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From "Laing, Michael" <michael.la...@nytimes.com>
Subject Re: Write request in Cassandra?
Date Fri, 21 Aug 2015 15:53:51 GMT
2 is more correct.

On Fri, Aug 21, 2015 at 11:48 AM, ibrahim El-sanosi <
ibrahimsabattt@gmail.com> wrote:

> Dear folks,
>
>
> I have doubt on how Cassandra performs a write request; I have two
> scenarios, please read them and ensure which one is correct?
>
>
> Assume we have cluster consists of 4 nodes N1, N2, N3, and N4. As
> Cassandra distributes the nodes in ring topology, the nodes links as
> following:
>
> N-->N-->N3-->N4-->N1
>
> Also we have replication factor equal to 3 (RF=3), and consistency level
> equals to ALL (CL=ALL).
>
> Client sends write request, W, to coordinator, say N4. The partitioner has
> determined the primary node of W is N1.
>
> What will happen now?
>
>
> *Scenario 1**:* coordinator sends W to N1. Upon receiving W, N1 stores it
> locally (in commitLog and memtable, *please forget about internal process*)
> and acknowledges the coordinator N4. Then N1 sends a copy of W to N2
> (because N2 is next node in ring from N1 prospective). Upon receiving W, N2
> stores it locally and sends acknowledgement to N4. Then N2 sends a copy of
> W to N3 (because N3 is next node in ring from N2 prospective). Upon
> receiving W, N3 stores it locally and acknowledges the Coordinator N4.
> Finally as soon as coordinator, N4, receives an acknowledgement from all
> nodes (N1, N2, and N3), it replays to the client.
>
> Note that: if scenario 1 correct, then the latency will be 4 rounds (N4-->
> N1-->N2-->N3-->N4 ----client).
>
>
>
> *Scenario 2:*  coordinator, N4, broadcasts W to N1, N2, and N3 (N4-->N1,
> N4-->N2, N4-->N3). Then replicas (N1, N2, and N3) store W locally and
> acknowledge to N4.  When N4 receives all ACKs, it replays to client.
>
>
>
> Can anyone confirm which scenario is correct in Cassandra?
>
>
>
> Regards?
>
>
>
> Ibrahim
>

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