[Zope] question about use zc.async in Zope2

[eastxing]

Hi all,
In Plone2.5.5(with Zope2.9.6-final) I use zasync as my schedule task
framework, it works well. Then I update my site to Plone3.1.7, and go
further update to use Zope2.11.2 and ZODB3.9.0. Now I want to replace zasync
with zc.async.

As I know in Zope3 there is a event 'zope.app.appsetup.IDatabaseOpenedEvent'
that you can subscribe to start your zc.async dispatcher. But in Zope2.11.2,
the event never be fired, so I have to find other point to start the
dispatcher.

My method is start it when you first use it, see the snip following:

===============
class AsyncTool( UniqueObject, SimpleItem, ActionProviderBase ):
    ...
    def addJob( self, *args ):
        ''' '''
        self._startZCAsync()

        portal = getToolByName(self,'portal_url').getPortalObject()

        # get queue from portal root
        queue = zc.async.interfaces.IQueue( portal )

        # insert a job
        job = queue.put( Job(...) )

    def _startZCAsync( self ):
        # create volatile attributes
        if not hasattr( self, '_v_start_zcasync' ):
            self._v_start_zcasync = 0

        if not self._v_start_zcasync:
            db = Zope2.DB

            # zc.async does not startup, start it now
            zc.async.configure.base()
            zc.async.configure.start( db, poll_interval=1, twisted=False )

            self._v_start_zcasync = 1
===============

It works, but fragile. Any of my jobs execute failure will cause the
'volatile' attribute '_v_start_zcasync' lost, and dispatcher start process
will run again, this cause error.

So I want to know the right time to start zc.async dispatcher in Zope2?

best regards
eastxing
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