Poettering: The Biggest Myths
Posted Feb 5, 2013 22:12 UTC (Tue) by Cyberax (✭ supporter ✭, #52523)
On the other hands, I did have problems when ejabberd started in parallel with the PostgreSQL database - there was no dependency in LSB headers because I was using an optional psql auth module.
This worked just fine until the day it suddenly stopped working because the ordering of services has changed and Postgres moved a little bit later into the boot process.
Posted Feb 5, 2013 22:16 UTC (Tue) by dlang (subscriber, #313)
One of the things you learn after several years of running large numbers of production systems is to not trust claims that a process will always work, whatever that process is.
Failures are generally not that common, but they do happen.
Posted Feb 5, 2013 22:24 UTC (Tue) by Cyberax (✭ supporter ✭, #52523)
Also, it might decrease the response time time for the first client.
SystemD nicely allows to do both.
Posted Feb 6, 2013 8:14 UTC (Wed) by paulj (subscriber, #341)
Posted Feb 6, 2013 9:18 UTC (Wed) by anselm (subscriber, #2796)
So? Systemd can deal with that, too – at least as well as System V init does. For example, systemd lets you express explicit forward and backward dependencies between services and will automatically construct a starting order based on that. With System V init, you either get to work out any dependencies yourself to set the magic numbers correctly by hand, or you use something like SUSE's insserv based on LSB metadata in the init scripts, where reverse dependencies are not an official feature.
Posted Feb 6, 2013 12:21 UTC (Wed) by HelloWorld (guest, #56129)
Posted Feb 6, 2013 16:52 UTC (Wed) by Cyberax (✭ supporter ✭, #52523)
Posted Feb 7, 2013 4:34 UTC (Thu) by paulj (subscriber, #341)
Hardly smoke signals.
Posted Feb 7, 2013 5:38 UTC (Thu) by khim (subscriber, #9252)
Hardly smoke signals.
It is a smoke signal - from GPS stateliness. It can easily be converted to something systemd understands if you'll create a specialized daemon (in a "true UNIX fashion" people like to preach here so much) which will convert it to signals systemd understands.
And you need such daemon anyway because you need to decide how often you poll, if you use just GPS or if you want to use WiFi, too, etc. It's not as if GPS satellites can pull some trigger on your PC or smartphone which means you'll need some complex logic anyway.
Posted Feb 7, 2013 5:50 UTC (Thu) by dlang (subscriber, #313)
Just because systemd doesn't handle something doesn't mean that the something is worthless or stupid.
Posted Feb 7, 2013 8:00 UTC (Thu) by rahulsundaram (subscriber, #21946)
Posted Feb 7, 2013 8:39 UTC (Thu) by paulj (subscriber, #341)
Posted Feb 7, 2013 12:33 UTC (Thu) by khim (subscriber, #9252)
There'll be no bug report on systemd because the applications involved are likely already using some other system. E.g. DBus IPC.
If it used DBus IPC then it can be easily be used with systemd thus error report will be entirely unnecessary. I've said "convert it to signals systemd understands" and not "convert it to socket activation" exactly for this reason: because systemd handles many different activation requests and makes sure they don't conflict. Socket activation is just one of them (even if one of the most important ones).
Posted Feb 7, 2013 17:02 UTC (Thu) by paulj (subscriber, #341)
Bit of a dance, to have the IPC nexus hand-off this activation. It feels like really these should be part of one thing...
Posted Feb 7, 2013 8:25 UTC (Thu) by anselm (subscriber, #2796)
The GPS daemon provides the current location on a socket. It is probably unreasonable to expect systemd to be able to deal with that sort of information directly (systemd detractors would immediately jump on features such as these and call them out as »bloat«, with some justification).
Hence, a reasonable way of handling this would be to write a (not very complicated) subsidiary daemon that listened to the GPS daemon's output and triggered various systemd actions based on them. This might be a good idea in any case in order to provide »smoothing« of the location data or additional rules (»tell me about bars in the vicinity but only during happy hour«). This daemon itself would of course be managed by systemd.
This approach should also please the Unix traditionalists who insist that programs should »do one job and do it well«.
Posted Feb 7, 2013 8:35 UTC (Thu) by paulj (subscriber, #341)
(FWIW, I have no opinion generally on systemd - I don't know enough about it. I'm just a tad sceptical of the wonder claims being made for socket activation based dependency resolution).
Posted Feb 7, 2013 8:59 UTC (Thu) by cortana (subscriber, #24596)
Posted Feb 7, 2013 12:41 UTC (Thu) by khim (subscriber, #9252)
So, go on, how do you expose that dependency in a way systemd can handle it. Tell me.
Well, as you've suggested: DBus activation looks like a natural fit for such a use case - and since systemd handles it just fine... I don't see what's your problem.
I'm just a tad sceptical of the wonder claims being made for socket activation based dependency resolution.
Socket activation covers 90% of usecases, but there are other ways to activate service. And the important thing of systemd is that they all can be used simultaneously. You can start some daemon at specific time (using time-based activation) and when your a leaving specific area (D-Bus based activation) and when some other service needs this particular daemon (socket-based activation). They don't conflict and handled correctly in all cases.
Socket activation is there to track dependencies between services on your own system: it's the simplest one to use and most robust one. But there are other to handle "smoke signals", too.
Posted Feb 7, 2013 12:19 UTC (Thu) by Cyberax (✭ supporter ✭, #52523)
Additionally, designer of such a crazy interface should be shot immediately to stop propagating bogosity.
Posted Feb 7, 2013 15:59 UTC (Thu) by dlang (subscriber, #313)
Then why is it that when people talk about doing exactly this, they get jumped by systemd people saying things like "why didn't you submit a bug report to get that capibility added to systemd" or "that's a stupid way to do things, you need to re-write your software to use systemd to do it"
This subthread started by the simple statement that socket-based activation was not always appropriate, with the acknowledgement that systemd could support this.
Posted Feb 7, 2013 16:03 UTC (Thu) by Cyberax (✭ supporter ✭, #52523)
If you have a compelling use-case of some exotic activation system for which it makes sense to add core support then doing a bugreport might be a good idea.
And in other cases it might be a good idea to simply rewrite the offending code.
Posted Feb 5, 2013 22:43 UTC (Tue) by raven667 (subscriber, #5198)
If we had the time we could skim through http://cgit.freedesktop.org/systemd/systemd/tree/src/core/ and see if we can identify where it keeps that state and how it is updated to see if there are any bugs.
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