[Zope-dev] Re: Event & Timer Service for Zope 2.8

[Dylan Jay]

Chris McDonough wrote:
> On Sat, 2005-01-29 at 05:49, Andrew Veitch wrote:
> 
>>I noticed there was a thread last month on an event service for Zope.  
>>Did that reach any conclusion?
>>
>> From what I can see the present products are:
>>
>>- http://www.zope.org/Members/lstaffor/Xron - Zope 2.6 only, not being  
>>developed
>>- http://zope.org/Members/naumen/NauScheduler - shipped with one of the  
>>Zope CMS's works fine with 2.7
>>- http://dev.legco.biz/products/ZopeScheduler and  
>>http://dev.legco.biz/products/timerservice - works fine with 2.7 too
>>- http://cvs.zope.org/Products/Event/ - event service without a time  
>>service, not sure of development status
>>- http://mail.zope.org/pipermail/zope-cmf/2005-January/021747.html -  
>>Dieter's project to be based on Python's sched module
>>-  
>>http://api.nuxeo.org/CPS3/public/
>>CPSCore.EventServiceTool.EventServiceTool-class.html - CPS's event  
>>service
>>-  
>>http://cvs.sourceforge.net/viewcvs.py/*checkout*/mailmanager/
>>mailmanager/MailCheckThread.py?rev=1.15 - our custom scheduler for  
>>MailManager
>>- Zope X3's event service
> 
> 
> It has proven (at least for me) to be useful to make the distinction
> between "event service", "scheduler", and "clock" in the past.  I tend
> to think of each this way:
> 
> Event Service:
> 
> Framework software that allows objects to communicate between each other
> indirectly by sending and receiving "events".  An event service's
> responsibility is only to receive and send messages.  Note that it is
> not within the scope of an event service's responsibility to run tasks
> at timed or scheduled intervals.
> 
> Scheduler:
> 
> Software that ensures tasks may be run at timed or scheduled intervals.
> 
> Clock:
> 
> Software that intermittently "pokes" Zope, telling it to wake up and "do
> something" every so often.  A scheduler needs a clock.  A clock can be
> implemented either as an external source (e.g. wget every 10 seconds) or
> internally (by coopting the asyncore loop).
> 
> Schedulers usually require clocks, but neither clocks nor schedulers
> require event services.  Similarly, event services require neither a
> clock nor a scheduler.
> 
> 
>>It would be great to some consolidation of all of this into the core. I  
>>would also strongly favour a timer service along with the event  
>>service. I know that the argument has always been that operating  
>>systems provide time services but this is not very convenient for cross  
>>platform development and on some Windows platforms it's difficult.
> 
> 
> If we use the definitions above, I think what you want is a "scheduler"
> and a "clock" to be available for Zope, but not necessarily an event
> service.  Don't want to be pedantic about it, but when you use the term
> "event service" it tends to mean different things to different people.
> 
> To that end, I'd suggest using the Scheduler product at
> http://cvs.zope.org/Products/Scheduler.  Note that this product *does*
> currently depend on the "Event" product at
> http://cvs.zope.org/Products/Event but it's only by accident (the Event
> product is usable separately).  There is an external clock
> implementation that ships with the Scheduler product.
> 
> I agree with you that there should be a Zope "internal" clock.  I
> created one not too long ago, but it's a bit invasive.  Read
> http://www.plope.com/Members/chrism/more_scheduling and
> http://www.plope.com/Members/chrism/scheduling_service for an overview
> of the strategy I was thinking of when I did it.
> 
> Personally, I think maybe the internal clock service can also be
> implemented as a Product and not added to Zope proper.  It would be
> great if someone were to maybe volunteer to take the work that I did
> towards it and productify it.

+1 for using http://cvs.zope.org/Products/Scheduler
I think the time of a non-core scheduler needs to come to an end. Its 
crazy the number of implementations out there.\
I'm using the zope cvs version and I've enhanced it to make it 
serialised and non reentrant. I'd like to check that in.

In order to simplify installation I do think a Product version of a 
clock would be required. Perhaps the clock can optional be promoted from 
outside to ensure its thread is still alive?

I'm willing to have a crack at it as long as that effort isn't going to 
yet another non-core scheduler/clock.


Dylan.