Hi Jun, The 2 diagrams you are comparing come from versions of Cassandra that are significantly different - 1.2 in the first case and 2.1 in the second case, so it's not surprising that there are differences. since you haven't qualified your question with the Cassandra version you are asking about, I would assume that the 2.1 example is more representative of what you would be likely to see. In any case, it's best to use a consistent version for your documentation because Cassandra changes quite rapidly with many of the releases.
As far as choosing the coordinator node, I don't think there's a way to force it, nor would it be a good idea to do so. In order to make a reasonable selection of coordinators, you would need a lot of internal knowledge about load on the nodes in the cluster and you'd need to also handle certain classes of failures and retries, so you would end up duplicating what is already being done for you internally. Steve On Fri, Jan 29, 2016 at 9:11 AM, Jun Wu <wuxiaomi...@hotmail.com> wrote: > Hi there, > > I have some questions about the replicas selection. > > Let's say that we have 2 data centers: DC1 and DC2, the figure also be > got from link here: > https://docs.datastax.com/en/cassandra/1.2/cassandra/images/write_access_multidc_12.png. > There're > 10 nodes in each data center. We set the replication factor to be 3 and 3 > in each data center, which means there'll be 3 and 3 replicas in each data > center. > > (1) My first question is how to choose which 3 nodes to write data to, > in the link above, the 3 replicas are node 1, 2, 7. But, is there any > mechanism to select these 3? > > (2) Another question is about the remote coordinator, the previous > figure shows that node 10 in DC1 will write data to node 10 in DC 2, then > node 10 in DC2 will write 3 copies to 3 nodes in DC2. > > But, another figure from datastax shows different method, the figure > can be found here, > https://docs.datastax.com/en/cassandra/2.1/cassandra/dml/architectureClientRequestsMultiDCWrites_c.html. > It > shows that node 10 in DC 1 will send directly 3 copies to 3 nodes in DC2, > without using remote coordinator. > > I'm wondering which case is true, because in multiple data center, the > time duration for these two methods varies a lot. > > Also, is there any mechanism to select which node to be remote > coordinator? > > Thanks! > > Jun > -- Steve Robenalt Software Architect sroben...@highwire.org <bza...@highwire.org> (office/cell): 916-505-1785 HighWire Press, Inc. 425 Broadway St, Redwood City, CA 94063 www.highwire.org Technology for Scholarly Communication
