Thanks for your response, i am understanding that the application will
typically send the queries to coordinator nodes,if that coordinator does not
respond within a certain time frame then the application will probably resend
the query to a different coordinator node, under the assumption that the
primary coordinator is no longer alive.
thanks
On Saturday, January 12, 2019, 11:31:40 AM PST, Andy Tolbert
<andrew.tolb...@datastax.com> wrote:
Hi Amit,
a) If queries are submitted to co-ordinator nodes (i assume this includes
writes as well as reads) then: -- is this the approach also followed for the
initial data load?
Writes get sent to all replica nodes, and then the coordinator responds to the
client as soon as enough replicas have responded to achieve the configured
consistency level.
-- some select queries may not have restrictions on all the partition key
columns, and Cassandra would reject such a query, but we we utilize ALLOW
FILTER then the query will execute, since there is no way to determine which
node to send the query to, it will be sent to all the nodes that could
potentially have results. In such a case it would seem that the co-ordinator
would gather the results from all the nodes and return it to the application.
Correct, if the data is on multiple ranges, the coordinator will make queries
to as many replicas needed to cover those ranges and will then gather those
results. Using tracing (blog post) is a good way to get insights into what
replicas are involved in your queries.
Application does not know which nodes may have the data, so it can not directly
send the data to the right nodes. Even if application had the data, it may not
be able to perform load balancing.
Most client drivers have a nice optimization called token-aware load balancing
(i.e. DataStax Java Driver's TokenAwarePolicy), where if the driver is able to
infer which partition is being accessed, it will prioritize coordinators that
have that data. This determination will typically work if all parts of your
partition key are bind parameters in your statement (requirements).
Does the coordinator perform load balancing? I imagine it would have to ...
The coordinator utilizes a dynamic snitch to determine where to route read
queries.
Thanks,Andy
On Sat, Jan 12, 2019 at 9:14 AM amit sehas <cu...@yahoo.com.invalid> wrote:
Thanks for your response, this leads to some further questions:
a) If queries are submitted to co-ordinator nodes (i assume this includes
writes as well as reads) then: -- is this the approach also followed for the
initial data load? -- some select queries may not have restrictions on all
the partition key columns, and Cassandra would reject such a query, but we we
utilize ALLOW FILTER then the query will execute, since there is no way to
determine which node to send the query to, it will be sent to all the nodes
that could potentially have results. In such a case it would seem that the
co-ordinator would gather the results from all the nodes and return it to the
application.
b) This seems as if this is a 3 tier architecture. Application sends query to
coordinator. coordinator sends it to the right nodes.Application does not know
which nodes may have the data, so it can not directly send the data to the
right nodes. Even if application had the data, it may not be able to perform
load balancing. Does the coordinator perform load balancing? I imagine it would
have to ...
thanks
On Saturday, January 12, 2019, 3:32:53 AM PST, Rajesh Kishore
<rajesh10si...@gmail.com> wrote:
Application would send request to one of the node(called as coordinating node)
& this coordinating node is aware of where your result lies(considering you
have modelled your DB correctly, it should not result in scatter& gather kind
of stuff) and thus delegate the query to respective node, so it does follow
client server architecture & your assumption is correct.
As per my knowledge , generally application should be unaware where your result
lies & must not be tied to a specific node because it would have bigger
implications when stuffs like re-balancing would occur. So, your application
should be unaware where your data lies (in which node I meant), but obviously
keeping application in same region as that of cassandra cluster would make
sense, can't comment much on cloud deployment.
Thanks,Rajesh
On Sat, Jan 12, 2019 at 8:54 AM amit sehas <cu...@yahoo.com.invalid> wrote:
I am new to Cassandra, i am wondering how the Cassandra applications are
deployed in the cloud. Does Cassandra have a client server architecture and the
application is deployed as a 3rd tier that sends over queries to the clients,
which then submit them to the Cassandra servers? Or does the application
submit the request directly to any of the Cassandra server which then decides
where the query will be routed to, and then gathers the response and returns
that to the application.
Does the application accessing the data get deployed on the same nodes in the
cloud as the Cassandra cluster itself? Or on separate nodes? Are there any
best practices available in this regard?
thanks
