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From Anthony John <chirayit...@gmail.com>
Subject Re: How does Cassandra handle failure during synchronous writes
Date Wed, 23 Feb 2011 21:57:50 GMT
Seems to me that the explanations are getting incredibly complicated - while
I submit the real issue is not!

Salient points here:-
1. To be guaranteed data consistency - the writes and reads have to be at
Quorum CL or more
2. Any W/R at lesser CL means that the application has to handle the
inconsistency, or has to be tolerant of it
3. Writing at "ANY" CL - a special case - means that writes will always go
through (as long as any node is up), even if the destination nodes are not
up. This is done via hinted handoff. But this can result in inconsistent
reads, and yes that is a problem but refer to pt-2 above
4. At QUORUM CL R/W - after Quorum is met, hinted handoffs are used to
handle that case where a particular node is down and the write needs to be
replicated to it. But this will not cause inconsistent R as the hinted
handoff (in this case) only applies after Quorum is met - so a Quorum R is
not dependent on the down node being up, and having got the hint.

Hope I state this appropriately!

HTH,

-JA

On Wed, Feb 23, 2011 at 3:39 PM, Ritesh Tijoriwala <
tijoriwala.ritesh@gmail.com> wrote:

> > Read repair will probably occur at that point (depending on your config),
> which would cause the newest value to propagate to more replicas.
>
> Is the newest value the "quorum" value which means it is the old value that
> will be written back to the nodes having "newer non-quorum" value or the
> newest value is the real new value? :) If later, than this seems kind of odd
> to me and how it will be useful to any application. A bug?
>
> Thanks,
> Ritesh
>
>
> On Wed, Feb 23, 2011 at 12:43 PM, Dave Revell <dave@meebo-inc.com> wrote:
>
>> Ritesh,
>>
>> You have seen the problem. Clients may read the newly written value even
>> though the client performing the write saw it as a failure. When the client
>> reads, it will use the correct number of replicas for the chosen CL, then
>> return the newest value seen at any replica. This "newest value" could be
>> the result of a failed write.
>>
>> Read repair will probably occur at that point (depending on your config),
>> which would cause the newest value to propagate to more replicas.
>>
>> R+W>N guarantees serial order of operations: any read at CL=R that occurs
>> after a write at CL=W will observe the write. I don't think this property is
>> relevant to your current question, though.
>>
>> Cassandra has no mechanism to "roll back" the partial write, other than to
>> simply write again. This may also fail.
>>
>> Best,
>> Dave
>>
>>
>> On Wed, Feb 23, 2011 at 10:12 AM, <tijoriwala.ritesh@gmail.com> wrote:
>>
>>> Hi Dave,
>>> Thanks for your input. In the steps you mention, what happens when client
>>> tries to read the value at step 6? Is it possible that the client may see
>>> the new value? My understanding was if R + W > N, then client will not see
>>> the new value as Quorum nodes will not agree on the new value. If that is
>>> the case, then its alright to return failure to the client. However, if not,
>>> then it is difficult to program as after every failure, you as an client are
>>> not sure if failure is a pseudo failure with some side effects or real
>>> failure.
>>>
>>> Thanks,
>>> Ritesh
>>>
>>> <quote author='Dave Revell'>
>>>
>>> Ritesh,
>>>
>>> There is no commit protocol. Writes may be persisted on some replicas
>>> even
>>> though the quorum fails. Here's a sequence of events that shows the
>>> "problem:"
>>>
>>> 1. Some replica R fails, but recently, so its failure has not yet been
>>> detected
>>> 2. A client writes with consistency > 1
>>> 3. The write goes to all replicas, all replicas except R persist the
>>> write
>>> to disk
>>> 4. Replica R never responds
>>> 5. Failure is returned to the client, but the new value is still in the
>>> cluster, on all replicas except R.
>>>
>>> Something very similar could happen for CL QUORUM.
>>>
>>> This is a conscious design decision because a commit protocol would
>>> constitute tight coupling between nodes, which goes against the Cassandra
>>> philosophy. But unfortunately you do have to write your app with this
>>> case
>>> in mind.
>>>
>>> Best,
>>> Dave
>>>
>>> On Tue, Feb 22, 2011 at 8:22 PM, tijoriwala.ritesh <
>>> tijoriwala.ritesh@gmail.com> wrote:
>>>
>>> >
>>> > Hi,
>>> > I wanted to get details on how does cassandra do synchronous writes to
>>> W
>>> > replicas (out of N)? Does it do a 2PC? If not, how does it deal with
>>> > failures of of nodes before it gets to write to W replicas? If the
>>> > orchestrating node cannot write to W nodes successfully, I guess it
>>> will
>>> > fail the write operation but what happens to the completed writes on X
>>> (W
>>> > >
>>> > X) nodes?
>>> >
>>> > Thanks,
>>> > Ritesh
>>> > --
>>> > View this message in context:
>>> >
>>> http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/How-does-Cassandra-handle-failure-during-synchronous-writes-tp6055152p6055152.html
>>> > Sent from the cassandra-user@incubator.apache.org mailing list archive
>>> at
>>> > Nabble.com.
>>> >
>>>
>>> </quote>
>>> Quoted from:
>>>
>>> http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/How-does-Cassandra-handle-failure-during-synchronous-writes-tp6055152p6055408.html
>>>
>>
>>
>

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