[collectd] Dynamic interval values for Python Plugins

[Horacio Sanson]

I am writing a python plugin that checks the number of available upgrades
and security upgrades on Debian servers (based on apt-check tool).

The thing is that I want this check to be executed once every day unless
there are upgrades available in which case I want to reduce the interval to
a lower value. Once the upgrades are installed I want the interval to
change back to once a day.

This is what I tried in my plugin:

def read_callback():
    global callback_id

    res = apt_check()
    upgrades = collectd.Values(plugin=NAME, type='apt')
    upgrades.values = [ res[0], res[1] ]
    upgrades.dispatch()

    #collectd.info("Apt-Check %s %s " % (res[0], res[1]))

    if(res[0] > 0 or res[1] > 0):
      collectd.unregister_read(callback_id)
      callback_id = collectd.register_read(read_callback, SHORT_INTERVAL)
    else:
      collectd.unregister_read(callback_id)
      callback_id = collectd.register_read(read_callback, INTERVAL)

In my read callback, after I get the number of upgrades, if there are
upgrades (res[0] or res[1] > 0) I unregister the read callback and register
it again with a shorter iterval. If there are no upgrades I register it
with a large interval (e.g. 24 hours).


My questions are:

- Is is necessary to unregister the callback each time? or is the
register_read method indemponent and can be called multiple times without
causing memory leaks?

- It seems that the interval variable in the register_read method is
ignored. If I un-comment the info call I can see the read callback is
invoked at increasing intervals starting from 3 secs. No matter what are
the values of INTERVAL and SHORT_INTERVAL.

- Is there a better way in collectd to support dynamic intervals that
depend on the values being monitored?


regards,
Horacio
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