Advanced request tracing in Cassandra 1.2

Cassandra automatically saves trace sessions to the system_traces keyspace for reference, which expires after 24 hours. The system_traces keyspace contains two tables: sessions and events. Cqlsh's rendering of a traced request omits request parameters and event thread, but these can be useful when probabilistic tracing is enabled. Activity is a simple text field that may change in future Cassandra releases. Nodetool settraceprobability allows coordinators to trace a proportion of all requests it handles, which can help identify intermittent query slowness. However, tracing requests requires significant resources and should be used judiciously, starting with a small fraction like 0.001 and increasing only if necessary.