Understanding read from a single partition in Cassandra - cassandra

Question

I have a 3 node setup, Node1 (172.30.56.60), Node2 (172.30.56.61) and Node3 (172.30.56.62),
It has the single partition data of 100K, the partition is framed by nodeip.
Please find the token / partition value for the nodeip - 172.30.56.60
cqlsh:qnapstat> SELECT token(nodeip) FROM nodedata WHERE nodeip = '172.30.56.60' LIMIT 5;
system.token(nodeip)
----------------------
222567180698744628
222567180698744628
222567180698744628
222567180698744628
222567180698744628
As per the ./nodetool ring value provided below, '172.30.56.60' only will return the data to the coordinator since the value from 173960939250606057 to 239923324758894350 is handled bu the node 172.30.56.60. Note : This is my understanding
172.30.56.60 rack1 Up Normal 32.72 MiB 100.00% 173960939250606057
172.30.56.62 rack1 Up Normal 32.88 MiB 100.00% 239923324758894351
172.30.56.61 rack1 Up Normal 32.84 MiB 100.00% 253117576269706963
172.30.56.60 rack1 Up Normal 32.72 MiB 100.00% 273249439554531014
172.30.56.61 rack1 Up Normal 32.84 MiB 100.00% 295635292275517104
172.30.56.62 rack1 Up Normal 32.88 MiB 100.00% 301162927966816823
I have two questions here,
1) When I try to execute the following query, Does it mean that Coordinator (say 172.30.56.61) reads all the data from the 172.30.56.60?
2) Is that after receiving all the 100 K entries in the coordinator, Coordinator will perform the aggregation for 100K, If so does it keeps all 100K entries in memory in 172.30.56.61?
SELECT Max(readiops) FROM nodedata WHERE nodeip = '172.30.56.60';

Answer 1

There is nice tool called CQL TRACING that can help you understand and see the flow of events once a SELECT query is executed.
cqlsh> INSERT INTO test.nodedata (nodeip, readiops) VALUES (1, 10);
cqlsh> INSERT INTO test.nodedata (nodeip, readiops) VALUES (1, 20);
cqlsh> INSERT INTO test.nodedata (nodeip, readiops) VALUES (1, 30);
cqlsh> select * from test.nodedata ;
nodeip | readiops
--------+-----------
1 | 10
1 | 20
1 | 30
(3 rows)
cqlsh> SELECT MAX(readiops) FROM test.nodedata WHERE nodeip = 1;
system.max(readiops)
-----------------------
30
(1 rows)
Now let's set cqlsh> TRACING ON and run the same query again.
cqlsh> TRACING ON
Now Tracing is enabled
cqlsh> SELECT MAX(readiops) FROM test.nodedata WHERE nodeip = 1;
system.max(readiops)
----------------------
30
(1 rows)
Tracing session: 4d7bf970-eada-11e7-a79d-000000000003
activity | timestamp | source | source_elapsed
-----------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------+--------------+----------------
Execute CQL3 query | 2017-12-27 07:48:44.404000 | 172.16.0.128 | 0
read_data: message received from /172.16.0.128 [shard 4] | 2017-12-27 07:48:44.385109 | 172.16.0.48 | 9
read_data handling is done, sending a response to /172.16.0.128 [shard 4] | 2017-12-27 07:48:44.385322 | 172.16.0.48 | 222
Parsing a statement [shard 1] | 2017-12-27 07:48:44.404821 | 172.16.0.128 | --
Processing a statement [shard 1] | 2017-12-27 07:48:44.404913 | 172.16.0.128 | 93
Creating read executor for token 6292367497774912474 with all: {172.16.0.128, 172.16.0.48, 172.16.0.115} targets: {172.16.0.48} repair decision: NONE [shard 1] | 2017-12-27 07:48:44.404966 | 172.16.0.128 | 146
read_data: sending a message to /172.16.0.48 [shard 1] | 2017-12-27 07:48:44.404972 | 172.16.0.128 | 152
read_data: got response from /172.16.0.48 [shard 1] | 2017-12-27 07:48:44.405497 | 172.16.0.128 | 676
Done processing - preparing a result [shard 1] | 2017-12-27 07:48:44.405535 | 172.16.0.128 | 715
Request complete | 2017-12-27 07:48:44.404722 | 172.16.0.128 | 722
As for your questions:
The Coordinator passes the query to the replica, if RF = 1 or (RF > 1 and CL=ONE), than it will receive the reply from 1 replica, but if (RF > 1 and CL > 1), than it needs to receive replies from multiple replicas and compare the answers, so there's also orchestration done on the Coordinator side.
The way it is actually done is a data request to the fastest replica (using the snitch) and a digest request to the other replicas needed to satisfy the CL.
And then the coordinator need to hash the responses from the data and digest requests and compare them.
If the partition is hashed into a specific node, it will reside in that node (assuming RF=1) and information will be read only from that node.
The Client sends with the query the page size, so the reply itself is returned in bulks (default=5000), which can be set from the client side.
I recommend watching this youtube clip on Cassandra read path for more details.