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

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.
> 
>> 
>> 
>> Background: Dynamic and static are the two types of transactions in the IndexedDB spec.  Static declare what resources they want access to before they begin, which means that they can be implemented via objectStore level locks.  Dynamic decide at commit time whether the transaction was serializable.  This leaves implementations with two options:
>> 
>> 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.
>  
> 
>> 
>> 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?

>> 
>> J
> 
> Can you please respond to the rest?  I really don't see the point of dynamic transactions for v1.

There has been previous discussions on this DL about the need for dynamic locking [1], MVCC [2] and incremental locking [3] . Did you have anything new to add to that discussion?

[1] http://lists.w3.org/Archives/Public/public-webapps/2010JanMar/0197.html
[2] http://lists.w3.org/Archives/Public/public-webapps/2010JanMar/0322.html
[3] http://lists.w3.org/Archives/Public/public-webapps/2009OctDec/1080.html