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Re: [IndexedDB] IDBRequest.abort on writing requests

From: Jonas Sicking <jonas@sicking.cc>
Date: Tue, 13 Jul 2010 23:28:51 -0700
Message-ID: <AANLkTimcr6GdmhVrHXms_v-91_ibxTgLXqIJLdlL5D2M@mail.gmail.com>
To: Jeremy Orlow <jorlow@chromium.org>
Cc: Webapps WG <public-webapps@w3.org>
On Tue, Jul 13, 2010 at 11:12 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
> On Tue, Jul 13, 2010 at 9:41 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>>
>> On Tue, Jul 13, 2010 at 1:17 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
>> > On Tue, Jul 13, 2010 at 8:25 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>> >>
>> >> Hi All,
>> >>
>> >> Sorry if this is something that I've brought up before. I know I meant
>> >> to bring this up in the past, but I couldn't find any actual emails.
>> >>
>> >> One thing that we discussed while implementing IndexedDB was what to
>> >> do for IDBRequest.abort() or "writing" requests. For example on the
>> >> request object returned from IDBObjectStore.remove() or
>> >> IDBCursor.update().
>> >>
>> >> Ideal would of course be if it would cancel the write operation,
>> >> however this isn't always possible. If the call to .abort() comes
>> >> after the write operation has already executed in the database, but
>> >> before the 'success' event has had a chance to fire. What's worse is
>> >> that other write operations might already have been performed on top
>> >> of the aborted request. Consider for example the following code:
>> >>
>> >> req1 = myObjectStore.remove(12);
>> >> req2 = myObjectStore.add({ id: 12, name: "Benny Andersson" });
>> >> .... do other stuff ....
>> >> req1.abort();
>> >>
>> >> In this case, even if the database supported aborting a specific
>> >> operation, it's very hard to say what the correct thing to do with
>> >> operations performed after it. As far as I know, databases generally
>> >> don't support rolling back a given operation, only rolling back to a
>> >> specific point, i.e. rolling back a given operation and all operations
>> >> performed after it.
>> >>
>> >> We could say that abort() signals some sort of error if the operation
>> >> has already been performed in the database, however that makes abort()
>> >> very racy.
>> >>
>> >> Instead we concluded that the best thing to do was to specify that
>> >> IDBRequest.abort() should throw if called on a modifying request. If
>> >> this sounds good I'll make this change to the spec.
>> >
>> > I'd be fine with that.
>> > Or we could remove abort all together.  I can't really think of what
>> > types
>> > of operations you'd really want to abort until (at least) we have some
>> > sort
>> > of join language or other mechanism to do really expensive read-only
>> > calls.
>>
>> I think there are expensive-ish read-only calls. Indexes are
>> effectively a join mechanism since you'll hit one b-tree to do the
>> index lookup, and then a second b-tree to look up the full object in
>> the objectStore.
>
> But each individual call (the scope of canceling an IDBRequest) is pretty
> short.
>
>>
>> I don't really feel strongly either way. I think abort() isn't too
>> hard to implement, but also doesn't provide a ton of value. At least
>> not, like you say, until we add expensive calls like getAll or
>> multi-step joins.
>
> I agree that when we look at adding such calls we may want to add an abort
> on just IDBRequest, but until then I don't think it's a very useful feature.
>  And being easy to add is not a good reason to lock ourselves into
> a particular design in the future.  I think we should remove it until
> there's a good reason for it to exist.
>
>>
>> > Or we could take abort off IDBRequest and instead put a rollback on
>> > transactions (and not do the modify limitation).
>>
>> I definitely think we should have IDBTransaction.abort() no matter
>> what. And that should allow rolling back write operations.
>
> Agreed.  In which case it seems as though being able to abort individual
> operations isn't that important...especially given what we just talked about
> above.
> So can we just get rid of abort() on IDBRequest?

I don't feel strongly either way. We'll probably keep them in the
mozilla implementation since we have experimental
objectStore.getAll(key) and index.getAllObjects(key) implementations,
which both probably count as long-running.

/ Jonas
Received on Wednesday, 14 July 2010 06:29:45 GMT

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