Multiple services in one file

The real power of Compose shows when an app has more than one service. You list each service under services, and Compose runs them together on a shared network where they find each other by name.

A web and cache pair

Here is an app with two services, a web server and a Redis cache:

services:
  web:
    image: nginx:1.27-alpine
    ports:
      - "8080:80"

  cache:
    image: redis:7-alpine

Two services, web and cache. Compose starts both, creates one network for the app, and attaches both to it. web can reach cache at the hostname cache on Redis's port 6379 - no --network, no links, nothing extra. The service name is the address, the same rule you learned for user-defined networks, applied automatically.

Only publish what needs the outside

Notice web publishes a port but cache does not. That is deliberate. The web server needs to be reachable from the host, so it maps 8080. The cache is only ever talked to by web, over the internal network, so it needs no published port at all. Publishing it would only expose it needlessly.

This is a good habit: publish ports only for the services that must be reached from outside - usually just the front door - and let the rest talk to each other privately on the Compose network.

Each service is still a container

Nothing new is happening under the hood. Each service is a container like the ones you ran by hand, with the same images, environment, and volumes. Compose is coordinating them - starting them together, wiring the network, giving them names. Everything you know about containers still applies to each service.