What is the difference between “expose” and “publish” in Docker?

In Docker, the terms "expose" and "publish" refer to different aspects of port management for containerized applications. Here is a detailed explanation of the differences between the two:

Expose

1. Definition: The EXPOSE instruction in a Dockerfile or the --expose flag in the docker run command is used to indicate that a container listens on a specified port or ports. This is primarily a documentation mechanism that informs users and other containers about which ports the application inside the container is set to use.

2. Functionality: Exposing a port does not make it accessible from outside the Docker host. It only makes the port available for inter-container communication within the same Docker network. This is useful for services that need to communicate with each other within a Docker environment.

3. Usage:

   • In a Dockerfile:

     EXPOSE 8080

   • Using the docker run command:

     docker run --expose 8080 my_container

4. Purpose: The primary purpose of EXPOSE is to document which ports the containerized application will use, making it easier to manage and understand the container's configuration.

Publish

1. Definition: The --publish (or -p) flag in the docker run command is used to map a container's port to a port on the Docker host. This makes the container's service accessible from outside the Docker host, allowing external clients to connect to the containerized application.

2. Functionality: Publishing a port creates a firewall rule on the Docker host that maps a container port to a host port. This allows external traffic to reach the container through the specified host port.

3. Usage:

   • Using the docker run command:

     docker run -p 8080:80 my_container

     This command maps port 80 in the container to port 8080 on the Docker host.

4. Purpose: The primary purpose of --publish is to enable external access to the containerized application by binding a container port to a host port.

Key Differences

• Scope: EXPOSE is used for inter-container communication within the same Docker network, while --publish is used to make a container's port accessible from outside the Docker host.
• **Implementat...