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From A J <s5a...@gmail.com>
Subject Re: New Chain for : Does Cassandra use vector clocks
Date Fri, 25 Feb 2011 16:21:35 GMT
Though you are not really implying that, I am not selling anything. I
don't work for VoltDB. I had other issues for my use case with the
software when I was evaluating it (their claim of durability is weak
according to me. Though it does not matter I'd rather they call
themselves NOSQL. they just give lip-service to SQL)
I'd rather not drink any sort of kool-aid, get all sides (whatever the
motive of the sides be) and be the judge myself for what I want to do.

The thread was by someone who seems to be having difficulty wrapping
head around the gives and takes of cassandra. maybe something else is
better for their use case.

Peace :)


On Fri, Feb 25, 2011 at 10:39 AM, Jonathan Ellis <jbellis@gmail.com> wrote:
> That article is heavily biased by "I am selling a competitor to Cassandra."
>
> First, read Coda's original piece if you haven't:
> http://codahale.com/you-cant-sacrifice-partition-tolerance/
>
> Then, Jeff Darcy's response: http://pl.atyp.us/wordpress/?p=3110
>
> On Thu, Feb 24, 2011 at 2:56 PM, A J <s5alye@gmail.com> wrote:
>> While we are at it, there's more to consider than just CAP in distributed :)
>> http://voltdb.com/blog/clarifications-cap-theorem-and-data-related-errors
>>
>> On Thu, Feb 24, 2011 at 3:31 PM, Edward Capriolo <edlinuxguru@gmail.com> wrote:
>>> On Thu, Feb 24, 2011 at 3:03 PM, A J <s5alye@gmail.com> wrote:
>>>> yes, that is difficult to digest and one has to be sure if the use
>>>> case can afford it.
>>>>
>>>> Some other NOSQL databases deals with it differently (though I don't
>>>> think any of them use atomic 2-phase commit). MongoDB for example will
>>>> ask you to read from the node you wrote first (primary node) unless
>>>> you are ok with eventual consistency. If the write did not make to
>>>> majority of other nodes, it will be rolled-back from the original
>>>> primary when it comes up again as a secondary.
>>>> In some cases, you still could server either new value (that was
>>>> returned as failed) or the old one. But it is different from Cassandra
>>>> in the sense that Cassandra will never rollback.
>>>>
>>>>
>>>>
>>>> On Thu, Feb 24, 2011 at 2:47 PM, Anthony John <chirayithaj@gmail.com>
wrote:
>>>>> The leap of faith here is that an error does not mean a clean backing
out to
>>>>> prior state - as we are used to with databases. It means that the operation
>>>>> in error could have gone through partially
>>>>>
>>>>> Again, this is not an absolutely unfamiliar territory and can be dealt
with.
>>>>> -JA
>>>>> On Thu, Feb 24, 2011 at 1:16 PM, A J <s5alye@gmail.com> wrote:
>>>>>>
>>>>>> >>but could be broken in case of a failed write<<
>>>>>> You can think of a scenario where R + W >N still leads to
>>>>>> inconsistency even for successful writes. Say you keep W=1 and R=N
.
>>>>>> Lets say the one node where a write happened with success goes down
>>>>>> before it made to the other N-1 nodes. Lets say it goes down for
good
>>>>>> and is unrecoverable. The only option is to build a new node from
>>>>>> scratch from other active nodes. This will lead to a write that was
>>>>>> lost and you will end up serving stale copy of it.
>>>>>>
>>>>>> It is better to talk in terms of use cases and if cassandra will
be a
>>>>>> fit for it. Otherwise unless you have W=R=N and fsync before each
>>>>>> write commit, there will be scope for inconsistency.
>>>>>>
>>>>>>
>>>>>> On Thu, Feb 24, 2011 at 1:25 PM, Anthony John <chirayithaj@gmail.com>
>>>>>> wrote:
>>>>>> > I see the point - apologies for putting everyone through this!
>>>>>> > It was just militating against my mental model.
>>>>>> > In summary, here is my take away - simple stuff but - IMO -
important to
>>>>>> > conclude this thread (I hope):-
>>>>>> > 1. I was splitting hair over a failed ( partial ) Q Write. Such
an event
>>>>>> > should be immediately followed by the same write going to a
connection
>>>>>> > on to
>>>>>> > another node ( potentially using connection caches of client
>>>>>> > implementations
>>>>>> > ) or a Read at CL of All. Because a write could have partially
gone
>>>>>> > through.
>>>>>> > 2. Timestamps are used in determining the latest version ( correcting
>>>>>> > the
>>>>>> > false impression I was propagating)
>>>>>> > Finally, wrt "W + R > N for Q CL statement" holds, but could
be broken
>>>>>> > in
>>>>>> > case of a failed write as it is unsure whether the new value
got written
>>>>>> > on
>>>>>> >  any server or not. Is that a fair characterization ?
>>>>>> > Bottom line - unlike traditional DBMS, errors do not ensure
automatic
>>>>>> > cleanup and revert back, app code has to follow up if  immediate
- and
>>>>>> > not
>>>>>> > eventual -  consistency is desired. I made that leap in almost
all cases
>>>>>> > - I
>>>>>> > think - but the case of a failed write.
>>>>>> > My bad and I can live with this!
>>>>>> > Regards,
>>>>>> > -JA
>>>>>> >
>>>>>> > On Thu, Feb 24, 2011 at 11:50 AM, Sylvain Lebresne
>>>>>> > <sylvain@datastax.com>
>>>>>> > wrote:
>>>>>> >>
>>>>>> >> On Thu, Feb 24, 2011 at 6:33 PM, Anthony John <chirayithaj@gmail.com>
>>>>>> >> wrote:
>>>>>> >>>
>>>>>> >>> Completely understand!
>>>>>> >>> All that I am quibbling over is whether a CL of quorum
guarantees
>>>>>> >>> consistency or not. That is what the documentation says
- right. IF
>>>>>> >>> for a CL
>>>>>> >>> of Q read - it depends on which node returns read first
to determine
>>>>>> >>> the
>>>>>> >>> actual returned result or other more convoluted conditions
, then a
>>>>>> >>> Quorum
>>>>>> >>> read/write is not consistent, by any definition.
>>>>>> >>
>>>>>> >> But that's the point. The definition of consistency we are
talking
>>>>>> >> about
>>>>>> >> has no meaning if you consider only a quorum read. The definition
>>>>>> >> (which is
>>>>>> >> the de facto definition of consistency in 'eventually consistent')
make
>>>>>> >> sense if we talk about a write followed by a read. And it
is
>>>>>> >> considering succeeding write followed by succeeding read.
>>>>>> >> And that is the statement the wiki is making.
>>>>>> >> Honestly, we could debate forever on the definition of consistency
and
>>>>>> >> whatnot. Cassandra guaranties that if you do a (succeeding)
write on W
>>>>>> >> replica and then a (succeeding) read on R replica and if
R+W>N, then it
>>>>>> >> is
>>>>>> >> guaranteed that the read will see the preceding write. And
this is what
>>>>>> >> is
>>>>>> >> called consistency in the context of eventual consistency
(which is not
>>>>>> >> the
>>>>>> >> context of ACID).
>>>>>> >> If this is not the definition of consistency you had in
mind then by
>>>>>> >> all
>>>>>> >> mean, Cassandra probably don't guarantee this definition.
But given
>>>>>> >> that the
>>>>>> >> paragraph preceding what you pasted state clearly we are
not talking
>>>>>> >> about
>>>>>> >> ACID consistency, but eventual consistency, I don't think
the wiki is
>>>>>> >> making
>>>>>> >> any unfair statement.
>>>>>> >> That being said, the wiki may not be always as clear as
it could. But
>>>>>> >> it's
>>>>>> >> an editable wiki :)
>>>>>> >> --
>>>>>> >> Sylvain
>>>>>> >>
>>>>>> >>>
>>>>>> >>> I can still use Cassandra, and will use it, luv it!!!
But let us not
>>>>>> >>> make
>>>>>> >>> this statement on the Wiki architecture section:-
>>>>>> >>> -------------------------------------------------------------
>>>>>> >>>
>>>>>> >>> More specifically: R=read replica count W=write replica
>>>>>> >>> count N=replication factor Q=QUORUM (Q = N / 2 +
1)
>>>>>> >>>
>>>>>> >>> If W + R > N, you will have consistency
>>>>>> >>>
>>>>>> >>> W=1, R=N
>>>>>> >>> W=N, R=1
>>>>>> >>> W=Q, R=Q where Q = N / 2 + 1
>>>>>> >>>
>>>>>> >>> Cassandra provides consistency when R + W > N (read
replica count
>>>>>> >>> + write
>>>>>> >>> replica count > replication factor).
>>>>>> >>>
>>>>>> >>> ----------------------------------------------------
>>>>>> >>>
>>>>>> >>> .
>>>>>> >>>
>>>>>> >>> On Thu, Feb 24, 2011 at 11:22 AM, Sylvain Lebresne
>>>>>> >>> <sylvain@datastax.com>
>>>>>> >>> wrote:
>>>>>> >>>>
>>>>>> >>>> On Thu, Feb 24, 2011 at 6:01 PM, Anthony John <chirayithaj@gmail.com>
>>>>>> >>>> wrote:
>>>>>> >>>>>
>>>>>> >>>>> If you are correct and you are probably closer
to the code - then CL
>>>>>> >>>>> of
>>>>>> >>>>> Quorum does not guarantee a consistency.
>>>>>> >>>>
>>>>>> >>>> If the operation succeed, it does (for some definition
of consistency
>>>>>> >>>> which is, following reads at Quorum will be guaranteed
to see the new
>>>>>> >>>> value
>>>>>> >>>> of a update at quorum). If it fails, then no, it
does not guarantee
>>>>>> >>>> consistency.
>>>>>> >>>> It is important to note that the word consistency
has multiple
>>>>>> >>>> meaning.
>>>>>> >>>> In particular, when we are talking of consistency
in Cassandra, we
>>>>>> >>>> are not
>>>>>> >>>> talking of the same definition as the C in ACID
>>>>>> >>>>
>>>>>> >>>> (see: http://www.allthingsdistributed.com/2007/12/eventually_consistent.html)
>>>>>> >>>>>
>>>>>> >>>>> On Thu, Feb 24, 2011 at 10:54 AM, Sylvain Lebresne
>>>>>> >>>>> <sylvain@datastax.com> wrote:
>>>>>> >>>>>>
>>>>>> >>>>>> On Thu, Feb 24, 2011 at 5:34 PM, Anthony
John
>>>>>> >>>>>> <chirayithaj@gmail.com>
>>>>>> >>>>>> wrote:
>>>>>> >>>>>>>>
>>>>>> >>>>>>>> >>Time stamps are not used
for conflict resolution - unless is is
>>>>>> >>>>>>>> >> part of the application
logic!!!
>>>>>> >>>>>>>
>>>>>> >>>>>>> >>What is you definition of conflict
resolution ? Because if you
>>>>>> >>>>>>> >> update twice the same column
(which
>>>>>> >>>>>>> >>I'll call a conflict), then
the timestamps are used to decide
>>>>>> >>>>>>> >> which
>>>>>> >>>>>>> >> update wins (which I'll call
a resolution).
>>>>>> >>>>>>> I understand what you are saying, and
yes semantics is very
>>>>>> >>>>>>> important
>>>>>> >>>>>>> here. And yes we are responding to the
immediate questions without
>>>>>> >>>>>>> covering
>>>>>> >>>>>>> all questions in the thread.
>>>>>> >>>>>>> The point being made here is that the
timestamp of the column is
>>>>>> >>>>>>> not
>>>>>> >>>>>>> used by Cassandra to figure out what
data to return.
>>>>>> >>>>>>
>>>>>> >>>>>> Not quite true.
>>>>>> >>>>>>>
>>>>>> >>>>>>> E.g. - Quorum is 2 nodes - and RF of
3 over N1/2/3
>>>>>> >>>>>>> A Quorum  Write comes and add/updates
the time stamp (TS2) of a
>>>>>> >>>>>>> particular data element. It succeeds
on N1 - fails on N2/3. So the
>>>>>> >>>>>>> write is
>>>>>> >>>>>>> returned as failed - right ?
>>>>>> >>>>>>> Now Quorum read comes in for exactly
the same piece of data that
>>>>>> >>>>>>> the
>>>>>> >>>>>>> write failed for.
>>>>>> >>>>>>> So N1 has TS2 but both N2/3 have the
old TS (say TS1)
>>>>>> >>>>>>> And the read succeeds - Will it return
TS1 or TS2.
>>>>>> >>>>>>> I submit it will return TS1 - the old
TS.
>>>>>> >>>>>>
>>>>>> >>>>>> It all depends on which (first 2) nodes
respond to the read (since
>>>>>> >>>>>> RF=3, that can any two of N1/N2/N3). If
N1 is part of the two that
>>>>>> >>>>>> makes the
>>>>>> >>>>>> quorum, then TS2 will be returned, because
cassandra will compare
>>>>>> >>>>>> the
>>>>>> >>>>>> timestamp and decide what to return based
on this. If N2/N3
>>>>>> >>>>>> responds
>>>>>> >>>>>> however, both timestamp will be TS1 and
so, after timestamp
>>>>>> >>>>>> resolution, it
>>>>>> >>>>>> will stil be TS1 that will be returned.
>>>>>> >>>>>> So yes timestamp is used for conflict resolution.
>>>>>> >>>>>> In your example, you could get TS1 back
because a failed write can
>>>>>> >>>>>> let
>>>>>> >>>>>> you cluster in an inconsistent state. You'd
have to retry the
>>>>>> >>>>>> quorum and
>>>>>> >>>>>> only when it succeeds can you be guaranteed
that quorum read will
>>>>>> >>>>>> always
>>>>>> >>>>>> return TS2.
>>>>>> >>>>>> This is because when a write fails, Cassandra
doesn't guarantee
>>>>>> >>>>>> that
>>>>>> >>>>>> the write did not made it in (there is no
revert).
>>>>>> >>>>>>
>>>>>> >>>>>>>
>>>>>> >>>>>>> Are we on the same page with this interpretation
?
>>>>>> >>>>>>> Regards,
>>>>>> >>>>>>> -JA
>>>>>> >>>>>>> On Thu, Feb 24, 2011 at 10:12 AM, Sylvain
Lebresne
>>>>>> >>>>>>> <sylvain@datastax.com> wrote:
>>>>>> >>>>>>>>
>>>>>> >>>>>>>> On Thu, Feb 24, 2011 at 4:52 PM,
Anthony John
>>>>>> >>>>>>>> <chirayithaj@gmail.com> wrote:
>>>>>> >>>>>>>>>
>>>>>> >>>>>>>>> Sylvan,
>>>>>> >>>>>>>>> Time stamps are not used for
conflict resolution - unless is is
>>>>>> >>>>>>>>> part of the application logic!!!
>>>>>> >>>>>>>>
>>>>>> >>>>>>>> What is you definition of conflict
resolution ? Because if you
>>>>>> >>>>>>>> update twice the same column (which
>>>>>> >>>>>>>> I'll call a conflict), then the
timestamps are used to decide
>>>>>> >>>>>>>> which
>>>>>> >>>>>>>> update wins (which I'll call a resolution).
>>>>>> >>>>>>>>
>>>>>> >>>>>>>>>
>>>>>> >>>>>>>>> You can have "lost updates"
w/Cassandra. You need to to use 3rd
>>>>>> >>>>>>>>> products - cages for e.g. -
to get ACID type consistency.
>>>>>> >>>>>>>>
>>>>>> >>>>>>>> Then again, you'll have to define
what you are calling "lost
>>>>>> >>>>>>>> updates". Provided you use a reasonable
consistency level,
>>>>>> >>>>>>>> Cassandra
>>>>>> >>>>>>>> provides fairly strong durability
guarantee, so for some
>>>>>> >>>>>>>> definition you
>>>>>> >>>>>>>> don't "lose updates".
>>>>>> >>>>>>>> That being said, I never pretended
that Cassandra provided any
>>>>>> >>>>>>>> ACID
>>>>>> >>>>>>>> guarantee. ACID relates to transaction,
which Cassandra doesn't
>>>>>> >>>>>>>> support. If
>>>>>> >>>>>>>> we're talking about the guarantees
of transaction, then by all
>>>>>> >>>>>>>> means,
>>>>>> >>>>>>>> cassandra won't provide it. And
yes you can use cages or the like
>>>>>> >>>>>>>> to get
>>>>>> >>>>>>>> transaction. But that was not the
point of the thread, was it ?
>>>>>> >>>>>>>> The thread
>>>>>> >>>>>>>> is about vector clocks, and that
has nothing to do with
>>>>>> >>>>>>>> transaction (vector
>>>>>> >>>>>>>> clocks certainly don't give you
transactions).
>>>>>> >>>>>>>> Sorry if I wasn't clear in my mail,
but I was only responding to
>>>>>> >>>>>>>> why
>>>>>> >>>>>>>> so far I don't think vector clocks
would really provide much for
>>>>>> >>>>>>>> Cassandra.
>>>>>> >>>>>>>> --
>>>>>> >>>>>>>> Sylvain
>>>>>> >>>>>>>>
>>>>>> >>>>>>>>>
>>>>>> >>>>>>>>> -JA
>>>>>> >>>>>>>>>
>>>>>> >>>>>>>>> On Thu, Feb 24, 2011 at 7:41
AM, Sylvain Lebresne
>>>>>> >>>>>>>>> <sylvain@datastax.com>
wrote:
>>>>>> >>>>>>>>>>
>>>>>> >>>>>>>>>> On Thu, Feb 24, 2011 at
3:22 AM, Anthony John
>>>>>> >>>>>>>>>> <chirayithaj@gmail.com>
wrote:
>>>>>> >>>>>>>>>>>
>>>>>> >>>>>>>>>>> Apologies : For some
reason my response on the original mail
>>>>>> >>>>>>>>>>> keeps bouncing back,
thus this new one!
>>>>>> >>>>>>>>>>>
>>>>>> >>>>>>>>>>> > From the other
hand, the same article says:
>>>>>> >>>>>>>>>>> > "For conditional
writes to work, the condition must be
>>>>>> >>>>>>>>>>> > evaluated at all
update
>>>>>> >>>>>>>>>>> > sites before the
write can be allowed to succeed."
>>>>>> >>>>>>>>>>> >
>>>>>> >>>>>>>>>>> > This means, that
when doing such an update CL=ALL must be
>>>>>> >>>>>>>>>>> > used
>>>>>> >>>>>>>>>>>
>>>>>> >>>>>>>>>>> Sorry, but I am confused
by that entire thread!
>>>>>> >>>>>>>>>>> Questions:-
>>>>>> >>>>>>>>>>> 1. Does Cassandra implement
any kind of data locking - at any
>>>>>> >>>>>>>>>>> granularity whether
it be row/colF/Col ?
>>>>>> >>>>>>>>>>
>>>>>> >>>>>>>>>> No locking, no.
>>>>>> >>>>>>>>>>
>>>>>> >>>>>>>>>>>
>>>>>> >>>>>>>>>>> 2. If the answer to
1 above is NO! - how does CL ALL prevent
>>>>>> >>>>>>>>>>> conflicts. Concurrent
updates on exactly the same piece of
>>>>>> >>>>>>>>>>> data on different
>>>>>> >>>>>>>>>>> nodes can still mess
each other up, right ?
>>>>>> >>>>>>>>>>
>>>>>> >>>>>>>>>> Not sure why you are taking
CL.ALL specifically. But in any CL,
>>>>>> >>>>>>>>>> updating the same piece
of data means the same column value. In
>>>>>> >>>>>>>>>> that case,
>>>>>> >>>>>>>>>> the resolution rules are
the following:
>>>>>> >>>>>>>>>>   - If the updates have
a different timestamp, keep the one
>>>>>> >>>>>>>>>> with
>>>>>> >>>>>>>>>> the higher timestamp. That
is, the more recent of two updates
>>>>>> >>>>>>>>>> win.
>>>>>> >>>>>>>>>>   - It the timestamps
are the same, then it compares the values
>>>>>> >>>>>>>>>> (byte comparison) and keep
the highest value. This is just to
>>>>>> >>>>>>>>>> break ties in
>>>>>> >>>>>>>>>> a consistent manner.
>>>>>> >>>>>>>>>> So if you do two truly concurrent
updates (that is from two
>>>>>> >>>>>>>>>> place
>>>>>> >>>>>>>>>> at the same instant), then
you'll end with one of the update.
>>>>>> >>>>>>>>>> This is the
>>>>>> >>>>>>>>>> column level.
>>>>>> >>>>>>>>>> However, if that simple
conflict detection/resolution mechanism
>>>>>> >>>>>>>>>> is
>>>>>> >>>>>>>>>> not good enough for some
of your use case and you need to keep
>>>>>> >>>>>>>>>> two
>>>>>> >>>>>>>>>> concurrent updates, it is
easy enough. Just make sure that the
>>>>>> >>>>>>>>>> update don't
>>>>>> >>>>>>>>>> end up in the same column.
This is easily achieved by appending
>>>>>> >>>>>>>>>> some unique
>>>>>> >>>>>>>>>> identifier to the column
name for instance. And when reading,
>>>>>> >>>>>>>>>> do a slice and
>>>>>> >>>>>>>>>> reconcile whatever you get
back with whatever logic make sense.
>>>>>> >>>>>>>>>> If you do
>>>>>> >>>>>>>>>> that, congrats, you've roughly
emulated what vector clocks
>>>>>> >>>>>>>>>> would do. Btw, no
>>>>>> >>>>>>>>>> locking or anything needed.
>>>>>> >>>>>>>>>> In my experience, for most
things the timestamp resolution is
>>>>>> >>>>>>>>>> enough. If the same user
update twice it's profile picture on
>>>>>> >>>>>>>>>> you web site
>>>>>> >>>>>>>>>> at the same microsecond,
it's usually fine to end up with one
>>>>>> >>>>>>>>>> of the two
>>>>>> >>>>>>>>>> pictures. In the rare case
where you need something more
>>>>>> >>>>>>>>>> specific, using the
>>>>>> >>>>>>>>>> cassandra data model usually
solves the problem easily. The
>>>>>> >>>>>>>>>> reason for not
>>>>>> >>>>>>>>>> having vector clocks in
Cassandra is that so far, we haven't
>>>>>> >>>>>>>>>> really found
>>>>>> >>>>>>>>>> much example where it is
no the case.
>>>>>> >>>>>>>>>>
>>>>>> >>>>>>>>>> --
>>>>>> >>>>>>>>>> Sylvain
>>>>>> >>>>>>>>>
>>>>>> >>>>>>>>
>>>>>> >>>>>>>
>>>>>> >>>>>>
>>>>>> >>>>>
>>>>>> >>>>
>>>>>> >>>
>>>>>> >>
>>>>>> >
>>>>>> >
>>>>>
>>>>>
>>>>
>>>
>>>
>>> Just to make a note the "EVENTUAL" in eventual consistency could be a
>>> time that is less then 1ms.
>>>
>>> I have a program that demonstrates that "eventual" means if i write
>>> data at the weakest level, and read it back from a random another node
>>> as soon as possible. 99% I see the update. I can share the code if you
>>> would like.
>>>
>>> Remember http://en.wikipedia.org/wiki/Spacetime
>>> ...but there is no reference frame in which the two events can occur
>>> at the same time...
>>>
>>> As to MongoDB references ....Yes! most of the noSQL work differently.
>>> They each approach CAP
>>> http://www.julianbrowne.com/article/viewer/brewers-cap-theorem in a
>>> different way.
>>>
>>> Cassandra does not lock (it is no secret). But remember, you can not
>>> have it all pick 2/3 from CAP.
>>>
>>
>
>
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support
> http://www.datastax.com
>

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