Re: [IndexedDB] READ_ONLY vs SNAPSHOT_READ transactions

On Thu, Aug 12, 2010 at 10:22 PM, Pablo Castro
<Pablo.Castro@microsoft.com>wrote:

> We currently have two read-only transaction modes, READ_ONLY and
> SNAPSHOT_READ. As we map this out to implementation we ran into various
> questions that made me wonder whether we have the right set of modes.
>
> It seems that READ_ONLY and SNAPSHOT_READ are identical in every aspect
> (point-in-time consistency for readers, allow multiple concurrent readers,
> etc.), except that they have different concurrency characteristics, with
> READ_ONLY blocking writers and SNAPSHOT_READ allowing concurrent writers
> come and go while readers are active. Does that match everybody's
> interpretation?
>

Yup.

Assuming that interpretation, then I'm not sure if we need both. Should we
> consider having only READ_ONLY, where transactions are guaranteed a stable
> view of the world regardless of the implementation strategy, and then let
> implementations either block writers or version the data? I understand that
> this introduces variability in the reader-writer interaction. On the other
> hand, I also suspect that the cost of SNAPSHOT_READ will also vary a lot
> across implementations (e.g. mvcc-based stores versus non-mvcc stores that
> will have to make copies of all stores included in a transaction to support
> this mode).


This seems reasonable to me.

J