In Docker Compose, services define application components, networks determine which components can communicate, and volumes provide storage that can outlast a container. Those three parts work together in a Compose file: Compose creates the containers and related resources, then lets you manage the application as a project.
What is a service in Docker Compose?
A service is the configuration for an application component, such as a web server, application process, or database. It specifies an image and runtime settings; Compose uses that definition to create one or more containers. The service is the reusable configuration, not the individual running container.
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Service names also matter at runtime. When two services share a Compose network, a container can generally reach the other service by its service name, rather than by a container IP address. For example, an application service named app can address a database service named db using db as the hostname, provided both are on a shared network.
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By default, Compose creates a project network using the bridge driver and attaches services to it. Services on that shared network can discover one another through Compose’s internal DNS using their service names. This default is convenient for small stacks because you do not need to define a network just to let the application reach its database.
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Containers on the same network can communicate with each other without publishing a port on the host. Publishing is a separate choice: it makes a container port reachable from outside the Compose network, for example by mapping host port 443 to container port 8043. Use published ports for access from the host or other external clients, not as a prerequisite for service-to-service communication.
When should I use custom networks?
Use separate networks when some components should not share a direct network path. A common arrangement puts a proxy and application on a frontend network, and the application and database on a backend network. The application joins both networks; the proxy and database do not share one. That means they cannot communicate through those networks directly, while the application can reach both.
An internal: true network has no default gateway for external connectivity. It does not guarantee that a service is isolated from the internet if that service is also attached to a regular network: it may still have external connectivity through that other network. External networks can be used to connect services from different Compose projects, but the network must already exist before you run docker compose up.
What is the difference between a named volume and a bind mount?
Both named volumes and bind mounts make files available inside a container, but they differ in where the data lives and who manages the location.
| Mount type | Storage location and management | Typical use |
|---|---|---|
| Named volume | A storage area managed by the container engine, declared at the top level of the Compose file and mounted by a service. | Persistent application data, such as a database’s files. Multiple services can use the same volume when each is explicitly configured to mount it. |
| Bind mount | A host path mapped into the container. It can be configured directly under a service when it is only used there. | Files managed on the host, including source files in a development setup. |
Choose a named volume when the container engine should manage persistent application storage. Choose a bind mount when the container needs access to a particular host directory. A bind mount exposes that host path inside the container, so be deliberate about which path you share.
Compose supports other mount types as well, including tmpfs and npipe. The right choice depends on the data and runtime environment; a named volume is not interchangeable with a host-directory mapping.
How do services, networks, and volumes fit in a Compose file?
This example shows a database volume and a segmented network. The application and database share the backend network, while a proxy and application share the frontend network. The proxy can reach the application, and the application can reach the database by service name. The database’s data is stored in the named volume.
services:
proxy:
image: example-proxy
networks:
- frontend
app:
image: example-app
networks:
- frontend
- backend
db:
image: example-database
volumes:
- db-data:/var/lib/example
networks:
- backend
volumes:
db-data:
networks:
frontend:
backend:The images, mount destination, and service behavior in this illustrative configuration must match the application you are actually deploying. The important relationships are the resource declarations and which services are attached to each network.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What happens when network settings are omitted?
If a service has no networks setting, or its setting is empty, Compose connects it to the project’s implicit default network. This makes the default shared-network behavior useful for simple applications. Add explicit network assignments when you need to control which services share connectivity.
The service reference also allows network_mode values such as host or none. A service cannot set both network_mode and the networks attribute; they are alternative networking configurations.
How do you start and inspect a Compose application?
The Compose Specification is Docker’s recommended file format. Docker documents it as the merged successor to the legacy 2.x and 3.x file formats and identifies support in Compose V2, beginning with Compose CLI version 1.27.0. That boundary is format-history context, not a recommendation to install an old CLI; consult Docker’s current Compose overview for installation guidance.
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| Command | Purpose |
|---|---|
docker compose up |
Starts the services defined in the Compose file. |
docker compose ps |
Lists the services and their current status. |
docker compose logs |
Displays container output. |
docker compose down |
Stops and removes running services. |
For a guided working example that covers a Flask and Redis application, health checks, Compose Watch, named-volume persistence, multi-file setup, and stack inspection, see Docker’s Compose Quickstart. Consult the official references for services, networks, and volumes when checking supported fields and configuration details.
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