I believe Ignite updates values during the commit phase, so it's not possible to get a dirty read even if you do not acquire distributed locks on reads with READ_COMMITTED isolation. But I would let other community members who are more knowledgeable in this topic to add there comments, as it's possible that I'm missing something.
As for documentation, looks like semantic of "lock" there always implies that it is help until transaction is committed or rolled back. Probably it makes sense to clarify this as well. And yes, please disregard the REPEATABLE_READ point. I misread you initial message a little bit. -Val On Wed, Jul 25, 2018 at 11:25 AM John Wilson <sami.hailu...@gmail.com> wrote: > And no. I'm not describing REPEATABLE_READ. I'm describing > https://en.wikipedia.org/wiki/Isolation_(database_systems)#Dirty_reads and > how READ_COMMITTED isolation can avoid dirty reads. > > On Wed, Jul 25, 2018 at 11:20 AM, John Wilson <sami.hailu...@gmail.com> > wrote: > > > I agree with your description. But the documentation https:// > > apacheignite.readme.io/docs/transactions is misleading in stating that > no > > read locks are acquired (says "READ_COMMITTED - Data is read without a > lock > > and is never cached in the transaction itself."). Which should be wrong. > > Read locks are acquired but they are released as soon as the read is > > complete (and they are not held until the transaction commits or rolls > > back). > > > > Thanks, > > > > On Wed, Jul 25, 2018 at 10:33 AM, Valentin Kulichenko < > > valentin.kuliche...@gmail.com> wrote: > > > >> Hi John, > >> > >> Read committed isolation typically implies that lock on read is not held > >> throughout the transaction lifecycle, i.e. released right after read is > >> completed (in Ignite this just means that no lock is required at all). > >> This > >> semantic allows second read to get an updated value that was already > >> committed by another transaction. See Wikipedia description for example: > >> > https://en.wikipedia.org/wiki/Isolation_(database_systems)#Read_committed > >> > >> What you're describing is REPEATABLE_READ isolation which guarantees > that > >> subsequent reads return the same value. In Ignite it's achieved by > >> acquiring lock on read and releasing it only on commit/rollback. > >> > >> -Val > >> > >> On Wed, Jul 25, 2018 at 10:12 AM John Wilson <sami.hailu...@gmail.com> > >> wrote: > >> > >> > Hi, > >> > > >> > Consider the following transaction where we read key 1 twice. > >> > > >> > try (Transaction tx = Ignition.ignite().transactions > >> ().txStart(PESSIMISTIC, > >> > READ_COMMITTED)) { > >> > cache.get(1); > >> > //... > >> > cache.get(1); > >> > tx.commit(); > >> > } > >> > > >> > According to the documentation here, > >> > https://apacheignite.readme.io/docs/transactions, data is read > without > >> a > >> > lock and is never cached. If that is the case, then how do we avoid a > >> dirty > >> > read on the second cache.get(1)? Another uncommitted transaction may > >> update > >> > the key between the first and second reads. > >> > > >> > In most RDMS, a READ_COMMITTED isolation level, acquires locks for > both > >> > read and writes. The read lock is released after a read while the > write > >> > lock is held until the transaction completes. So for the above > example, > >> I > >> > expect a read lock on each cache.get(1). > >> > > >> > > >> > Thanks, > >> > > >> > > > > >
