Re: process supervisor - considerations for docker

From: Dreamcat4 <>
Date: Wed, 25 Feb 2015 11:32:37 +0000

Thank you for moving my message Laurent.

Sorry for the mixup r.e. the mailing lists. I have subscribed to the
correct list now (for s6 specific).

On Wed, Feb 25, 2015 at 11:30 AM, Laurent Bercot
<> wrote:
> (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).
>> * can take directly command and arguments. With argv[] like this:
>> "process_supervisor" "my_program_or_script" "my program or script
>> arguments…"
>> * will pass on all ENV vars to "my_program_or_script" faithfully
>> * will run as PID 1 inside the linux namespace
>> * where my_program_or_script may spawn BOTH child AND non-child
>> (orphaned) 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
>> * 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
>> 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 - 11:32:37 UTC

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