2 is more correct. On Fri, Aug 21, 2015 at 11:48 AM, ibrahim El-sanosi < ibrahimsaba...@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 >
