Applause for ZODB! And a question: How do you get pending change info from a transaction object?

Hi Jim,

I read much of what you wrote in these Google discussions and I want you to 
know that I am mightily impressed with your philosophies and industry 
perspectives and I am sympathetic about your frustrations. Rest assured, as 
CPU technology goes beyond silicon (beyond 4ghz per thread) and as RAM 
continues to explode, the masses for MANY business purposes will adopt 
things like ZODB for the same reasons that they went beyond assembly to C 
and beyond C to Java and better: Python. The day is approaching when the 
time-to-market advantage will put players who choose things like ZODB way 
above their competition. Don't let forum activity fool you. You have more 
people eyeing your achievements than you think. As for me, I have been 
gifted with the great blessing that ZODB is a perfect fit for my new 
project.

Now, here is what I am trying to accomplish. Perhaps you know of entirely 
better ways to do what I am doing. Feel free to share! I am working toward 
the development of a websocket connection in a Tornado app that is keeping 
track of multiple client persistent connections. I have chosen ZODB to be 
my source of truth for all data of the application. As soon as the ZODB 
transaction commits, I want all clients to get sent a message to inform 
them of changes to all of the specific objects that they are listening to 
changes for.

Keeping track of user client sockets and telling each of them to maintain a 
list of object ids to pay attention to isn't going to be a hard thing from 
what I can tell. I just need a way to hear from ZODB what the object ids 
are that have changed when a commit happens. I came across Synchronizers 
and can see where beforeCompletion might somehow be useful to acquire the 
object ids that are about to be changed and afterCompletion could be used 
to fire off the event and pass the list of changed object ids (and could 
acquire the new object states at that time to send to the sockets asking 
for it).

With the following:
transaction.manager.registerSynch(MySynch)

The MySynch beforeCompletion method that I implement would tell MySynch 
about objects being changed in some temporary class variable. Then, the 
MySynch afterCompletion that I implement would ultimately have access to 
sockets and send those sockets the changes they are interested in knowing 
about. (Most importantly I think I need the object ids from transaction 
because there should be a way to re-fetch the current values via just the 
ids, right?)

Now, the documentation shows that the beforeCompletion method is passed the 
transaction object and only the transaction object. What I need to know 
from the transaction object is what object ids are getting ready to be 
changed. I probably don't need to know what all of the changes are yet, 
because I might be able to fetch all that in the afterCompletion--though it 
could still be helpful to also know what the changes are. In short, how do 
I get ids for objects that are about to change from the transaction object?

I haven't proven any of this out yet, so maybe my whole approach is flawed 
or inefficient. Maybe you know of a much better approach, or even a more 
awesome set of supporting technologies for all this. If so, don't hesitate 
to share! :)

Many thanks,
David

P.S. I initially addressed this message to Jim in my introduction. But 
anyone is free to chime in!

Also for your interest, the only thing that hindered my decision to jump 
into Python/ZODB a few years ago was the lack of static typing in Python. 
Just had to have it. But now, many thanks to Python 3.6!!! :)

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