Running multiple applications in one docker container

Question

This question is part of my continuing exploration of Docker and in some ways follows up on one of my earlier questions. I have now understood how one can get a full applic

Answer 1

@Bryan's answer is solid, particularly in relation to the overheads of a container that just runs one process being low.

That said, you should at least read the arguments at https://phusion.github.io/baseimage-docker/, which makes a case for having containers with multiple processes. Without them, docker is light on provision for:

• process supervision
• cron jobs
• syslog

baseimage-docker runs an init process which fires up a few processes besides the main one in the container.

For some purposes this is a good idea, but also be aware that for instance having a cron daemon and a syslog daemon per container adds up a bit more overhead. I expect that as the docker ecosystem matures we'll see better solutions that don't require this.

Answer 2

A container is basically a process. There is no technical issue with running 500 processes on a decent-sized Linux system, although they will have to share the CPU(s) and memory.

The cost of a container over a process is some extra kernel resources to manage namespaces, file systems and control groups, and some management structures inside the Docker daemon, particularly to handle stdout and stderr.

The namespaces are introduced to provide isolation, so that one container does not interfere with any others. If your groups of 5 containers form a unit that does not need this isolation then you can share the network namespace using --net=container. There is no feature at present to share cgroups, AFAIK.

What is wrong with what you suggest:

• it is not "the Docker way". Which may not be important to you.
• you have to maintain the scripting to get it to work, worry about process restart, etc., as opposed to using an orchestrator designed for the task
• you will have to manage conflicts in the filesystem, e.g. two processes need different versions of a library, or they both write to the same output file
• stdout and stderr will be intermingled for the five processes