cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Narendra Sharma <narendra.sha...@gmail.com>
Subject Re: How does Cassandra handle failure during synchronous writes
Date Thu, 24 Feb 2011 00:47:57 GMT
Remember the simple rule. Column with highest timestamp is the one that will
be considered correct EVENTUALLY. So consider following case:

Cluster size = 3 (say node1, node2 and node3), RF = 3, Read/Write CL =
QUORUM
a. QUORUM in this case requires 2 nodes. Write failed with successful write
to only 1 node say node1.
b. Read with CL = QUORUM. If read hits node2 and node3, old data will be
returned with read repair triggered in background. On next read you will get
the data that was written to node1.
c. Read with CL = QUORUM. If read hits node1 and node2/node3, new data that
was written to node1 will be returned.

HTH!

Thanks,
Naren


On Wed, Feb 23, 2011 at 3:36 PM, Ritesh Tijoriwala <
tijoriwala.ritesh@gmail.com> wrote:

> Hi Anthony,
> I am not talking about the case of CL ANY. I am talking about the case
> where your consistency level is  R + W > N and you want to write to W nodes
> but only succeed in writing to X ( where X < W) nodes and hence fail the
> write to the client.
>
> thanks,
> Ritesh
>
> On Wed, Feb 23, 2011 at 2:48 PM, Anthony John <chirayithaj@gmail.com>wrote:
>
>> Ritesh,
>>
>> At CL ANY - if all endpoints are down - a HH is written. And it is a
>> successful write - not a failed write.
>>
>> Now that does not guarantee a READ of the value just written - but that is
>> a risk that you take when you use the ANY CL!
>>
>> HTH,
>>
>> -JA
>>
>>
>> On Wed, Feb 23, 2011 at 4:40 PM, Ritesh Tijoriwala <
>> tijoriwala.ritesh@gmail.com> wrote:
>>
>>> hi Anthony,
>>> While you stated the facts right, I don't see how it relates to the
>>> question I ask. Can you elaborate specifically what happens in the case I
>>> mentioned above to Dave?
>>>
>>> thanks,
>>> Ritesh
>>>
>>>
>>> On Wed, Feb 23, 2011 at 1:57 PM, Anthony John <chirayithaj@gmail.com>wrote:
>>>
>>>> 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
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

Mime
View raw message