Docker Compose (Media Engineering Architecture & Deployment)

# Docker Compose

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Learn the basics of [Docker Compose][docker-compose], a tool for defining and
running **multi-container applications**.

This material is part of [architecture & deployment course](https://github.com/MediaComem/comem-archidep) for [Media Engineering](https://heig-vd.ch/formations/bachelor/filieres/ingenierie-des-medias).

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**You will need**

* A [Docker][docker] account
* [Docker Desktop][docker-desktop] installed on your machine
  * *[Docker Compose][docker-compose] is included with Docker Desktop*

**Recommended reading**

* [Docker](../docker/)

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## The Docker philosophy

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A container is **ephemeral**: The same container may not run in perpetuity. Many
replicas may be launched. New containers will replace crashed ones.

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* Any code running in your container should be included in the image. **If the
  code needs revision, update the image** and run a new container.
* Pass **configuration** via environment variables.
* Do not manage **persistent storage** within containers. Delegate storage to a
  third-party service, another container (e.g. running a database), or mount a
  volume to keep data when a container is created again.

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### The Docker philosophy: isolation

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> Docker containers are **isolated services**, not VM replacements.

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### The Docker philosophy: microservice architecture

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A container should have **only one mission**. Containers allow subdividing the
functions of a system into smaller collaborating pieces.

* A Docker image should **contain what it needs** to provide its service and run
  as quickly as possible, and nothing else! Minimize your dependencies. The
  simpler it is, the more reusable and portable it is.
* For maximum efficiency and isolation, each container should address one
specific area of concern and **delegate other functions to other containers**,
  e.g. a web application container will delegate storage to a separate database
  container.

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## What is Docker Compose?

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Docker Compose is a tool for defining and running multi-container applications,
making it easy to manage **services, networks, and volumes** in a single,
comprehensible [YAML][yaml] configuration file called the **Compose file**.

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```yml
services:
  # Application service
  app:
    build: .
    depends_on:
      - db
    environment:
      DB_URL: postgres://db:5432/app
    ports:
      - "8080:80"
    restart: always
  # Database service
  db:
    image: postgres:16.1-alpine
    environment:
      POSTGRES_DB: awesome-db
      POSTGRES_USER: example
      POSTGRES_PASSWORD: changeme
    restart: always
    volumes:
      - "dat:/var/lib/postgresql/data"
volumes:
  dat:
```

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---
### Compose services

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The main unit of work with Docker Compose is a **service**: A service is an
**abstract definition of a computing resource within an application** which can
be scaled or replaced independently from other components. Services are defined
in the **Compose file**.

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```yml
app:
  build: .
  depends_on:
    - db
  environment:
    DB_URL: postgres://db:5432/app
  ports:
    - "8080:80"
  restart: always
```

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**Services are backed by one or multiple containers**, run by the platform
according to specified requirements. As services are backed by containers, they
are defined by a Docker image and set of runtime arguments. All containers
within a service are identically created with these arguments.

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### The Docker Compose command line

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Docker Compose is also a **Docker subcommand**. Based on a Compose file, it can:

* Start, stop, and rebuild services:
 * `docker compose up [service]`
 * `docker compose stop [service]`
 * `docker compose build [service]`
* View the status of running services:
 * `docker compose ps [service]`
* Stream the log output of running services:
 * `docker compose logs [--follow] [service]`
* Run a one-off command on a service:
 * `docker-compose run <service> <command> [args...]`

> See `docker compose help` for more commands.

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### Other Compose concepts

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**Networks** are the layer that allow services to communicate with each other.
Compose lets you configure named networks that can be reused across multiple
services for greater control and security.

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**Volumes** are persistent data stores implemented by the container engine.
Compose offers a neutral way for services to mount volumes, and configuration
parameters to allocate them to infrastructure.

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**Configs** allow services to adapt their behaviour without the need to rebuild
a Docker image. Services can only access configs when explicitly granted in the
Compose file. Configs are mounted as files into the file system of a service's
container.

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**Secrets** are a flavor of Configs focusing on sensitive data, with specific
constraint for this usage. Services can only access secrets when explicitly
granted in the Compose file. Secrets are either read from files or from the
environment.

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## Why use Docker Compose?

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* **Simplified control**: Orchestrate multi-container applications in a single
  file, making your application environment easy to replicate.
* **Efficient collaboration**: Compose files are easy to share, facilitating
  collaboration among developers, operations teams, and other stakeholders.
* **Rapid application development**: Compose caches its configuration. When you
  restart a service that has not changed, Compose re-uses the existing
  containers. Re-using containers means that you can make changes to your
  environment very quickly.
* **Portability across environments**: Compose supports variables to customize
  your containers for different environments or users.

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## Going further

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The following tools are (completely) out of scope for this course, but
interesting to learn about if you want to go further with Docker:

* [Traefik][traefik] is a **reverse proxy developed to integrate with
  microservices** (like Docker Compose services). Using it, you can get rid of
  nginx and its site configuration files. Traefik can interrogate the Docker
  Daemon about running containers and [configure itself
  automatically](https://doc.traefik.io/traefik/providers/docker/).
* [Docker Swarm][swarm] can network a cluster of Docker engines together across
  multiple servers, allowing you to **aggregate separate machines into one giant
  pool of resources**. You can then simply deploy Compose services to the swarm
  and containers will be automatically spawned on one of the cluster's machines.
* If you want to go even further into large-scale Docker deployments, look at
  [Kubernetes][k8s], an open-source system for **automating deployment, scaling,
  and management of containerized applications**. It groups containers that make
  up an application into logical units for easy management and discovery.

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## References

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* [Docker Compose][docker-compose]
* [Containers philosophy](https://dev.to/iblancasa/containers-philosophy-2714)
* [The First Thing You Should Know When Learning About Docker Containers](https://medium.com/factualopinions/the-first-thing-you-should-know-when-learning-about-docker-containers-e0de29ddb6c3)

[docker]: https://www.docker.com
[docker-compose]: https://docs.docker.com/compose/
[docker-desktop]: https://www.docker.com/products/docker-desktop/
[compose-configs]: https://docs.docker.com/compose/compose-file/08-configs/
[compose-networks]: https://docs.docker.com/compose/compose-file/06-networks/
[compose-secrets]: https://docs.docker.com/compose/compose-file/09-secrets/
[compose-volumes]: https://docs.docker.com/compose/compose-file/07-volumes/
[k8s]: https://kubernetes.io
[swarm]: https://docs.docker.com/engine/swarm/
[traefik]: https://traefik.io
[yaml]: https://yaml.org