Could you please explain to me what daemon is? For instance, Docker Daemon. In Plain English. How can I think about it using real world things/objects to understand its functionality? What's the best analogy, mental model that you're using to understand it? Thank you.
21 Answer
The strict definition of a daemon in computing is any process that is not attached to an input/output terminal. So any process which runs in the background. Daemons often have a trick with fork() (called daemonize) which disassociates the daemon from the existing session, allowing it to keep running after the user that started it logs off. Compare it with a Windows service (kinda).
In a wider sense, it's mostly used to denote "server" processes, which communicate only over network connections, and not with some sort of UI in a terminal or GUI.
The Docker daemon is the part of Docker that runs continuously in the background and manages all interactions with the kernel. The docker command line client only interacts with the Docker daemon, but it doesn't do anything by itself. It usually communicates with the Docker daemon through a socket, at /var/run/docker.sock, and therefore is a good example of a daemon that doesn't work only through network communications.
