Trace reads at different consistency levels
Running and tracing queries that read data at different consistency levels.
After performing the setup steps, run and trace queries that read data at different consistency levels. The tracing output shows that using three replicas on a five-node cluster, a consistency level of ONE processes responses from one of three replicas, QUORUM from two of three replicas, and ALL from three of three replicas.
Tip: For more information on tracing data, see this post on the DataStax Support Blog,
which explains in detail how to locate data on disk.
Procedure
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On the cqlsh command line, create a keyspace that specifies using three
replicas for data distribution in the cluster.
cqlsh> CREATE KEYSPACE cycling_alt WITH replication = {'class':'SimpleStrategy', 'replication_factor':3}; -
Create a table, and insert some values:
cqlsh> USE cycling_alt; cqlsh> CREATE TABLE cycling_alt.tester ( id int PRIMARY KEY, col1 int, col2 int ); cqlsh> INSERT INTO cycling_alt.tester (id, col1, col2) VALUES (0, 0, 0); -
Turn on tracing and use the CONSISTENCY command to check that the
consistency level is ONE, the default.
cqlsh> TRACING on; cqlsh> CONSISTENCY;The output should be:Current consistency level is 1. -
Query the table to read the value of the primary key.
cqlsh> SELECT * FROM cycling_alt.tester WHERE id = 0;The output includes tracing information:id | col1 | col2 ----+------+------ 0 | 0 | 0 (1 rows) Tracing session: 65bd3150-0109-11e6-8b46-15359862861c activity | timestamp | source | source_elapsed ------------------------------------------------------------------------------+----------------------------+-----------+---------------- Execute CQL3 query | 2016-04-12 16:50:55.461000 | 127.0.0.1 | 0 Parsing SELECT * FROM cycling_alt.tester WHERE id = 0; [SharedPool-Worker-1] | 2016-04-12 16:50:55.462000 | 127.0.0.1 | 276 Preparing statement [SharedPool-Worker-1] | 2016-04-12 16:50:55.462000 | 127.0.0.1 | 509 Executing single-partition query on tester [SharedPool-Worker-2] | 2016-04-12 16:50:55.463000 | 127.0.0.1 | 1019 Acquiring sstable references [SharedPool-Worker-2] | 2016-04-12 16:50:55.463000 | 127.0.0.1 | 1106 Merging memtable contents [SharedPool-Worker-2] | 2016-04-12 16:50:55.463000 | 127.0.0.1 | 1159 Read 1 live and 0 tombstone cells [SharedPool-Worker-2] | 2016-04-12 16:50:55.463000 | 127.0.0.1 | 1372 Request complete | 2016-04-12 16:50:55.462714 | 127.0.0.1 | 1714The tracing results list all the actions taken to complete the
SELECTstatement. -
Change the consistency level to QUORUM to trace what happens during a read with
a QUORUM consistency level.
cqlsh> CONSISTENCY quorum; cqlsh> SELECT * FROM cycling_alt.tester WHERE id = 0;id | col1 | col2 ----+------+------ 0 | 0 | 0 (1 rows) Tracing session: 5e3601f0-0109-11e6-8b46-15359862861c activity | timestamp | source | source_elapsed ------------------------------------------------------------------------------+----------------------------+-----------+---------------- Execute CQL3 query | 2016-04-12 16:50:42.831000 | 127.0.0.1 | 0 Parsing SELECT * FROM cycling_alt.tester WHERE id = 0; [SharedPool-Worker-1] | 2016-04-12 16:50:42.831000 | 127.0.0.1 | 259 Preparing statement [SharedPool-Worker-1] | 2016-04-12 16:50:42.831000 | 127.0.0.1 | 557 Executing single-partition query on tester [SharedPool-Worker-3] | 2016-04-12 16:50:42.832000 | 127.0.0.1 | 1076 Acquiring sstable references [SharedPool-Worker-3] | 2016-04-12 16:50:42.832000 | 127.0.0.1 | 1182 Merging memtable contents [SharedPool-Worker-3] | 2016-04-12 16:50:42.832000 | 127.0.0.1 | 1268 Read 1 live and 0 tombstone cells [SharedPool-Worker-3] | 2016-04-12 16:50:42.832000 | 127.0.0.1 | 1632 Request complete | 2016-04-12 16:50:42.832887 | 127.0.0.1 | 1887 -
Change the consistency level to ALL and run the SELECT statement again.
cqlsh> CONSISTENCY ALL; cqlsh> SELECT * FROM cycling_alt.tester WHERE id = 0;id | col1 | col2 ----+------+------ 0 | 0 | 0 (1 rows) Tracing session: 6c4678b0-0109-11e6-8b46-15359862861c activity | timestamp | source | source_elapsed ------------------------------------------------------------------------------+----------------------------+-----------+---------------- Execute CQL3 query | 2016-04-12 16:51:06.427000 | 127.0.0.1 | 0 Parsing SELECT * FROM cycling_alt.tester WHERE id = 0; [SharedPool-Worker-1] | 2016-04-12 16:51:06.427000 | 127.0.0.1 | 324 Preparing statement [SharedPool-Worker-1] | 2016-04-12 16:51:06.427000 | 127.0.0.1 | 524 Read-repair DC_LOCAL [SharedPool-Worker-1] | 2016-04-12 16:51:06.428000 | 127.0.0.1 | 1016 Executing single-partition query on tester [SharedPool-Worker-3] | 2016-04-12 16:51:06.428000 | 127.0.0.1 | 1793 Acquiring sstable references [SharedPool-Worker-3] | 2016-04-12 16:51:06.429000 | 127.0.0.1 | 1886 Merging memtable contents [SharedPool-Worker-3] | 2016-04-12 16:51:06.429000 | 127.0.0.1 | 1951 Read 1 live and 0 tombstone cells [SharedPool-Worker-3] | 2016-04-12 16:51:06.429000 | 127.0.0.1 | 2176 Request complete | 2016-04-12 16:51:06.429391 | 127.0.0.1 | 2391