[ZODB-Dev] ZEO, FileStorage, and Undo

[Shane Hathaway]

Jeremy Hylton wrote:
> Yes.  Every FileStorage method that reads the database needs to hold
> the lock to protect the file position.  The read-only methods call
> seek() and read() on the file object.  If two read-only methods
> executed at the same time, there's no guarantee they would read the
> data they wanted to read.

Alternatively, could undoLog() open its own file (pointed to the same 
filename, but opened read-only) to do the scanning?

> 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.  If I sent you a patch, would
> you want to give it a try?

If you go ahead with this method, you could avoid the performance hit by 
releasing and reacquiring the lock every 20 transactions or so.

Shane