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From Anton Vinogradov ...@apache.org>
Subject Re: Consistency check and fix (review request)
Date Tue, 16 Apr 2019 09:47:21 GMT
Andrey, thanks for tips

>> You can perform consistency check using idle verify utility.
Could you please point to utility's page?
According to its name, it requires to stop the cluster to perform the check?
That's impossible at real production when you should have downtime less
that some minutes per year.
So, the only case I see is to use online check during periods of moderate
activity.

>> Recovery tool is good idea
This tool is a part of my IEP.
But recovery tool (process)
- will allow you to check entries in memory only (otherwise, you will warm
up the cluster incorrectly), and that's a problem when you have
persisted/in_memory rate > 10:1
- will cause latency drop for some (eg. 90+ percentile) requests, which is
not acceptable for real production, when we have strict SLA.
- will not guarantee that each operation will use consistent data,
sometimes it's extremely essential
so, the process is a cool idea, but, sometime you may need more.

Ivan, thanks for analysis

>> why it comes as an on-demand enabled proxy but not as a mode enabled by
some configuration property
It's a bad idea to have this feature permanently enabled, it slows down the
system by design.
Customer should be able to change strategy on the fly according to time
periods or load.
Also, we're going to use this proxy for odd requests or for every 5-th,
10-th, 100-th request depends on the load/time/SLA/etc.
The goal is to perform as much as possible gets-with-consistency operations
without stopping the cluster and never find a problem :)

>> As for me it will be great if we can improve consistency guarantees
provided by default.
Once you checked backups you decreased throughput and increased latency.
This feature requred only for some financial, nuclear, health systems when
you should be additionally sure about consistency.
It's like a
- read from backups
- data modification outside the transaction
- using FULL_ASYNC instead of FULL_SYNC,
sometimes it's possible, sometimes not.

>> 1. It sounds suspicious that reads can cause writes (unexpected
deadlocks might be possible).
Code performs writes
- key per additional transaction in case original tx was OPTIMISTIC ||
READ_COMMITTED,
- all keys per same tx in case original tx was PESSIMISTIC &&
!READ_COMMITTED, since you already obtain the locks,
so, deadlock should be impossible.

>> 2. I do not believe that it is possible to implement a (bugless?)
feature which will fix other bugs.
It does not fix the bugs, it looks for inconsistency (no matter how it
happened) and reports using events (previous state and how it was fixed).
This allows continuing processing for all the entries, even inconsistent.
But, each such fix should be rechecked manually, for sure.

On Tue, Apr 16, 2019 at 11:39 AM Павлухин Иван <vololo100@gmail.com> wrote:

> Anton,
>
> Thank you for your effort for improving consistency guarantees
> provided by Ignite.
>
> The subject sounds really vital. Could you please elaborate why it
> comes as an on-demand enabled proxy but not as a mode enabled by some
> configuration property (or even as a default behavior)? How do you see
> the future development of such consistency checks? As for me it will
> be great if we can improve consistency guarantees provided by default.
>
> Also thinking loud a bit:
> 1. It sounds suspicious that reads can cause writes (unexpected
> deadlocks might be possible).
> 2. I do not believe that it is possible to implement a (bugless?)
> feature which will fix other bugs.
> 3. A storage (or database) product (Ignite in our case) consistency is
> not equal to a user application consistency. So, it might be that
> introduced checks are insufficient to make business applications
> happy.
>
> пн, 15 апр. 2019 г. в 19:27, Andrey Gura <agura@apache.org>:
> >
> > Anton,
> >
> > I'm trying tell you that this proxy can produce false positive result,
> > incorrect result and just hide bugs. What will the next solution?
> > withNoBugs proxy?
> >
> > You can perform consistency check using idle verify utility. Recovery
> > tool is good idea but user should trigger this process, not some cache
> > proxy implementation.
> >
> > On Mon, Apr 15, 2019 at 5:34 PM Anton Vinogradov <av@apache.org> wrote:
> > >
> > > Seems, we already fixed all bugs caused this feature, but there is no
> > > warranty we will not create new :)
> > > This proxy is just checker that consistency is ok.
> > >
> > > >> reaching bugless implementation
> > > Not sure it's possible. Once you have software it contains bugs.
> > > This proxy will tell you whether these bugs lead to inconsistency.
> > >
> > > On Mon, Apr 15, 2019 at 5:19 PM Andrey Gura <agura@apache.org> wrote:
> > >
> > > > 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
> > > > > > > >> > > >
> > > > > > > >>
> > > > > > > >
> > > > > >
> > > >
>
>
>
> --
> Best regards,
> Ivan Pavlukhin
>

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