Docker packages applications into containers and provides the tools to build, run, connect, and distribute them. The key to understanding its architecture is to separate Docker Engine—the client, API, and daemon that manage Docker objects—from Docker Compose, which describes how an application’s services fit together.
What Docker does
Docker is a platform for packaging and running applications in containers. An application’s image supplies its files and dependencies; when Docker runs that image, it creates a container with runtime settings. Docker also provides components for building images, managing containers, connecting services, and distributing images through registries.
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A container is not automatically a separate virtual machine. Docker describes it as a process running on a host with its own filesystem, networking, and isolated process tree. This isolation helps keep application processes and resources separate, while the container remains a process on the host.
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How Docker Engine works
Docker Engine follows a client-server model. The docker command-line interface (CLI) sends requests through Docker APIs to the dockerd daemon. The daemon performs the requested work and manages Docker objects such as images, containers, networks, and volumes. The CLI and daemon may run on the same machine or communicate across machines.
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- CLI: The interface a user invokes to request Docker operations.
- API: The interface through which clients and the daemon communicate.
- Daemon: The long-running service that carries out operations and manages Docker objects.
This distinction matters: the CLI is not the container runtime’s entire architecture, and Compose is not a replacement for the Engine daemon. Compose is a higher-level tool that expresses an application’s services; Engine provides the machinery that manages the resulting Docker objects. See Docker’s Docker Engine documentation and Docker overview.
Dockerfile, image, and container: the build-to-run lifecycle
These terms describe different stages of an application’s lifecycle. A Dockerfile contains instructions for building an image; an image is a read-only template; and a container is a runnable instance of an image with runtime configuration.
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| Component | Role | When it matters |
|---|---|---|
| Dockerfile | Instructions used to build an image. | During image creation. |
| Image | A read-only template used to create containers. | As the basis for running an application. |
| Container | A running instance of an image, with runtime settings. | While the application process runs. |
| Compose file | Configuration describing application services and related resources. | When defining a multi-container application. |
Building an image from a Dockerfile creates image layers. When Docker starts a container, it adds a writable layer for runtime changes. Those changes do not become part of the original image, and changes that are not kept in persistent storage disappear when the container is removed. Use a volume for data that needs to outlast a container.
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Docker Compose is a tool for applications made up of multiple containers. A Compose file—usually named compose.yaml—describes services and their configuration, along with related networks and volumes. A service represents an application component and can be implemented by one or more containers created from the same image and configuration. Compose groups an application’s resources into a project, while services communicate through networks.
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For example, a web application might have one service for its web process and another for its database. The Compose file expresses those components and how they are configured and connected. The services are not themselves containers: Compose uses the description to create and manage the containers and supporting resources.
Docker’s guidance is that “each container should do one thing and do it well,” but Docker also notes that exceptions exist. Treat that as a useful design principle, not an absolute rule. For the application model, see the official Compose documentation, Compose application model, and Compose services reference.
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The simplest distinction is that a Dockerfile tells Docker how to build an image, while a Compose file describes application services and how they run together. They solve related but different problems and can be used in the same project: the Dockerfile defines an image’s build, and Compose defines the services that use images and their connections.
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| Dockerfile | Compose file | |
|---|---|---|
| Defines | Image-building instructions. | Application services, configuration, networks, and volumes. |
| Focus | How to create an image. | How application components run and connect. |
| Typical file | Dockerfile |
compose.yaml |
Docker’s guide to Compose explains the distinction between Compose and Dockerfiles.
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Images and registries
A registry stores and distributes images. Docker Hub is a public registry, and Docker uses it by default unless configured otherwise. The image is the packaged template; the registry is a place to store or obtain that template. Neither is the same thing as a running container.
How to get Compose
The supported installation route depends on the operating system and how Docker is installed. Docker Desktop includes Docker Engine, the CLI, and Compose. On Linux, Compose can be installed as a plugin alongside Engine and the CLI. Docker’s current installation overview treats standalone Compose as a legacy route and does not recommend it.
Check Docker’s Compose installation instructions for the current steps for your operating system. Installation methods and licensing terms can change; use the applicable official documentation rather than assuming one set of instructions applies everywhere.
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A quick mental model
- Write a Dockerfile to describe an image build.
- Build an image, the read-only template.
- Run the image as a container, a process with runtime settings and isolation on the host.
- Use Compose when an application’s services, networks, or volumes need to be described together.
- Use persistent storage for data that must survive container removal.
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