[ZODB-Dev] Storage API change: Checking for reading out-of-date data

[Jim Fulton]

On Mon, Aug 30, 2010 at 6:39 PM, Shane Hathaway <shane at hathawaymix.org> wrote:
> On 08/30/2010 03:36 PM, Jim Fulton wrote:
>>
>> The tricky thing about this is the last paragraph.  If the method
>> doesn't raise an error, then there can't be updates to the object
>> until after the transaction commits.  For most current
>> implementations, this implies that the storage lock is help when this
>> is called.  For ZEO, some special care will be necessary because the
>> storage lock isn't acquired until the very end of the first phase of
>> 2-phase commit.
>>
>> I'm particularly concerned about the impact on RelStorage.
>
> Thanks for calling attention to this.  Like ZEO, RelStorage doesn't acquire
> the commit lock until the end of the first phase of 2-phase commit, for
> performance reasons.  Therefore, if I understand correctly, ZEO and
> RelStorage will have to maintain a list of OID/serials that the transaction
> is depending on, then check that list after the commit lock is acquired (but
> still in the first phase).  If anything has changed, we should raise a
> ReadConflictError.  Does that sound right?

Yes

> BTW, perhaps the method name and signature could be as simple as
> "require(oid, serial)".  It simply declares to the storage that the
> transaction requires a specific version of an object.

While I'm open to better names, I prefer a longer more descriptive name.

Jim

-- 
Jim Fulton