Docker Composeネットワークでコンテナを接続する

Running many containers and wiring their networks by hand is tedious. Docker Compose describes your whole stack in one YAML file and creates a dedicated network so services can find each other.

When you run docker compose up, Compose creates a single user-defined bridge network for the project. Every service joins it automatically, so no manual docker network create is needed.

On a Compose network, each service is reachable by its service name via built-in DNS. A service named db is reachable at the hostname db from any other service.

Two services on one network: a web app and a database.

Inside the web container, the connection string points to the service name, not an IP:

You can declare named networks to isolate groups of services.

In the example above, web can reach api but NOT db, because they share no network. This limits the blast radius if a frontend container is compromised.

expose makes a port reachable only to other containers on the network. ports publishes it to the host. Internal services like databases should usually only be exposed, never published.

You can give a service extra DNS names with aliases, handy when migrating from an old hostname.

Set a network as external: true to attach services to a network created outside this Compose file, sharing it across projects.

List and inspect the network Compose created to confirm which containers are attached.

Test what you have learned.

You learned that Docker Compose creates a network per project, lets services talk by service name, and supports custom networks for isolation, expose vs ports, aliases, and external networks.