Connector is a module which hooks into the query path and extract information for diagnostics. Bytecode instrumentation is used to augment existing Cassandra code with additional functionality. It uses low priority threads to execute the diagnostics information extraction with minimal performance impact to the target code (Cassandra node or application/driver).
Tracing of queries enables reporting the actual queries for some measurement. Currently only module that supports it is Slow Query Module which is reporting queries above some threshold and you can turn on tracing optionally to view actual query which took longer than configured threshold. In future there might be more modules which use tracing to get the idea which query caused some metric (for example module which is reading too many tombstones, or module which is reading row with number of columns above certain threshold).
When LogReporter for Slow Query Module is used (which reports the measurements and tracing information in Cassandra's log), slow query tracing is displayed like this:
INFO [cassandra-diagnostics-connector-0] 2017-03-23 14:23:58,998 LogReporter.java:35 - Measurement SLOW_QUERY [time=1490275438931, value=50.0, tags={host=SmartCat-Inspiron-5559, statementType=SELECT}, fields={sta
tement=select * from typestest where name = ? and choice = ? LIMIT 100, client=/127.0.0.1:58908}]
The slow query, in this example is select * from typestest where name = ? and choice = ? LIMIT 100 and it came from the 127.0.0.1:58908 client. The query is reported because it is above configured slow query treshlod (configuration option: slowQueryThresholdInMilliseconds in SlowQueryModule).
The most important configuration options for the connector are the ones that enable connector to connect to Cassandra JMX:
jmxHost- Node JMX hostjmxPort- Node JMX port
If JMX authentication in Cassandra is enabled, jmxUsername and jmxPassword must also be configured:
jmxAuthEnabled: true
jmxUsername: "username" # actual jmx username
jmxPassword: "password" # actual jmx passwordBeside that, there are other configuration options:
queuedEventsOverflowThreshold- Configured threshold for queue size, above this threshold all events will be dropped until the number of queued events is dropped toqueuedEventsRelaxThreshold.queuedEventsRelaxThreshold- Lower threshold bound for event queue size. After the queue was previously in overflow state, new events will be queued only when the number of queued events drop below this value.enableTracing- Whether to enable tracing or not. It is useful for various modules when debugging. Note that this can impact the performance. The idea is that tracing should be turned on only when needed and turned off once it is not needed anymore.
The connector comes with sensible default values:
connector:
jmxHost: "127.0.0.1" # optional
jmxPort: 7199 # optional
jmxAuthEnabled: false # optional
queuedEventsOverflowThreshold: 1000 # optional
queuedEventsRelaxThreshold: 700 # optional
enableTracing: false # optional