On 19 September 2013 09:06, Radim Vansa <[email protected]> wrote: > I think that Read Committed isolation level is not obliged to present > you with up-to-date committed data - the only fact is that it can, but > application must not rely on that. It's lower isolation level. > Nevertheless, I think that lower isolation level should mean better > performance. I would be strongly against imposing any additional > overhead that could slow it down. I should give it some test, but if it > currently does not offer any advantage against RR, I don't see any > reason for this to exist.
+1 I also think the lower level of isolation is not necessarily a promise, more something people can select when they are able to handle it in their apps / business requirements. As long as this is properly documented it should be fine. I agree with your point that it's undesirable that depending on the node topology (i.e. on which node you happen to run a test) the experienced isolation might seem different, but again let's just document it clearly. > > Regarding the bug: Thanks a lot for pointing this out, I should adapt > the testsuite to use different object types as values (if I understand > the scenario correctly). > > Radim > > On 09/19/2013 12:03 AM, William Burns wrote: >> I was recently refactoring code dealing with isolation levels and >> found how ReadCommitted is implemented and I have a few concerns I >> wanted to bring up. >> >> ReadCommitted read operations work by storing a reference to the value >> from the data store in its caller's context. Thus whenever another >> transaction is committed that updates the data store value any context >> that has that reference now sees the latest committed value. This >> works well for Local and Replicated caches since all data stores are >> updated with the latest value upon completion of the transaction. >> However Distributed caches only the owners see the update in their >> data store and thus any non owner will still have the old value they >> previously read before the commit occurred. >> >> This seems quite inconsistent that Distributed caches run in a mix of >> Repeatable Read/Read Committed depending on what node and what key you >> are using. >> >> To operate properly we could track requests similar to how it works >> for L1 so we can tell non owners to clear out their context values for >> values they read remotely that they haven't updated (since Read >> Committed writes should return the same written value). That seems >> like quite a bit of additional overhead though. >> >> I am wondering is it worth it to try to keep Read Committed isolation >> level though? It seems that Repeatable Read would be simpler and most >> likely more performant as you wouldn't need all the additional remote >> calls to get it to work properly. Or is it okay that we have >> different isolation levels for some keys on some nodes? This could be >> quite confusing if a user was using a local and remote transaction and >> a transaction may not see the other's committed changes when they >> expect to. >> >> What do you guys think? >> >> - Will >> >> P.S. >> >> I also found a bug with Read Committed for all caches where if you do >> a write that changes the underlying InternalCacheEntry to a new type, >> that reads won't see subsequent committed values. This is caused >> because the underlying data is changed to a new reference and a read >> would still be holding onto a reference of the old InternalCacheEntry. >> This can happen when using the various overridden put methods for >> example. We should have a good solution for it, but may not be >> required if we find that Read Committed itself is flawed beyond >> saving. >> _______________________________________________ >> infinispan-dev mailing list >> [email protected] >> https://lists.jboss.org/mailman/listinfo/infinispan-dev > > > -- > Radim Vansa <[email protected]> > JBoss DataGrid QA > > _______________________________________________ > infinispan-dev mailing list > [email protected] > https://lists.jboss.org/mailman/listinfo/infinispan-dev _______________________________________________ infinispan-dev mailing list [email protected] https://lists.jboss.org/mailman/listinfo/infinispan-dev
