[z3-five] local utilities

[Martijn Faassen]

Hi there,

This is a note to discuss what I'm up to, as I think I need help. :) I'm 
trying to get local utilities (aka tools) working.

Why do I want to get these working? As I've announced in a previous 
note, I have code checked in that sends Zope 3 events. It's possible to 
catch these events by factories; basically module-level functions.

For this event-handling to become really useful though -- for instance 
to build an archetypes-style unique id service, or even the Zope 3 
catalog, we need ZODB-level objects that can track these events and 
store information.

The Zope 3 pattern for this, I understand, is to get a local utility and 
sending the events to this.

How does Zope 3 know that a local utility is even there? By ask the 
site, which is a global associated with the thread.

How does Zope 3 know the current site? During traversal an event is sent 
if a site object is encountered. This event his handled (globally) and 
the thing is set.

What constitutes a site in Zope? It's an IPossibleSite, a simple enough 
interface to support, that becomes an ISite, if a ISiteManager is set in 
it. The site logic can be found in zope.app.site.

Unfortunately ISiteManager is *not* a simple interface; it inherits from 
IServiceService, IComponentManager and registration.IRegistry. The 
SiteManager implementation in zope.app.site.service is rather large. To 
make matters more complicated, it's also implementing 
IRegisterableContainerContainer in addition to ISiteManager. Backporting 
this class to Zope 2 seems rather daunting.

Things in zope.app.site.servicecontainer give me some hope though -- it 
turns out that the site manager only needs to provide IServiceService to 
be accepted. This is a somewhat smaller, though of course a highly 
abstract interface, which may make it harder to figure out what is required.

So, todo:

* backport the stuff in zope.app.site.servicecontainer to Zope 2. That 
should be pretty simple.

* make our own implementation of a local IServiceService that does the 
'right thing', whatever that is. There is all kinds of fallback behavior 
on sites 'higher up' and finally the global site which we'll need to 
make sure we support.

* make Zope 2's traverser machinery send out the right event as soon as 
an ISite is encountered. I hope we can get away with doing a traversal 
hook in the site class (i.e. our 
zope.app.site.servicecontainer.ServiceManagerContainer, which implements 
IPossibleSite) that sends this event.

And some hopes:

* we don't care right now about any user interface to the site manager. 
I hope that means we don't need to care about all kinds of stuff.

* I hope we can get away with ignoring the whole registration logic. I 
think that is only used by services, while we're right now only 
interested in local utilities.

I hope that others can jump in with some answers, suggestions, testing 
strategies, and maybe even code. :)

Regards,

Martijn