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From Sylvain Lebresne <sylv...@datastax.com>
Subject Re: New Chain for : Does Cassandra use vector clocks
Date Thu, 24 Feb 2011 17:50:42 GMT
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
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

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