Why does Docker have a daemon?

Question

I recently discovered rkt, a competitor container runtime to Docker. It seems like rkt does not need a daemon. For me, rkt is like running any other command and it works easily with systemd (or other init systems).

This makes me wonder about the utility of Docker's daemon.

Why does Docker need a daemon ? What does the daemon provide that would not be possible without it ? Is its only goal to remove the need for an init system like systemd (as can be seen in the Rancher OS) ?

Answer 1

Docker was designed as a client/server application which allows you to have remote access to the docker API. This allows tools like the classic container based swarm that were effectively a reverse proxy to a cluster of docker hosts.

The daemon also provides a place for shared state. It's restarting containers according to their restart policy. But it's also managing networks and volumes that may be shared between multiple containers.

Lastly, with the introduction of swarm mode, the daemon is also the central location for these tools that would otherwise be run as their own daemons with tools like kubernetes.

If you need a daemon-less solution but otherwise like docker, then consider using runc which is the runtime environment that docker uses for each container by default.

This doesn't involve the init inside the container. If you need that, docker now includes an optional init that you can enable per container. And you've always had the option to include your own init, like tini, if you needed something to cleanup zombie processes.