On Thu, Aug 12, 2010 at 2:46 PM, Nikunj Mehta <nik...@o-micron.com> wrote: > > On Aug 12, 2010, at 2:22 PM, Pablo Castro 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? > > That is the intention.
That was my understanding too. >> 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). > > The main reason to separate the two was to correctly set expectations. It > seems fine to postpone this feature to a future date. Sounds good to me. / Jonas
