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Re: [IndexedDB] Current editor's draft

From: Jonas Sicking <jonas@sicking.cc>
Date: Tue, 27 Jul 2010 08:38:28 -0700
Message-ID: <AANLkTinJJ57J93Fxi2J35J1MBZXycps4h4rghfz7cRAQ@mail.gmail.com>
To: Jeremy Orlow <jorlow@chromium.org>
Cc: Nikunj Mehta <nikunj@o-micron.com>, Pablo Castro <Pablo.Castro@microsoft.com>, Andrei Popescu <andreip@google.com>, public-webapps <public-webapps@w3.org>
On Tue, Jul 27, 2010 at 3:14 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
> On Tue, Jul 27, 2010 at 12:22 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>> On Sat, Jul 24, 2010 at 8:29 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
>> >> >> And is it
>> >> >> only possible to lock existing rows, or can you prevent new records
>> >> >> from being created?
>> >> >
>> >> > There's no way to lock yet to be created rows since until a
>> >> > transaction
>> >> > ends, its effects cannot be made visible to other transactions.
>> >>
>> >> So if you have an objectStore with auto-incrementing indexes, there is
>> >> the possibility that two dynamic transactions both can add a row to
>> >> said objectStore at the same time. Both transactions would then add a
>> >> row with the same autogenerated id (one higher than the highest id in
>> >> the table). Upon commit, how is this conflict resolved?
>> >>
>> >> What if the objectStore didn't use auto-incrementing indexes, but you
>> >> still had two separate dynamic transactions which both insert a row
>> >> with the same key. How is the conflict resolved?
>> >
>> > I believe a common trick to reconcile this is stipulating that if you
>> > add
>> > 1000 "rows" the id's may not necessarily be 1000 sequential numbers.
>> >  This
>> > allows transactions to increment the id and leave it incremented even if
>> > the
>> > transaction fails.  Which also means that other transactions can be
>> > grabbing
>> > an ID of their own as well.  And if a transaction fails, well, we've
>> > wasted
>> > one possible ID.
>> This does not answer the question what happens if two transactions add
>> the same key value though?
> If you're using optimistic transactions, whichever commits first succeeds.
>  Not sure with the pessimistic/lock-based implementations.

That doesn't count as serializable, does it? It's not a dead-lock
hazard, but a race condition where whoever commits last experiences
dataloss unless properly handling the commit error.

>> >> >> And is it possible to only use read-locking for
>> >> >> some rows, but write-locking for others, in the same objectStore?
>> >> >
>> >> > An implementation could use shared locks for read operations even
>> >> > though
>> >> > the object store might have been opened in READ_WRITE mode, and later
>> >> > upgrade the locks if the read data is being modified. However, I am
>> >> > not keen
>> >> > to push for this as a specced behavior.
>> >>
>> >> What do you mean by "an implementation could"? Is this left
>> >> intentionally undefined and left up to the implementation? Doesn't
>> >> that mean that there is significant risk that code could work very
>> >> well in a conservative implementation, but often cause race conditions
>> >> in a implementation that uses narrower locks? Wouldn't this result in
>> >> a "race to the bottom" where implementations are forced to eventually
>> >> use very wide locks in order to work well in websites?
>> >>
>> >> In general, there are a lot of details that are unclear in the dynamic
>> >> transactions proposals. I'm also not sure if these things are unclear
>> >> to me because they are intentionally left undefined, or if you guys
>> >> just haven't had time yet to define the details?
>> >>
>> >> As the spec is now, as an implementor I'd have no idea of how to
>> >> implement dynamic transactions. And as a user I'd have no idea what
>> >> level of protection to expect from implementations, nor what
>> >> strategies to use to avoid bugs.
>> >>
>> >> In all the development I've done deadlocks and race conditions are
>> >> generally unacceptable, and instead strategies are developed that
>> >> avoids them, such as always grab locks in the same order, and always
>> >> grab locks when using shared data. I currently have no idea what
>> >> strategy to recommend in IndexedDB documentation to developers to
>> >> allow them to avoid race conditions and deadlocks.
>> >>
>> >> To get clarity in these questions, I'd *really* *really* like to see a
>> >> more detailed proposal.
>> >
>> > I think a detailed proposal would be a good thing (maybe from Pablo or
>> > Nikunj since they're who are really pushing them at this point), but at
>> > the
>> > same time, I think you're getting really bogged down in the details,
>> > Jonas.
>> > What we should be concerned about and speccing is the behavior the user
>> > sees.  For example, can any operation on data fail due to transient
>> > issues
>> > (like deadlocks, serialization issues) or will the implementation shield
>> > web
>> > developers from this?  And will we guarantee 100% serializable
>> > semantics?
>> >  (I strongly believe we should on both counts.)  How things are
>> > implemented,
>> > granularity of locks, or even if an implementation uses locks at all for
>> > dynamic transactions should be explicitly out of scope for any spec.
>> >  After
>> > all, it's only the behavior users care about.
>> If we can guarantee no deadlocks and 100% serializable semantics, then
>> I agree, it doesn't matter beyond that. However I don't think the
>> current proposals for dynamic transactions guarantee that. In fact, a
>> central point of the dynamic transactions proposal seems to be that
>> the author can grow the lock space dynamically, in an author defined
>> order. As long as that is the case you can't prevent deadlocks other
>> than by forbidding multiple concurrent (dynamic) transactions.
> There has been a lot of talk on list about how you could implement dynamic
> transactions with locks and I agree that I don't see how what's been
> mentioned can be guaranteed to be serializable and/or not create deadlocks.
>  But my point is: why are we talking about this??  The spec should talk
> about the semantics and behavior not the underlying engine, right?
> I think we need to take a BIG step back here and talk about what the
> behavior should be from the user point of view and leave implementation
> details to the implementations.  Otherwise we're going to continue getting
> nowhere in this discussion.
> I guess the best way forward is for Nikunj and/or Pablo to come up with a
> concrete proposal for the behavior/semantics (leave implementation out of
> it, please) and then we can discuss the merits of that.  If we can't settle
> on something soon, I suggest we take them out of the spec for the time being
> since what's there is pretty half baked.

That's exactly what I was trying to ask for too :)

/ Jonas
Received on Tuesday, 27 July 2010 15:39:24 UTC

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