Re: process supervisor - considerations for docker

From: Gorka Lertxundi <>
Date: Wed, 25 Feb 2015 15:58:07 +0100


After that great john's post, I tried to solve exactly your same problems. I
created my own base image based primarily on John's and Phusion's base

See my thoughts below.

2015-02-25 12:30 GMT+01:00 Laurent Bercot <>:

> (Moving the discussion to the list.
> The original message is quoted below.)
> Hi Dreamcat4,
> Thanks for your detailed message. I'm very happy that s6 found an
> application in docker, and that there's such an interest for it!
> is indeed the right place to reach me and
> discuss the software I write, but for s6 in particular and process
> supervisors in general, is the better
> place - it's full of people with process supervision experience.
> Your message gives a lot of food for thought, and I don't have time
> right now to give it all the attention it deserves. Tonight or
> tomorrow, though, I will; and other people on the supervisionlist
> will certainly have good insights.
> Cheers!
> -- Laurent
> On 25/02/2015 11:55, Dreamcat4 wrote:
>> Hello,
>> Now there is someone (John Regan) who has made s6 images for docker.
>> And written a blog post about it. Which is a great effort - and the
>> reason I've come here. But it gives me a taste of wanting more.
>> Something a bit more foolproof, and simpler, to work specifically
>> inside of docker.
>> From that blog post I get a general impression that s6 has many
>> advantages. And it may be a good candidate for docker. But I would be
>> remiss not to ask the developers of s6 themselves not to try to take
>> some kind of a personal an interest in considering how s6 might best
>> work inside of docker specifically. I hope that this is the right
>> mailing list to reach s6 developers / discuss such matters. Is this
>> the correct mailing list for s6 dev discussions?
>> I've read and read around the subject of process supervision inside
>> docker. Various people explain how or why they use various different
>> process supervisors in docker (not just s6). None of them really quite
>> seem ideal. I would like to be wrong about that but nothing has fully
>> convinced me so far. Perhaps it is a fair criticism to say that I
>> still have a lot more to learn in regards to process supervisors. But
>> I have no interest in getting bogged down by that. To me, I already
>> know more-or-less enough about how docker manages (or rather
>> mis-manages!) it's container processes to have an opinion about what
>> is needed, from a docker-sided perspective. And know enough that
>> docker project itself won't fix these issues. For one thing because of
>> not owning what's running on the inside of containers. And also
>> because of their single-process viewpoint take on things. Andy way.
>> That kind of political nonsense doesn't matter for our discussion. I
>> just want to have a technical discussion about what is needed, and how
>> might be the best way to solve the problem!
>> In regards of s6 only, currently these are my currently perceived
>> shortcomings when using it in docker:
>> * it's not clear how to pass in programs arguments via CMD and
>> ENTRYPOINT in docker
> - in fact i have not seen ANY docker process supervisor solutions
>> show how to do this (except perhaps phusion base image)
>> * it is not clear if ENV vars are preserved. That is also something
>> essential for docker.
>> * s6 has many utilities s6-*
>> - not clear which ones are actually required for making a docker
>> process supervisor
> * s6 not available yet as .deb or .rpm package
>> - official packages are helpful because on different distros:
>> + standard locations where to put config files and so on may
>> differ.
>> + to install man pages too, in the right place
> * s6 is not available as official single pre-compiled binary file for
>> download via wget or curl
>> - which would be the most ideal way to install it into a docker
>> container
>> ^^ Some of these perceived shortcomings are more important /
>> significant than others! Some are not in the remit of s6 development
>> to be concerned about. Some are mild nit-picking, or the ignorance of
>> not-knowning, having not actually tried out s6 before.
>> But my general point is that it is not clear-enough to me (from my
>> perspective) whether s6 can actually satisfy all of the significant
>> docker-specific considerations. Which I have not properly stated yet.
>> So here they are listed below…
>> A good process supervisor for docker should ideally:
>> * be a single pre-compiled binary program file. That can be downloaded
>> by curl/wget (or can be installed from .deb or .rpm).
I always include s6, execline, s6-portable-utils in my base images, they
are pretty
small and manageable. You could find/build them using this builder

I try to keep this repo as updated as possible so that when Laurent
releases a
version I publish a new release in github:

>> * can take directly command and arguments. With argv[] like this:
>> "process_supervisor" "my_program_or_script" "my program or script
>> arguments…"
What do you want to achieve with that? Get into the container to debug? If
just using CMD with [ "/init" ] is enough and allows to anyone get into the
without running any init process / supervisor.

I cannot imagine why this would be useful, but if it was required,
registering a
new service in runtime into s6 could be a possible solution. This service
include your custom program or script.

>> * will pass on all ENV vars to "my_program_or_script" faithfully
I take care of ENV's in my init process.

Concretely, as phusion does, I dump all ENV's into

And then make use of them in this script which internally uses s6-envdir to
environment variables in the spawned process.

So all container init.d, service run and service finish scripts will always
environment variables accessible.

* will run as PID 1 inside the linux namespace
OK. s6 can run as pid 1, so no problem. The init script I mentioned
responsibility to s6 so it runs as a init/pid1/supervisor perfectly.

> * where my_program_or_script may spawn BOTH child AND non-child
>> (orphaned) processes
Laurent correct me if I'm wrong but, this is an intrinsic feature of s6. As
it can run
as pid 1 it inherits all orphaned child processes.

>> * when "process_supervisor" (e.g. s6 or whatever) receives a TERM signal
>> * it faithfully passes that signal to "my_program_or_script"
>> * it also passes that signal to any orphaned non-child processes too
yes, it can:

> * when my_program_or_script dies, or exits
>> * clean up ALL remaining non-children orphaned processes afterwards
>> * which share the same linux namespace
>> - this is VERY important for docker, as docker does not do this
this should be your concrete service concern. For example, imagine you have
a load
balancer with two supervised processes, nginx (main) & confd. Probably you
want to
have confd always running, no matter why this process could die, but you
want it up.
Instead, if your main process dies, you want your container to die too. So
that should
be implemented in you specific use case:

For example:

> So to ENSURE these things:
>> * to pass full command line arguments and ENV vars to
>> "my_program_or_script"
>> * ensure that when "my_program_or_script" exits, (crashes or normal exit)
>> * no processes are left running in that linux namespace
>> * ensure that when the service reveives a TERM, (docker stop)
>> * no processes are left running in that linux namespace
>> * ensure that when the service reveives a KILL, (docker stop)
>> * no processes are left running in that linux namespace
>> BUT in addition, any extra configurability should be entirely optional:
>> * to add supplemental run scripts (for support programs like cron and
>> rsyslog etc)
>> * which is what most general process supervisors consider as their
>> main mechanism for starting services
>> SO
>> * if "my_program_or_script" is supplied as an argument, THAT is the
>> main process running inside the docker container.
>> * if no "my_program_or_script" argument is supplied, then use
>> whatever other conventional ways to determine the main process from
>> the directories of run scripts.
>> Current solutions for docker seem to be "too complex" for various
>> reasons. Such as:
>> * no mandatory ssh server (phusion baseimage fails)
>> * no python dependancy (supervisor fails, and phusion baseimage fails)
>> * no mandatory cron or rsyslog (current s6 base image fails, and
>> phusion base image fails)
>> * no "hundreds of cli tools" - most of which may never be used
>> (current s6 base image fails)
>> * no awkward intermediate bootstrap script required to write ENV vars
>> and cmdline args to a file (runit fails)
>> So for whichever of those reasons, it feels the problem of docker
>> remains un-satisfied. And not fully addressed by any individual
>> solution. What's the best course of action? How can these problems be
>> solved into one single tool?
>> I am hoping that the person who wrote this page:
>> Might be able to comment on some these above ^^ docker-specific
>> considerations? If so please, it would really be appreciated.
>> Also cc'ing the author of the docker s6 base images. Who perhaps can
>> comment about some of the problems he has encountered. Many thanks for
>> any comments again (but please reply on the mailing list).
>> Kind Regards
>> dreamcat4
Received on Wed Feb 25 2015 - 14:58:07 UTC

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