Thanks for the explanation. My question is
* How the client driver would determine which cassandra node is
considered "local". Is it auto discovered (if so, how?) or manually
specified somewhere?
* Whether local_xxx consistencies always fail when a partition is not
replicated in the local DC, as specified in its replication strategy.
Perhaps I should ask the node.js client authors about this.
On Monday, March 28, 2016 07:47 PM, Eric Stevens wrote:
> Local quorum works in the same data center as the coordinator node,
but when an app server execute the write query, how is the coordinator
node chosen?
It typically depends on the driver, and decent drivers offer you
several options for this, usually called load balancing strategy. You
indicate that you're using the node.js driver (presumably the DataStax
version), which is documented here:
http://docs.datastax.com/en/developer/nodejs-driver/3.0/common/drivers/reference/tuningPolicies.html
I'm not familiar with the node.js driver, but I am familiar with the
Java driver, and since they use the same terminology RE load
balancing, I'll assume they work the same.
A typical way to set that up is to use TokenAware policy with
DCAwareRoundRobinPolicy as its child policy. This will prefer to
route queries to the primary replica (or secondary replica if the
primary is offline) in the local datacenter for that query if it can
be discovered automatically by the driver, such as with prepared
statements.
Where the replica discovery can't be accomplished, TokenAware defers
to the child policy to choose the host. In the case of
DCAwareRoundRobinPolicy that means it iterates through the hosts of
the configured local datacenter (defaulted to the DC of the seed nodes
if they're all in the same DC) for each subsequent execution.
On Fri, Mar 25, 2016 at 2:04 PM X. F. Li <lixf...@gmail.com
<mailto:lixf...@gmail.com>> wrote:
Hello,
Local quorum works in the same data center as the coordinator
node, but
when an app server execute the write query, how is the coordinator
node
chosen?
I use the node.js driver. How do the driver client determine which
cassandra nodes are in the same DC as the client node? Does it use
private network IP [192.168.x.x etc] to auto detect, or must I
manually
provide a localBalancing policy by `new DCAwareRoundRobinPolicy(
localDcName )`?
If a partition is not available in the local DC, i.e. if the local
replica node fails or all replica nodes are in remote DC, will local
quorum fail? If it doesn't fail, there is no guarantee that it all
queries on a partition will be directed to the same data center,
so does
it means strong consistency cannot be expected?
Another question:
Suppose I have replication factor 3. If one of the node fails, will
queries with ALL consistency fail if the queried partition is on the
failed node? Or would they continue to work with 2 replicas during the
time while cassandra is replicating the partitions on the failed
node to
re-establish 3 replicas?
Thank you.
Regards,
X. F. Li
