Re: s6 as a systemd alternative

From: Jan Bramkamp <>
Date: Fri, 30 Jun 2017 23:13:24 +0200

The s6-svscan and s6-supervise are very simple (and elegant) and in a
way do less than runsvdir and runsv: they don't go around allocating
resources at runtime.

On 30.06.2017 22:38, Steve Litt wrote:
> On Fri, 30 Jun 2017 19:50:17 +0000
> "Laurent Bercot" <> wrote:
>>> The runsv executable is pretty robust, so it's unlikely to die.
>> Yadda yadda yadda. Most daemons are also unlikely to die, so
>> following your reasoning, I wonder why we're doing supervision in the
>> first place. Hint: we're doing supervision because we are not content
>> with "unlikely". We want "impossible".
> You want impossible. I'm quite happy with unlikely. With my use
> case, rebooting my computer doesn't ruin my whole day. If it *did* ruin
> my whole day, my priorities would be changed and I'd switch to s6.
>>> As far
>>> as somebody killing it accidentally or on purpose with the kill
>>> command, that's a marginal case. But if it were *really* important to
>>> protect against, fine, have one link dir per early longrun, and run
>>> an individual runsvdir on each of those link directories.
>> And you just increased the length of the chain while adding no
>> guarantee at all, because now someone can just kill that runsvdir
>> first and then go down the chain, like an assassin starting with the
>> bodyguards of the bodyguards of the important people. Or the assassin
>> might just use a bomb and blow up the whole house in one go: kill -9
>> -1.
>> The main point of supervision is to provide an absolute guarantee
>> that some process tree will always be up, no matter what gets killed
>> in what order, and even if everything is killed at the same time.
> To me, the preceding isn't the main point of supervision. Supervision
> benefits I value more are:
> * Run my daemon in foreground, so homegrown daemons have no need to
> self-background.
> * Consistent and easy handling of log files.
> * Under almost all circumstances, dead daemons get restarted.
> * Simple config and troubleshooting, lots of test points.
> * POSIX methodologies ensure I can easily do special stuff with it.
> * Ability to base process dependency on whether the dependee is
> *really* doing its job.
>> You
>> can only achieve that guarantee by rooting your supervision tree in
>> process 1.
> Yes.
>> With runit, only the main runsvdir is supervised - and even then it
>> isn't really, because when it dies runit switches to stage 3 and
>> reboots the machine. Which is probably acceptable behaviour, but
>> still not supervision.
> If we're going to get into definitions, then let me start by saying
> what I want is daemontools that comes up automatically when the machine
> is booted. Whether or not that's supervision isn't something I care
> about.
>> And everything running outside of that main
>> runsvdir is just hanging up in the air - they can be easily killed
>> and will not return.
> Wellllll, if they kill the runsv that's true, but if they kill the
> daemon, no. Either way, I'm willing to live with it.
>> By adding supervisors to supervisors, you are making probabilistic
>> statements, and hoping that nobody will kill all the processes in the
>> wrong order. But hope is not a strategy. There is, however, a strategy
>> that works 100% of the time, and that is also more lightweight because
>> it doesn't require long supervisor chains: rooting the supervision
>> tree in process 1. That is what an s6-based init does, and it
>> provides real, strong supervision; and unlike with runit, the machine
>> is only rebooted when the admin explicitly decides so.
> I completely understand your point. I just don't need that level of
> indestructibility.
>> If you're not convinced: *even systemd* does better than your
>> solution. systemd obviously has numerous other problems, but it does
>> the "root the supervision tree in process 1" thing right.
> LOL, my whole point is I don't necessarily think "root the supervision
> tree in process 1" is right, at least for my use case. I *enjoy* having
> a tiny, do-almost-nothing PID1.
> Like I said before, if losing control of the system during special
> circumstances would ruin my whole day, I'd change my priorities and use
> s6.
>> I appreciate your enthusiasm for supervision suites. I would
>> appreciate it more if you didn't stop halfway from understanding
>> everything they bring, and if you didn't paint your unwillingness to
>> learn more as a technical argument, which it definitely is not, while
>> flippantly dismissing contributions from people who know what they
>> are talking about.
> But I didn't flippantly dismiss anybody or any contributions. I
> pointed out that one can, and I'll use different verbiage now, respawn
> daemons early in the boot, before some of the one-shots had started.
> I'm not an enemy of s6. I'm not an enemy of anything you apply the word
> "supervision" to. I think I understand your reasons for doing what you
> do. It's just that with my current use case, I've traded some of s6's
> process and boot security (you know what I mean) for a simpler PID1 and
> a standalone daemon respawner.
> If and when I get a use case requiring more durability of processes and
> what runs them, I'll for sure use s6 for that.
> SteveT
> Steve Litt
> June 2017 featured book: The Key to Everyday Excellence
Received on Fri Jun 30 2017 - 21:13:24 UTC

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