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From Nikolay Izhikov <nizhi...@apache.org>
Subject Re: Continuous queries and MVCC
Date Fri, 17 Aug 2018 14:10:05 GMT
Hello, Vladimir.

> 1) *Event ordering*. 

I vote to keep things as is.

> 2) *Initial query*
> With MVCC it is now
> technically possible to query data on certain snapshot and then receive
> only events happened after this snapshot

I think it a usefull extension for current implementation.

В Пт, 17/08/2018 в 13:18 +0300, Vladimir Ozerov пишет:
> Folks,
> 
> As you know we are developing multi-version concurrency control for Ignite
> caches, which is essentially new mode for transactional cache with snapshot
> semantics. One of the most important applications of this mode would be
> fully transactional SQL The question is how to implement continuous query
> semantics with MVCC. Several interesting things here.
> 
> 1) *Event ordering*. Currently we guarantee order of updates for specific
> key. But for fair transactional mode user may want to get transactional
> ordering instead of key ordering. If there are several dependent
> transactions, I may want to receive their updates in order. E.g.:
> TX1: transfer(A -> B)
> TX2: transfer(B -> C)
> TX3: transfer(C -> D)
> 
> If user receives update on key D (TX3), should we guarantee that he already
> received updates for all keys in TX1 and TX2? My opinion is that we'd
> better to leave things as is and guarantee only per-key ordering. Only one
> reason - implementation complexity.
> 
> 2) *Initial query*. We implemented it so that user can get some initial
> data snapshot and then start receiving events. Without MVCC we have no
> guarantees of visibility. E.g. if key is updated from V1 to V2, it is
> possible to see V2 in initial query and in event. With MVCC it is now
> technically possible to query data on certain snapshot and then receive
> only events happened after this snapshot. So that we never see V2 twice. Do
> you think we this feature will be interesting for our users?
> 
> Please share your thoughts.
> 
> Vladimir.
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