Instruction 03-docker-compose-fundamentals: Reading

Docker Compose describes a containerized application as a model. Instead of entering a long docker run command for every container, you record the desired images, commands, networks, mounts, ports, and health checks in a YAML file. Compose then translates that model into Docker resources carrying labels that identify their project and service. This makes an application easier to review, repeat, troubleshoot, and remove as a unit.

The top-level services mapping contains the application's service definitions. In this lesson, web runs NGINX and serves a host-managed HTML file, while client runs a small BusyBox container used to test internal connectivity. Both services join labnet. Compose provides network-scoped DNS, so the client can request http://web without knowing the web container's changing IP address. Container IP addresses are implementation details and should not be placed in application configuration when a stable service name is available.

The web port declaration uses 127.0.0.1:8080:80. Port 80 is the container-side listening port, 8080 is the host-side port, and 127.0.0.1 limits the listener to the host's loopback interface. Internal traffic from client to web does not travel through port 8080; it uses the shared Docker network and the container port directly. This distinction is central to understanding Compose networking.

The HTML file is bind-mounted read-only. That demonstrates how host-managed content can be injected without rebuilding an image, while the read-only flag reduces accidental modification from inside the container. Bind mounts also create a dependency on a specific host path. Relative paths are convenient, but this lesson uses explicit paths so the mutation boundary remains clear. In a production workflow, application content is often built into a purpose-specific image rather than mounted from an administrator-managed directory.

Always validate the model before starting it. The config command resolves and checks the Compose input and can reveal YAML or schema mistakes without creating the service containers. Starting with up --detach reconciles the declared model and leaves the containers running in the background. The ps and logs commands then expose runtime state. A running process is not necessarily a functioning application, so web also has an HTTP health check. Health may briefly display as starting before becoming healthy.

Compose project names matter. This lab always supplies the project name class36 so resources are grouped predictably and do not depend on the current directory's name. The same project name and file should be used for every lifecycle command. The down command removes the project's containers and network, but it does not remove the source files or indiscriminately delete unrelated Docker resources. Compose is therefore both a documentation format and a lifecycle tool for a defined application boundary.