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RE: [IndexedDB] Dynamic Transactions (WAS: Lots of small nits and clarifying questions)

From: Pablo Castro <Pablo.Castro@microsoft.com>
Date: Thu, 22 Apr 2010 07:06:42 +0000
To: Nikunj Mehta <nikunj@o-micron.com>, Jeremy Orlow <jorlow@chromium.org>
CC: public-webapps WG <public-webapps@w3.org>
Message-ID: <F753B2C401114141B426DB383C8885E0498D3B69@TK5EX14MBXC124.redmond.corp.microsoft.com>
On Apr 21, 2010, 11:18 PM Nikunj Mehta wrote:

On Apr 21, 2010, at 5:11 PM, Jeremy Orlow wrote:

On Mon, Apr 19, 2010 at 11:44 PM, Nikunj Mehta <nikunj@o-micron.com> wrote:

On Mar 15, 2010, at 10:45 AM, Jeremy Orlow wrote:

On Mon, Mar 15, 2010 at 3:14 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
On Sat, Mar 13, 2010 at 9:02 AM, Nikunj Mehta <nikunj@o-micron.com> wrote:
On Feb 18, 2010, at 9:08 AM, Jeremy Orlow wrote:
>> 2) In the spec, dynamic transactions and the difference between static and dynamic are not very well explained.
>> Can you propose spec text?
>> In 3.1.8 of http://dev.w3.org/2006/webapi/WebSimpleDB/ in the first paragraph, adding a sentence would probably be good enough.  "If the scope is dynamic, the transaction may use any object stores or indexes in the database, but if another transaction touches any of the resources in a manner that could not be serialized by the implementation, a RECOVERABLE_ERR exception will be thrown on commit." maybe?
>> By the way, are there strong use cases for Dynamic transactions?  The more that I think about them, the more out of place they seem.
>> Dynamic transactions are in common place use in server applications. It follows naturally that client applications would want to use them. 
>> There are a LOT of things that are common place in server applications that are not in v1 of IndexedDB.
>> Consider the use case where you want to view records in entityStore A, while, at the same time, modifying another entityStore B using the records in entityStore A. Unless you use dynamic transactions, you will not be able to perform the two together.
>>...unless you plan ahead.  The only thing dynamic transactions buy you is not needing to plan ahead about using resources.
>> The dynamic transaction case is particularly important when dealing with asynchronous update processing while keeping the UI updated with data.

I strongly agree that dynamic transactions are important. Funnily enough we were considering proposing the other extreme, and drop all the static modes in favor of dynamic. This is not only about being able to transport server-service code to the client, but more in general about supporting modes of operation where the complete set of objects you'll use in a transaction is dependent upon things you'll only find out as you process the transaction; this includes the particular case where your application will make decisions based on data on the same database, so there is no way to plan ahead short of locking the whole thing. 

>> 1) Treat Dynamic transactions as "lock everything".
>> This is not consistent with the spec behavior. Locking everything is the static global scope.
>> I don't understand what you're trying to say in the second sentence.  And I don't understand how this is inconsistent with spec behavior--it's simply more conservative.

Of my main concerns around being overly conservative, and with the static locking model in general, is its impact on concurrency. While the client scenarios of IndexedDB don't have the same pressure for concurrency as server databases, things like synchronization and other background processing tasks do need a based level of concurrency to operate in a user-friendly way. 

>> 2) Implement MVCC so that dynamic transactions can operate on a consistent view of data.  (At times, we'll know a transaction is doomed long before commit, but we'll need to let it keep running since only .commit() can raise the proper error.)
>> MVCC is not required for dynamic transactions. MVCC is only required to open a database in the DETACHED_READ mode.
>> Since locks are acquired in the order in which they are requested, a failure could occur when an object store is being opened, but it is locked by another transaction. One doesn't have to wait until commit is invoked.
>> Am I missing something here?
>> If we really expect UAs to implement MVCC (or something else along those lines), I would expect other more advanced transaction concepts to be exposed. 
>> What precisely are you referring to? Why are these other more advanced transaction concepts required?
>> If we expect most v1 implementations to just use objectStore locks and thus use option 1, then is there any reason to include Dynamic transactions?
>> Why do you conclude that most implementations just use object store locks?

We were actually favoring use of the dynamic pattern. Note that other than the failure mode (which is a separate discussion we should have), you can do dynamic using regular locks instead of versioning if you follow the two-phase protocol[1]; that still results in a serializable schedule, although not with point-in-time consistency.

More in general, I'm a bit worried about the number of options around transactions. I understand the goal of creating an "error free" model where once you succeed at starting a transaction you know you won't fail due to concurrency work, but in the process we seem to be introducing a fair number of new concepts, which may be equally challenging for folks. Would it be worth it considering resetting this to a more traditional approach were you only have more or less the equivalent to dynamic (in that you don't say anything ahead, but you're still guaranteed a serializable schedule)? The downside is trickier failure modes when conflicting changes happen concurrently, but arguably there should be a lot less of that in a client application.


[1] http://en.wikipedia.org/wiki/Two_phase_locking
Received on Thursday, 22 April 2010 07:07:17 UTC

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