Docker Compose: What It Is, How It Works, How to Install, and Why Use It

Key takeaways

• Docker Compose is a Docker tool that lets you define and manage multi-container applications using a single configuration file.

• Docker Compose lets you start all the services specified in your configuration file using a single command: docker compose up.

• To install Docker Compose, you can download Docker Desktop to get Compose, Docker Engine, and Docker CLI, while Linux users with Docker Engine and Docker CLI can install Docker Compose as a plugin.

Docker Compose is used in local development environments, automated testing for CI/CD pipelines, and single-host deployments. Learn more about the benefits of Docker Compose in multi-container orchestration, essential Compose commands, and how it works using a single YAML file. 

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What is Docker Compose?

Docker Compose is a tool built on Docker, a platform for developing and deploying software applications in an isolated container environment. Docker Compose simplifies the management of multiple such containers by allowing you to define and run complex Docker applications using a single configuration file [1].

Normally, you’d use several docker run commands to manually start multiple containers, manage their networks, and connect containers to both networks and other containers defined in separate configuration files, which can be time-intensive. Using Docker Compose, you define your services (containers) and how to build them, the connections between these services, data sharing and storage, and other configurations in one YAML file. Then, using a single command, docker compose up, you can build, control, and run all the configurations defined in the YAML file.

You can store this file in your code repository so everyone on your development team can run the same configuration and environment every time, without having to rebuild from scratch. This ensures speed, consistency, and reproducibility when building interconnected applications such as servers, databases, and APIs [2]. 

What is the difference between Docker and Docker Compose?

The primary difference between Docker and Docker Compose is that Docker mainly works with individual containers, while Compose is a tool for managing multiple containers as a single stack. In normal Docker, you’ll have to individually configure your containers with commands like docker run, whereas Compose lets you store all your container specifications in a single YAML file and use commands like docker compose up to automatically start creating services and connecting with networks and other containers [3].

How does Docker Compose work?

At the center of Docker Compose is the Compose file, which is a configuration file written in YAML that specifies the containers and their settings, networks, dependencies, and environment variables. You’ll interact with the container applications through the Compose command-line interface (CLI), allowing you to start, stop, or rebuild your containers throughout the multi-container application’s life cycle.

In general, a Compose file, or a “compose.yaml” file, will include the following information [4]:

• Services: A service is a part of your application that includes the container settings and definition, which you can scale independently. You’ll define a service using a container image (or use the build command to create a new image) and runtime configurations like port mappings and environment variables. This dictates how every container within a single service is created and run.

• Networks: Various services communicate with one another via networks that connect the containers within each service. Docker creates a default network for containers to communicate with one another using the service names, but you can also explicitly define a specific network to use.

• Volumes: Volumes persistently store data from services and share it between containers, ensuring no data is lost during container restarts or rebuilding. Compose allows you to define how services use volumes and how to distribute volumes across the infrastructure.

After you’ve defined your Compose file, you’ll use the docker compose up command to run your application, and ti will;

1) Read YAML file

2) Create containers

3) Set up network

4) Pull or build images

5) Assign volumes

6) Start containers in order using the Docker Engine.

You can then continue to manage your application life cycle, like rebuilding or scaling containers, reviewing logs, and stopping services, through specific commands. Since Compose is declarative, where the Compose file defines the necessary configurations and application states every time, whenever you make a change in the YAML file, Compose compares the current state with the specified state in the file. It then merges changes and updates containers accordingly without requiring you to build everything again from the ground up. 

What are the essential Docker Compose commands?

You can use the Compose CLI to control the applications you create through your “compose.yaml” file. You’ll use the docker compose command along with its subcommands to specify tasks and manage your application life cycle. You might also use flags with your commands, designated by a hyphen for single letters and a double hyphen for multiple letters, to slightly adjust the behavior of a command.

Some useful Compose commands to know are listed in the table below.

What is Docker Compose used for?

Docker Compose supports the end-to-end management of multi-container applications across various stages of software development, from production to testing to deployment. 

Most modern applications involve multi-container dependencies, where you have to configure environment variables, storage, port bindings, etc., individually. For example, your app might run in one container but call a database stored in another container. Compose simplifies this process by allowing you to manage multiple services, like web servers, databases, cache layers, and back-end APIs, as a single stack through a YAML file. This reduces the need for manual configuration, supports the replication and reuse of stacks, and improves scalability without increasing complexity.

Compose also supports environment portability, where you can use the variables specified in your YAML file to customize any environment without modifying the file itself. Additionally, by caching container configurations, Compose allows you to restart services by reusing saved containers, enabling you to quickly change environments for application development. 

Since Compose streamlines multi-container management through a single Compose file and simple commands, it has applications in various aspects of application development [5].

Development environments 

Compose’s CLI allows you to quickly create an isolated environment for testing your application using the configurations and dependencies specified in the YAML file. This means that you can create and start containers for all your dependencies, like web servers, APIs, and databases, using a single command. You can also replicate your production environment for local development, ensuring consistency across teams and faster onboarding.

Automated testing 

Compose can provide an efficient approach for creating temporary testing environments for continuous integration and continuous delivery (CI/CD) workflows and automated test suites. By supporting a single, portable configuration file for setting up environments, Compose simplifies the process of creating and destroying testing environments across platforms.

Read more: What Is Continuous Integration?

Single-host deployments

Compose supports application deployment and management on a single Docker host with production environment-specific configurations. However, Compose is best suited for production environments of simple, small- to medium-sized applications. For large, distributed applications, using Compose with orchestration tools like Kubernetes or Docker Swarm might offer more reliable production management.

Is Docker Compose like Kubernetes?

While both Compose and Kubernetes help manage multi-container applications, the difference between them lies in scale. Compose, which uses Docker as the runtime engine, is best suited to local development and small-scale projects. Kubernetes supports various runtime engines and multi-host deployments, making it ideal for enterprise-grade production environments and complex container orchestration across thousands of nodes. Additionally, Kubernetes supports container auto-scaling in response to changes in demand, a feature unavailable in Compose [6].

How to install Docker Compose

The most straightforward way to install Docker Compose is to download Docker Desktop, which includes Docker Engine and Docker CLI along with Compose. You can install Docker Desktop directly from Docker’s website for Windows, Mac, and Linux setups. If you already have Docker Desktop, you can verify what version of Compose you have by selecting “About Docker Desktop” from the Docker menu or typing “docker compose version” on the CLI.

If you’re using a Linux system and you have Docker Engine and Docker CLI installed, you can download Compose as a plugin through the command line either manually or using Docker’s repository.

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