[ZODB-Dev] ZEO, FileStorage, and Undo

[Jim Fulton]

Jeremy Hylton wrote:
>   >> brinegar@purdue.edu wrote: Hello,
...


>   >> So I wrote a patch for undoLog in FileStorage to batch by days
>   >> instead of
>   >> # of transactions. This lets us show batches of 2 days at a time
>   >> # instead
>   >> of 20 transactions. Thus we never have to search more than 2 days
>   >> worth of the database. I also MonkeyPatched the manage_UndoForm
>   >> in App.UndoSupport.

That's a good idea, but even showing one day's worth could take a lot of
time on busy systems.

...

>   >> We have a ZEO with 3 Zeo Clients and 1 Zeo Server when someone
>   >> hits Undo the Zeo Server locks and all 3 clients sit waiting. It
>   >> appears from experience as a user that_lock_acquire locks reads
>   >> and writes.
> 
> Yes.  It's really a shame that this behavior locks up the ZEO server
> and prevents anyone else from making progress.  I've discussed this a
> few times as a worry, but I've never heard complaints about it in real
> life.  Well-- make that I had never heard until now.

I've complained about it. ;)


>   >> We have a system with potential for thousands of content
>   >> maintainers. It is undesirable to have our entire site die for 20
>   >> seconds everytime one of them clicks "Undo". We could disable
>   >> Undos, however the transactional database was one of the reasons
>   >> we choose Zope.
>   >>
>   >> Any suggestions? Is there a way to lock only for writes? So that
>   >> pages can still be served?
> 
> I think we could change the search process so that acquire and
> released the lock for each transaction.  That might make the undo a
> lot slower, but it would also allow other threads to make progress
> while undoLog() is running.  To implement this, we'd need to modify
> FileStorage to change its lock behavior and ZEO to run undoLog() in a
> separate thread.  These both sound easy. 

Sounds good.

Jim


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