To understand why Docker Compose is powerful, we must distinguish between two fundamental programming paradigms:
Imperative Programming
Declarative Programming
Imperative Programming: The How
You write code that explicitly defines the step-by-step sequence of actions the computer must take to reach a goal. It relies heavily on action verbs and control flow (loops, if/then statements).
Most major programming languages are designed this way: C, Java, Python, and Bash scripts.
Declarative Programming: The What
You write code that describes the desired end state without specifying the exact steps to get there. An underlying engine reads your description and handles the imperative execution automatically.
SQL is one example of this. While there are action words (SELECT, ADD, SORT), the intention is still about what you desire your data to be, not how to shape the data that way. HTML is another example where you describe the layout, not how to draw pixels on the screen.
Docker Compose
Why?
Dockerfile is a great mechanism for describing the steps for building container images.
This is imperative by nature (COPY this, RUN that, …)
It starts to become more complex when you want to arrange these containers into more complex infrastructure.
We want another layer of abstraction to help us declare what we want out of all these building blocks.
This is possible via Docker Compose.
In a nutshell
External, Python-based tool.
Open source.
Simple deployment workflow
Checkout code
Run docker compose up
Everything is up and running!
Overview of compose
Design of a container stack is declared/described in a YAML file typicalled named called docker-compose.yml.
Similar to docker build, -f flag can be used to specified non-standard compose file names.
Run docker compose up.
Compose automatically pulls images, builds containers, and starts them.
Compose can
Set up links, volumes, and other Docker options for the container stack.
Run containers in the background or in the foreground.
Example Compose File: Multiple Services
The ramcoin miner!
Run the following commands:
1
2
3
4
cd
git clone https://github.com/CSC468-WCU/ram_coin.git
cd ram_coin
docker compose up
Visit YOUR_CLOUDLAB_HEADNODE:YOUR_CHOSEN_PORT to see the deployed webserver.
Does it work?
Open another terminal, connect to your CloudLab node and run docker ps to see how many containers were deployed by the docker-compose.
Press Ctrl-C to stop the containers.
Sections of a compose file
Use cat or nano to view docker-compose.yaml file.
version used to be mandatory, now is optional (“2” or later).
Version 1 is legacy.
Version 2 has support for networks and volumes.
Version 3 has support for deployment options.
services is mandatory. A service is one or more replicas of the same image running as containers.
networks is optional and indicates to which networks containers should be connected. By default, containers will be connected on a private, per-compose-file network.
volumes is optional and can define volumes to be used and/or shared by the containers.
Containers in docker-compose.yaml
Each service in the YAML file must container either build or image.
build indicates a path containing a Dockerfile.
image indicates an image name (local or on registry).
If both are specified, an image will be built from the build directory and named image
Other parameters are optional and typically what you would add to docker run