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import cinit from unix.schottelius.org

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Signed-off-by: Nico Schottelius <nico@ikn.schottelius.org>
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Nico Schottelius 2009-09-16 12:53:45 +02:00
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cinit - Respawning and sleeping
===============================
Nico Schottelius <nico-cinit__@__schottelius.org>
0.1, for cinit 0.3, Initial Version from 2007-04-13
:Author Initials: NS
How cinit sleeps, before a service is respawned.
Introduction
------------
When a service, that is configured to be respawning, exits cinit restarts it.
Because services may be broken and thus cinit would try to restart it many
times in a second, it is necessary to sleep between restarts.
The big question: How to sleep?
------------------------------------
The first possibility is to sleep a fixed amount of time between restarts:
For instance one second. This is not a good solution, because this waits
one unecessary second if the process just crashed. It may be too less, if
the service is really broken.
The second possibility is to sleep a dynamic amount of time.
The next big question: How long to sleep?
-----------------------------------------
I think that as long as a service is broken we should not investigate to much
time into restarting it. So we can define a maximum amount of time to sleep
(for instance 30 seconds). The minimum amount of time to sleep is zero seconds.
When the service is first started, the sleep time defaults to the minimum
amount of time. Each time the service is restarted, the sleep time is adjusted:
sleep time (st) = Maximum sleep time (mst) / Time the process was running (tr)
- If tr is < 1 it is adjusted to 1 and thus st is set to mst.
- If tr is > mst, st is set to 0.
How to force restart of a service?
----------------------------------
Use cvsc (to be implemented).
Hom is it implemented?
----------------------
Everytime cinit has to respawn a service it has to fork() itself.
The sleep call is implemented in the fork, so cinit itself does not have
to care about it. After the sleep time is over or the fork() recieves
SIGALARM it executes the real service. cinit itself records as start time
the current time plus sleep time.