows container-based applications to be deployed easily and consistently, regardless of whether the target environment is a private data centre, the public cloud, or even a developer's personal laptop.
- Benefits: predictable environments, isolation between applications, portability, more granular resource control, improved efficiency, and better compute utilization.
Docker Image: The basis of a docker container. Represents a full application.
Docker Container: The standard unit in which the application service resides and executes.
Docker Engine: Creates, ships and runs Docker containers deployable on a physical or virtual, host locally, in a data centre or cloud service provider.
Repo (Docker Hub(Public) or Docker Trusted private): Cloud or server-based storage.

My mental model from object-oriented programming:

Docker works at the system level and helps keep runtime environments and dependency versions consistent.

Users can download an image from Docker Hub and start a container with the same environment defined by that image.
A modified container can be committed back into an image and shared with other users, which improves version control and reproducibility.
sudo service docker start — start the Docker service
docker build -t <imageName> .
- Build an image from a Dockerfile.
- -t: add a tag name for the image.
- -f: specify the Dockerfile path.
docker image rm <imageName>
- delete an IMAGE from the local image store
docker run -p 80:5000 —name <containerName> <imageName>
- Run a container from an image in the foreground.
docker run -d —rm -p 80:5000 —name <containerName> <imageName>
- Run a container from an image in the background and remove it automatically when it stops.
- -p: map a local port to a container port.
- -d: detach, run the container in the background, and print the container ID.
- —rm: remove the container when it stops.
- —name: set the container name.
docker stop <containerName>
- graceful shutdown, can be resumed
docker kill <containerName>
- force kill (like pulling the power cord)
docker ps -a
- list all the running containers including stopped containers
docker restart <containerName> or <containerID>
- restart a container

Dockerfile: a text document that defines how to assemble an image through an automated build.

FROM: initialize a new build stage with base IMAGE (from the local repository or public repository)
ENV: key - value pair
WORKDIR: working directory for any [RUN, CMD, ENTRYPOINT, COPY and ADD instructions] — sets the landing directory after logging into the terminal
COPY: COPY <src> <dest>
ADD: equivalent to COPY + automatically handles URLs and extracts tar archives
RUN: run commands in the current image (Linux) — supports both shell command format and exec format with String array
EXPOSE: the container expose a specific port to listen
ENTRYPOINT: cannot be overridden by commands after docker run (CMD provides additional arguments to ENTRYPOINT) → <ENTRYPOINT> <CMD>
CMD: specifies what to do after the container starts; when ENTRYPOINT is specified, CMD provides specific parameters; supports multiple instructions but only the last one takes effect (can be overridden by arguments after docker run, acts as default value)

PS: CMD runs at docker run time, RUN runs at docker build time

docker exec -it <containerName> /bin/bash
- operate inside an already-deployed container
- -it: run the command in the container with bash, keep interactive
docker container rm -f $(docker ps -aq)'
- Delete all Docker containers.
- -q → quiet: only display container ID
- -a → all
- -f → force
docker ps -q | xargs docker kill
- kill all running containers
- -q: only show container IDs
- -a all
- xargs: pass piped output as command arguments.
docker images -qa | xargs docker rmi -f
- list all images with only container IDs and force delete them
docker system prune
- clean up unused data (-a, -f for deeper cleanup, see docs)
docker inspect demo
- inspect the container
docker cp your_file demo:/
- copy a file into the container
docker <command> your_docker_account_name/image_name:<tag>
- tag can be treated as a version number; defaults to latest if not specified

docker-compose.yml: a file for quickly orchestrating groups of Docker containers.

service: individual application container instances, e.g. order microservice, inventory microservice, mysql container (one application container can include multiple running instances of the same container; each service has its own name, uses an image, and depends on other services)
project: a complete business unit composed of a group of related containers. 1 project = multiple services (container application instances)
Write the Dockerfile first, define each microservice and build the corresponding images
Use docker-compose to define a complete project, arranging all containers in the application, their order, and interactions
docker-compose up deploys everything in one command
docker-compose build: build all the Docker files
docker-compose up: run all the containers with dependencies
docker-compose down: stop

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