Service Ports Field

The ports field within a Compose service publishes one or more of that service's container ports to the host, making them reachable from outside the Compose-managed network, exactly as the -p flag would for an individual docker run command.

Publishing a Port

The most common form maps a specific host port to a specific container port.

services:
  api:
    ports:
      - "8080:8080"

This makes the API, listening on port 8080 inside its container, reachable at localhost:8080 from the host.

Mapping to a Different Host Port

The host-side and container-side ports don't need to match, allowing the same container port to be exposed differently depending on need.

services:
  api:
    ports:
      - "3000:8080"

The API still listens on port 8080 inside its container, but is reached at port 3000 on the host.

Why Most Services in a Stack Don't Need Published Ports

A service that's only ever reached by other services within the same Compose application — a database, for instance — generally doesn't need a ports entry at all, since Compose's automatic networking already makes it reachable to other services by name without any host-level publishing.

services:
  db:
    image: postgres:16
  api:
    ports:
      - "8080:8080"
    environment:
      - DATABASE_URL=postgres://db:5432/app

Here, only api (which needs to be reached from outside the Compose application, such as from a browser) has a ports entry; db is reachable by api through the network alone.

Publishing to a Specific Host Interface

A port can be bound specifically to a particular host interface, such as localhost only, restricting where the published port is actually reachable from.

services:
  api:
    ports:
      - "127.0.0.1:8080:8080"

Why the Ports Field Matters

Correctly using ports only where a service genuinely needs to be reachable from outside the application avoids unnecessarily exposing internal services, while still ensuring exactly the right entry points into the application are accessible as intended.