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Re: [IndexedDB] Interaction between transactions and objects that allow multiple operations

From: Jeremy Orlow <jorlow@chromium.org>
Date: Thu, 6 May 2010 11:17:53 +0100
Message-ID: <AANLkTikEGYkPTAAKR6h8wFJpaTjhL6w9Hzntu1Mxo2vA@mail.gmail.com>
To: Nikunj Mehta <nikunj@o-micron.com>
Cc: Shawn Wilsher <sdwilsh@mozilla.com>, Pablo Castro <Pablo.Castro@microsoft.com>, public-webapps WG <public-webapps@w3.org>
On Thu, May 6, 2010 at 9:14 AM, Nikunj Mehta <nikunj@o-micron.com> wrote:

>
> On May 4, 2010, at 7:17 PM, Pablo Castro wrote:
>
> > The interaction between transactions and objects that allow multiple
> operations is giving us trouble. I need to elaborate a little to explain the
> problem.
> >
> > You can perform operations in IndexedDB with or without an explicitly
> started transaction. When no transaction is present, you get an implicit one
> that is there for the duration of the operation and is committed and the end
> (or rolled-back if an error occurs).
>
> To provide context to those who might be missing it, an explicit
> transaction is active in an IndexedDB Database as long as it has not been
> explicitly committed or aborted. An implicit transaction's life time is
> under the control of the implementation and spans no more than the operation
> requested.
>
> >
> > There are a number of operations in IndexedDB that are a single step. For
> example, store.put() occurs either entirely in the current transaction (if
> the user started one explicitly) or in an implicit transaction if there
> isn't one active at the time the operation starts. The interaction between
> the operation and transactions is straightforward in this case.
> >
> > On the other hand, other operations in IndexedDB return an object that
> then allows multiple operations on it. For example, when you open a cursor
> over a store, you can then move to the next row, update a row, delete a row,
> etc. The question is, what is the interaction between these operations and
> transactions? Are all interactions with a given cursor supposed to happen
> within the transaction that was active (implicit or explicit) when the
> cursor was opened? Or should each interaction happen in its own transaction
> (unless there is a long-lived active transaction, of course)?
>
> The transactional context of a series of operations is the transaction that
> was created in the database. Each and every operation from that point on
> till one of the following happens is performed in that transaction:
>
> 1. The transaction is committed
> 2. The transaction is aborted
> 3. The database object goes out of scope.
>
> >
> > We have a few options:
> > a) make multi-step objects bound to the transaction that was present when
> the object is first created (or an implicit one if none was present). This
> requires new APIs to mark cursors and such as "done" so implicit
> transactions can commit/abort, and has issues around use of the database
> object while a cursor with an implicit transaction is open.
> >
> > b) make each interaction happen in its own transaction (explicit or
> implicit). This is quite unusual and means you'll get inconsistent reads
> from row to row while scanning unless you wrap cursor/index scans on
> transactions. It also probably poses interesting implementation challenges
> depending on what you're using as your storage engine.
> >
> > c) require an explicit transaction always, along the lines Nikunj's
> original proposal had it. We would move most methods from database to
> transaction (except a few properties such as version and such, which it may
> still be ok to handle implicitly from the transactions perspective). This
> eliminates this whole problem altogether at the cost of an extra step
> required always.
> >
> > We would prefer to go with option c) and always require explicit
> transactions. Thoughts?
>
> The current specification allows using an explicit transaction and once
> initiated, the explicitly created transaction is applicable for its life
> time as described above. IOW a) is the same as c)
>
> If you intend to perform multiple steps, then an explicit transaction
> appears to be in order unless the application can tolerate inconsistent
> results. Therefore b) is not a good idea for multi-step operations. In
> addition, it is not a good idea to create/commit explicit transactions for
> each operation.
>

Nikunj, I don't really understand your responses.  I'm pretty sure Pablo's
whole question revolved around implicit transactions and whether we should
get rid of them (option c), make cursors only available within transactions,
etc.  It looks like most of your response was clarifying how explicit
transactions should work?

There has been some discussion for nested transactions and the original
> proposal had support for those, but recall that some implementors were not
> convinced of the cost/benefit tradeoff on that one.
>

One of the strongest pieces of feedback I've gotten from Google developers
who have worked with gears/SQLDatabase is in support of open nested
transactions.  This is because it allows multiple layers of abstraction to
all use the database without needing a lot of code to coordinate between
them.  Implementing them should be fairly straight forward: just keep a
count of the nesting level and only commit the whole transaction if that
count becomes 0.  Aborts of course require a little extra work, but I don't
believe it'll be a major burden on implementors.


On Thu, May 6, 2010 at 9:18 AM, Nikunj Mehta <nikunj@o-micron.com> wrote:

>
> On May 5, 2010, at 1:56 PM, Shawn Wilsher wrote:
>
> > On 5/5/2010 1:09 PM, Jeremy Orlow wrote:
> >> I'd also worry that if creating the transaction were completely
> transparent
> >> to the user that they might not think to close it either.  (I'm mainly
> >> thinking about copy-and-paste coders here.)
> > I should have been more clear.  That statement goes along with the
> suggestion to make everything work off of a transaction - object stores,
> indexes, cursors, etc.  They'd have to know about the transaction because
> they'd have to use it.
>
> I feel that auto transaction creation is syntactic sugar and should be left
> to libraries. On the other hand, I'd be worried if we were developing for
> complex multi-tab applications and not explicitly managing transactions.
>

I agree with both points: implicit transactions really are just syntactic
sugar and can be handled by libraries (which we expect will also do things
like joins for users).

And concurrency is a problem.  Web developers assume run to completion
semantics.  To me, that means that at very least we should make the implicit
transaction last the length of the javascript execution (i.e. the same
semantics as the storage mutex...which exists because many browsers now have
multiple event loops running concurrently).  The problem with that is that
the final result may not be serializable and thus need to be aborted.

So the two options I see are to have implicit transactions essentially lock
the entire database (so we're guaranteed the transaction can be committed)
and have it last the length of the javascript execution (so essentially
we're creating another storage mutex).  Or we can get rid of implicit
transactions and make users think about them.

Given that we all implement LocalStorage and it's good for the super simple
use cases, I guess I don't see why we would put so much effort into hiding
the concept of a transaction from IndexedDB users.

J
Received on Thursday, 6 May 2010 10:18:45 GMT

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