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From Andrey Gura <ag...@apache.org>
Subject Re: Consistency check and fix (review request)
Date Mon, 15 Apr 2019 14:19:19 GMT
Method name is minor problem. I still believe that there is no need
for this proxy because there are no any guarantees about bugless
implementation this functionality. Better way is reaching bugless
implementation of current functionality.

On Mon, Apr 15, 2019 at 4:51 PM Anton Vinogradov <av@apache.org> wrote:
>
> Andrey,
>
> >> It means also that at least method name is bad.
> Agreed, already discussed with Aleksey Plekhanov.
> Decided that ".withConsistencyCheck()" is a proper name.
>
> >> What is the profit?
> This proxy allows to check (and fix) is there any consistency violation
> across the topology.
> The proxy will check all backups contain the same values as primary.
> So, when it's possible (you're ready to spend resources for this check) you
> will be able to read-with-consistency-check.
> This will decrease the amount of "inconsistency caused
> war/strikes/devastation" situations, which is important for financial
> systems.
>
> On Mon, Apr 15, 2019 at 3:58 PM Andrey Gura <agura@apache.org> wrote:
>
> > Anton,
> >
> > what does expression "withConsistency" mean? From user's standpoint it
> > means that all operations performed without this proxy are not
> > consistent. It means also that at least method name is bad.
> >
> > Are there any guarantees that withConsistency proxy will not contain
> > bugs that will lead to inconsistent write after inconsistency was
> > found? I think there are no such guarantees. Bugs still are possible.
> > So I always must use withConsistency proxy because I doesn't have
> > other choice - all ways are unreliable and withConsistency just sounds
> > better.
> >
> > Eventually we will have two different ways for working with cache
> > values with different bugs set. What is the profit?
> >
> >
> >
> > On Fri, Apr 12, 2019 at 2:49 PM Anton Vinogradov <av@apache.org> wrote:
> > >
> > > Folks,
> > >
> > > I've checked the tx benchmarks and found no performance drop.
> > > Also, see no issues at TC results.
> > > So, seems, code ready to be merged.
> > >
> > > Everyone interested, please share any objections about
> > > - public API
> > > - test coverage
> > > - implementation approach
> > >
> > > On Wed, Apr 3, 2019 at 5:46 PM Anton Vinogradov <av@apache.org> wrote:
> > >
> > > > Nikolay,
> > > >
> > > > This is not a PoC, but the final solution (I hope so:) ) required the
> > > > review.
> > > > LWW means Last Write Wins, detailed explanation can be found at IEP-31.
> > > >
> > > > On Wed, Apr 3, 2019 at 5:24 PM Nikolay Izhikov <nizhikov@apache.org>
> > > > wrote:
> > > >
> > > >> Hello, Anton.
> > > >>
> > > >> Thanks for the PoC.
> > > >>
> > > >> > finds correct values according to LWW strategy
> > > >>
> > > >> Can you, please, clarify what is LWW strategy?
> > > >>
> > > >> В Ср, 03/04/2019 в 17:19 +0300, Anton Vinogradov пишет:
> > > >> > Ilya,
> > > >> >
> > > >> > This is impossible due to a conflict between some isolation levels
> > and
> > > >> > get-with-consistency expectations.
> > > >> > Basically, it's impossible to perform get-with-consistency after
the
> > > >> other
> > > >> > get at !READ_COMMITTED transaction.
> > > >> > The problem here is that value should be cached according to
the
> > > >> isolation
> > > >> > level, so get-with-consistency is restricted in this case.
> > > >> > Same problem we have at case get-with-consistency after put,
so we
> > have
> > > >> > restriction here too.
> > > >> > So, the order matter. :)
> > > >> >
> > > >> > See OperationRestrictionsCacheConsistencyTest [1] for details.
> > > >> >
> > > >> > [1]
> > > >> >
> > > >>
> > https://github.com/apache/ignite/blob/8b0b0c3e1bde93ff9c4eb5667d794dd64a8b06f0/modules/core/src/test/java/org/apache/ignite/internal/processors/cache/consistency/OperationRestrictionsCacheConsistencyTest.java
> > > >> >
> > > >> > On Wed, Apr 3, 2019 at 4:54 PM Ilya Kasnacheev <
> > > >> ilya.kasnacheev@gmail.com>
> > > >> > wrote:
> > > >> >
> > > >> > > Hello!
> > > >> > >
> > > >> > > Sounds useful especially for new feature development.
> > > >> > >
> > > >> > > Can you do a run of all tests with cache.forConsistency(),
see if
> > > >> there are
> > > >> > > cases that fail?
> > > >> > >
> > > >> > > Regards,
> > > >> > > --
> > > >> > > Ilya Kasnacheev
> > > >> > >
> > > >> > >
> > > >> > > ср, 3 апр. 2019 г. в 16:17, Anton Vinogradov <av@apache.org>:
> > > >> > >
> > > >> > > > Igniters,
> > > >> > > >
> > > >> > > > Sometimes, at real deployment, we're faced with inconsistent
> > state
> > > >> across
> > > >> > > > the topology.
> > > >> > > > This means that somehow we have different values for
the same
> > key at
> > > >> > > > different nodes.
> > > >> > > > This is an extremely rare situation, but, when you
have
> > thousands of
> > > >> > > > terabytes of data, this can be a real problem.
> > > >> > > >
> > > >> > > > Apache Ignite provides a consistency guarantee, each
affinity
> > node
> > > >> should
> > > >> > > > contain the same value for the same key, at least eventually.
> > > >> > > > But this guarantee can be violated because of bugs,
see IEP-31
> > [1]
> > > >> for
> > > >> > > > details.
> > > >> > > >
> > > >> > > > So, I created the issue [2] to handle such situations.
> > > >> > > > The main idea is to have a special cache.withConsistency()
proxy
> > > >> allows
> > > >> > > > checking a fix inconsistency on get operation.
> > > >> > > >
> > > >> > > > I've created PR [3] with following improvements (when
> > > >> > > > cache.withConsistency() proxy used):
> > > >> > > >
> > > >> > > > - PESSIMISTIC && !READ_COMMITTED transaction
> > > >> > > > -- checks values across the topology (under locks),
> > > >> > > > -- finds correct values according to LWW strategy,
> > > >> > > > -- records special event in case consistency violation
found
> > > >> (contains
> > > >> > > > inconsistent map <Node, <K,V>> and last
values <K,V>),
> > > >> > > > -- enlists writes with latest value for each inconsistent
key,
> > so
> > > >> it will
> > > >> > > > be written on tx.commit().
> > > >> > > >
> > > >> > > > - OPTIMISTIC || READ_COMMITTED transactions
> > > >> > > > -- checks values across the topology (not under locks,
so
> > > >> false-positive
> > > >> > > > case is possible),
> > > >> > > > -- starts PESSIMISTIC && SERIALIZABLE (at separate
thread)
> > > >> transaction
> > > >> > >
> > > >> > > for
> > > >> > > > each possibly broken key and fixes it on a commit if
necessary.
> > > >> > > > -- original transaction performs get-after-fix and
can be
> > continued
> > > >> if
> > > >> > >
> > > >> > > the
> > > >> > > > fix does not conflict with isolation level.
> > > >> > > >
> > > >> > > > Future plans
> > > >> > > > - Consistency guard (special process periodically checks
we
> > have no
> > > >> > > > inconsistency).
> > > >> > > > - MVCC support.
> > > >> > > > - Atomic caches support.
> > > >> > > > - Thin client support.
> > > >> > > > - SQL support.
> > > >> > > > - Read-with-consistency before the write operation.
> > > >> > > > - Single key read-with-consistency optimization, now
the
> > collection
> > > >> > > > approach used each time.
> > > >> > > > - Do not perform read-with-consistency for the key
in case it
> > was
> > > >> > > > consistently read some time ago.
> > > >> > > >
> > > >> > > > [1]
> > > >> > > >
> > > >> > > >
> > > >> > >
> > > >> > >
> > > >>
> > https://cwiki.apache.org/confluence/display/IGNITE/IEP-31+Consistency+check+and+fix
> > > >> > > > [2] https://issues.apache.org/jira/browse/IGNITE-10663
> > > >> > > > [3] https://github.com/apache/ignite/pull/5656
> > > >> > > >
> > > >>
> > > >
> >

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