What does the first line in the "docker-compose.yml" file, that specifies the version mean? – Docker

The Problem:

In the first line of the docker-compose.yml file we specify a version, for example:

version: '3.9'

What exactly does this version specify?

When we run docker-compose --version in our terminal, we might see a different version number, so how do these two versions relate?

The Solutions:

Solution 1: Compose Specification Version

The first line in the `docker-compose.yml` file specifies the version of the Compose file format being used. The version number indicates which features and syntax are supported in the file.

The Python version of Compose requires explicit version specification, while the Go plugin version ignores it and uses the Compose Specification format. The Compose Specification format is mostly backward-compatible with version 2 and 3 formats.

Solution:

The top-level version property specifies the version of the Compose file schema, not the version of Docker Compose itself. It is used by Docker Compose to determine how to interpret the rest of the file. The version number does not correspond to the version of Docker Compose that you are using.

Solution 3: Backward Compatible Version Specification

The version specified in the first line of the "docker-compose.yml" file is for backward compatibility purposes. As per the Docker Compose specification (https://docs.docker.com/compose/compose-file/04-version-and-name/), this version property is only informative.

Essentially, this version number does not reflect the actual version of Docker Compose you are using. It serves as a mechanism to maintain compatibility with older versions of the software. By specifying a specific version in the "docker-compose.yml" file, you can ensure that your project will work with that version or any newer versions that support backward compatibility.

In your case, when you checked the version of Docker Compose using "docker-compose –version," the output indicated that you are using version 2.17.2. This does not conflict with the version specified in the "docker-compose.yml" file because Docker Compose allows for backward compatibility. Your project will still function as expected, even though the specified version in the "docker-compose.yml" file is higher than the actual version you are using.

Solution

The first line in the docker-compose.yml file specifies the version of the docker compose yalm file format. Each version corresponds to a specific Docker Engine version and may introduce new parameters or variables.

For example, Docker Compose version 3 is compatible with Docker Engine version 1.12 and later. It introduces several new features, such as:

• Service dependency management: The ability to define dependencies between services and ensure that they are started in the correct order.
• Health checks: The ability to define health checks for services and automatically restart them if they fail.
• Logging: The ability to configure logging for services and aggregate logs from all services into a single file.

Video Explanation: