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Re: [IndexedDB] Current editor's draft

From: Jonas Sicking <jonas@sicking.cc>
Date: Thu, 22 Jul 2010 17:18:03 -0700
Message-ID: <AANLkTils0nZWCxihvRQ6WLkx6uHGufICnxwbm1ZFDvN1@mail.gmail.com>
To: Pablo Castro <Pablo.Castro@microsoft.com>
Cc: Nikunj Mehta <nikunj@o-micron.com>, Jeremy Orlow <jorlow@chromium.org>, Andrei Popescu <andreip@google.com>, public-webapps <public-webapps@w3.org>
On Thu, Jul 22, 2010 at 4:41 PM, Pablo Castro
<Pablo.Castro@microsoft.com> wrote:
>
> From: Jonas Sicking [mailto:jonas@sicking.cc]
> Sent: Thursday, July 22, 2010 11:27 AM
>
>>> On Thu, Jul 22, 2010 at 3:43 AM, Nikunj Mehta <nikunj@o-micron.com> wrote:
>>> >
>>> > On Jul 16, 2010, at 5:41 AM, Pablo Castro wrote:
>>> >
>>> >>
>>> >> From: jorlow@google.com [mailto:jorlow@google.com] On Behalf Of Jeremy Orlow
>>> >> Sent: Thursday, July 15, 2010 8:41 AM
>>> >>
>>> >> On Thu, Jul 15, 2010 at 4:30 PM, Andrei Popescu <andreip@google.com> wrote:
>>> >> On Thu, Jul 15, 2010 at 3:24 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
>>> >>> On Thu, Jul 15, 2010 at 3:09 PM, Andrei Popescu <andreip@google.com> wrote:
>>> >>>>
>>> >>>> On Thu, Jul 15, 2010 at 9:50 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
>>> >>>>>>>> Nikunj, could you clarify how locking works for the dynamic
>>> >>>>>>>> transactions proposal that is in the spec draft right now?
>>> >>>>>>>
>>> >>>>>>> I'd definitely like to hear what Nikunj originally intended here.
>>> >>>>>>>>
>>> >>>>>>
>>> >>>>>> Hmm, after re-reading the current spec, my understanding is that:
>>> >>>>>>
>>> >>>>>> - Scope consists in a set of object stores that the transaction operates
>>> >>>>>> on.
>>> >>>>>> - A connection may have zero or one active transactions.
>>> >>>>>> - There may not be any overlap among the scopes of all active
>>> >>>>>> transactions (static or dynamic) in a given database. So you cannot
>>> >>>>>> have two READ_ONLY static transactions operating simultaneously over
>>> >>>>>> the same object store.
>>> >>>>>> - The granularity of locking for dynamic transactions is not specified
>>> >>>>>> (all the spec says about this is "do not acquire locks on any database
>>> >>>>>> objects now. Locks are obtained as the application attempts to access
>>> >>>>>> those objects").
>>> >>>>>> - Using dynamic transactions can lead to dealocks.
>>> >>>>>>
>>> >>>>>> Given the changes in 9975, here's what I think the spec should say for
>>> >>>>>> now:
>>> >>>>>>
>>> >>>>>> - There can be multiple active static transactions, as long as their
>>> >>>>>> scopes do not overlap, or the overlapping objects are locked in modes
>>> >>>>>> that are not mutually exclusive.
>>> >>>>>> - [If we decide to keep dynamic transactions] There can be multiple
>>> >>>>>> active dynamic transactions. TODO: Decide what to do if they start
>>> >>>>>> overlapping:
>>> >>>>>>   -- proceed anyway and then fail at commit time in case of
>>> >>>>>> conflicts. However, I think this would require implementing MVCC, so
>>> >>>>>> implementations that use SQLite would be in trouble?
>>> >>>>>
>>> >>>>> Such implementations could just lock more conservatively (i.e. not allow
>>> >>>>> other transactions during a dynamic transaction).
>>> >>>>>
>>> >>>> Umm, I am not sure how useful dynamic transactions would be in that
>>> >>>> case...Ben Turner made the same comment earlier in the thread and I
>>> >>>> agree with him.
>>> >>>>
>>> >>>> Yes, dynamic transactions would not be useful on those implementations, but the point is that you could still implement the spec without a MVCC backend--though it >> would limit the concurrency that's possible.  Thus "implementations that use SQLite would" NOT necessarily "be in trouble".
>>> >>
>>> >> Interesting, I'm glad this conversation came up so we can sync up on assumptions...mine where:
>>> >> - There can be multiple transactions of any kind active against a given database session (see note below)
>>> >> - Multiple static transactions may overlap as long as they have compatible modes, which in practice means they are all READ_ONLY
>>> >> - Dynamic transactions have arbitrary granularity for scope (implementation specific, down to row-level locking/scope)
>>> >
>>> > Dynamic transactions should be able to lock as little as necessary and as late as required.
>>>
>>> So dynamic transactions, as defined in your proposal, didn't lock on a
>>> whole-objectStore level? If so, how does the author specify which rows
>>> are locked? And why is then openObjectStore a asynchronous operation
>>> that could possibly fail, since at the time when openObjectStore is
>>> called, the implementation doesn't know which rows are going to be
>>> accessed and so can't determine if a deadlock is occurring? And is it
>>> only possible to lock existing rows, or can you prevent new records
>>> from being created? And is it possible to only use read-locking for
>>> some rows, but write-locking for others, in the same objectStore?
>
> That's my interpretation, dynamic transactions don't lock whole object stores. To me dynamic transactions are the same as what typical SQL databases do today.
>
> The author doesn't explicitly specify which rows to lock. All rows that you "see" become locked (e.g. through get(), put(), scanning with a cursor, etc.). If you start the transaction as read-only then they'll all have shared locks. If you start the transaction as read-write then we can choose whether the implementation should always attempt to take exclusive locks or if it should take shared locks on read, and attempt to upgrade to an exclusive lock on first write (this affects failure modes a bit).

What counts as "see"? If you iterate using an index-cursor all the
rows that have some value between "A" and "B", but another, not yet
committed, transaction changes a row such that its value now is
between "A" and "B", what happens?

> Regarding deadlocks, that's right, the implementation cannot determine if a deadlock will occur ahead of time. Sophisticated implementations could track locks/owners and do deadlock detection, although a simple timeout-based mechanism is probably enough for IndexedDB.
>
> I'm not sure why openObjectStore would need to be asynchronous in this context. In the past this was the case because metadata wasn't locked by the fact that you had an open database object, so openObjectStore involved I/O and possibly contentention against schema modification operations. Now that openObjectStore doesn't have to deal with contention (and implementations will probably cache the database catalog) there is no reason to make it async that I can think of.

Agreed, if locking isn't objectStore-wide then openObjectStore doesn't
need to be asynchronous.

> As for locking only existing rows, that depends on how much isolation we want to provide. If we want "serializable", then we'd have to put in things such as range locks and locks on non-existing keys so reads are consistent w.r.t. newly created rows.

Indeed. And if it's not serializable, how do we tell the author that
the world he/she saw, is no longer the current world?

/ Jonas
Received on Friday, 23 July 2010 00:18:58 GMT

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