Proposal: Integrate s6-rc into the overlay to handle service dependencies elegantly

[glerchundi]

Coming from #112

[dreamcat4]

Is this program s6-rc, also needed to start and stop services?

I can see in skanet documents there is a down file can put next to run in the services.d folder. Which makes avoid starting the service automatically. However I would also like to stop a service which was already running.

Or else should we make some shell script to do this without s6-rc program?

For example:

#!/bin/sh
# stop service

touch /etc/services.d/$1/down

# do something to kill supervised service
# it will try to restart the service but see "down" file, and know not to

and

#!/bin/sh
# start service

rm /etc/services.d/$1/down
# do something to start the supervised service

[skarnet]

Starting a machine with all services down and having a script that brings them up one by one is a viable solution. However, if you're going to do something like this, it means you have dependencies between services, so you'll want readiness notification, etc. etc. and ultimately it would be simpler to use s6-rc to get your machine running.

It's possible to use s6-rc with Docker, but I don't think s6-overlay is the project for this. Dependency management is a complex thing and it's difficult to provide a framework for it without saying "provide the compiled service database yourself", which would be a high entry barrier.

[Adirelle]

I have started a project around s6-rc for containers : https://github.com/Adirelle/s6rc-overlay

It started as a base image for building development images. I extracted the scripts and switched to just-containers/skaware for the binaries (thanks for the work). Right now, it is still in an early stage and lacks documentation but it should work.

But as @skarnet said, dependency management isn't easy.

[jjlorenzo]

Hi any progress on this? thanks

[skarnet]

I have a s6-frontend project in the making, which aims to integrate s6, s6-rc, and (when applicable, so not in containers) s6-linux-init together with a friendly interface and bundled policy for easy integration in distributions. It will be as lightweight as the rest of the stack, so usable in containers; however, I don't expect to release it before 2020 or 2021, so it doesn't help you for now.

[jjlorenzo]

Thanks so much @skarnet, I have a very single integration almost completed of s6 and s6-rc for docker containers. I know that some people don't like the idea of fat containers, but they work for me and I'm really grateful for you job that allows me to implement what I want with a minimal footprint.

Comparing to s6-overlay, it doesn't contain any abstraction specific to containers. I've used heavily s6-linux-init to understand how to put the pieces together.

I plan to release my work(progress) asap in case someone want to take a look.

[xZero707]

Hey, @skarnet. Any update on s6-frontend?

[skarnet]

Not yet - however, the new version of s6-overlay, v3, will be out very soon, and it includes s6-rc integration, so while waiting for s6-frontend, you'll be able to add dependencies to your services!

[xZero707]

Amazing! Thank you very much for getting back to us. Appreciate it!

[skarnet]

v3.0.0.0 is out! (Source only at the moment; pre-built tarballs to come as soon as CI/CD is set up.)
You can now write your own services as s6-rc source definition directories in /etc/s6-overlay/s6-rc.d; to start them automatically, make them a part of the user bundle by creating empty files with the name of your services in /etc/s6-overlay/s6-rc.d/user/contents.d.
Enjoy!