On Mon, Feb 6, 2017 at 12:29 AM, Yakov Zhdanov <yzhda...@apache.org> wrote:
> What users do not expect is that partitioned cache suddenly turns to > replicated with replicas on client machines running in virtual environment > on higher latency network. I am a bit surprised nobody sees it. I predict > we will soon start getting questions from our largest deployments =) > Yakov, I tend to agree with you, even though up until now we have not received any complaints in this regard. Let's add a new mode and call it, say, NEAR_ASYNC or DHT_SYNC. When this mode is turned on, the near caches will be updated asynchronously. I am assuming that we will take care of the back pressure as we always do. Can you please create a ticket? > > --Yakov > > 2017-02-06 15:20 GMT+07:00 Vladimir Ozerov <voze...@gridgain.com>: > > > Yakov, > > > > I think forcing reads in TX or from primary is not what users expect by > > default. So i would have this mode disabled by default. > > > > On Mon, Feb 6, 2017 at 10:40 AM, Yakov Zhdanov <yzhda...@apache.org> > > wrote: > > > > > >It still seems that outdated reads will be *more* outdated with async > > > mode. > > > >Also, is there a guarantee that a near-cache update will happen at > all, > > if > > > >you introduce the async mode? > > > > > > We have the same guarantees for continuous queries - updates are sent > to > > > listener and no ack is required on grid level protocol (communication > > > guaranteed delivery is used). If near node receives messages and > process > > > them, then the update should happen, if it does not receive messages it > > > should be thrown out of topology as message queue to it grows (slow > > client > > > limit) > > > > > > I do not want to operate "more outdated" or "less outdated" > definitions. > > To > > > me both of them are pretty much the same :) Want up to date reads - > read > > > from primary or in TX, all other options may be "outdated". > > > > > > > Is this going to be the default flag? > > > > > > Well, I don't want to take decision at the moment, but having DHT_SYNC > > > seems very good option to me. PRIMARY_SYNC may stay default. All I want > > is > > > to have opportunity to update near readers in async way. > > > > > > >Are you really suggesting that TX is committed without a guarantee > that > > > >near cache update happened? > > > > > > Do not see any issue here. You can ensure consistency and reread the > > value > > > in TX. Or you can enforce this by choosing FULL_SYNC for this update. > > > > > > Btw, is there any way to override configured writeSync mode? Seems > pretty > > > nice to have IgniteCache.withWriteSynchronizationMode(Mode m) > > > > > > >This would be a great optimization. It sounds like it could be done > > > >independently from sync or async updates of near caches, no? > > > > > > I think so > > > > > > >I still don't see it. It is still possible for a near node to be > alive, > > > but > > > >unresponsive. In this case, there is a possibility that a near cache > > will > > > >never be updated, even though the transaction has already been > > committed. > > > >This just does not seem kosher to me. > > > > > > See above example for continuous queries and slow client queue limit. > > > > > > Thanks! > > > > > > > > > > > > --Yakov > > > > > > 2017-02-04 12:12 GMT+07:00 Dmitriy Setrakyan <dsetrak...@apache.org>: > > > > > > > Hm... interesting. My questions are inline. > > > > > > > > On Mon, Jan 30, 2017 at 3:29 PM, Yakov Zhdanov <yzhda...@apache.org> > > > > wrote: > > > > > > > > > Guys, > > > > > > > > > > Currently when we do cache updates in FULL_SYNC mode we update near > > > > readers > > > > > (near cache entries) synchronously. This is quite big drawback in > > > > design, I > > > > > think. I get each near reader update at cost of 1 extra backup > > update. > > > I > > > > > think everyone understands that partitioned cache easily turns to > > > > > replicated once near readers number increases. In TX cache cost of > > such > > > > > updates doubles. > > > > > > > > > > I do not see any benefit on updating near entries in sync way. > > Outdated > > > > > reads can still be possible if I don't read from primary or in TX > > > > context. > > > > > > > > > > > > > It still seems that outdated reads will be *more* outdated with async > > > mode. > > > > Also, is there a guarantee that a near-cache update will happen at > all, > > > if > > > > you introduce the async mode? > > > > > > > > > > > > > > So, what I suggest: > > > > > 1. introduce flag for cahce - withSyncNearUpdates() or extra > > > > > CacheWriteSynchronizationMode.DHT_SYNC and make it default mode. > > > > > > > > > > > > > Is this going to be the default flag? > > > > > > > > > > > > > 2. Near entries are updated in async way > > > > > 2.1 in atomic mode together with backup updates > > > > > 2.2 in TX mode after tx is committed on primary > > > > > > > > > > > > Are you really suggesting that TX is committed without a guarantee > that > > > > near cache update happened? > > > > > > > > I would also suggest to exclude near readers from lock > > > acquisition/release > > > > > steps. Only force updates. Updates order will be ensured by single > > > > primary > > > > > node and > > > > > per-partition striping. > > > > > > > > > > > > > This would be a great optimization. It sounds like it could be done > > > > independently from sync or async updates of near caches, no? > > > > > > > > > > > > > 3. Near readers do not respond to primary. Once primary fails near > > > > readers > > > > > get invalidated, if primary is alive then communication recovery > > > ensures > > > > > that message will be delivered to near. > > > > > > > > > > > > > I still don't see it. It is still possible for a near node to be > alive, > > > but > > > > unresponsive. In this case, there is a possibility that a near cache > > will > > > > never be updated, even though the transaction has already been > > committed. > > > > This just does not seem kosher to me. > > > > > > > > > > > > > > > > > > Please share your thoughts. > > > > > > > > > > --Yakov > > > > > > > > > > > > > > >
