Last, but not least, Cassandra log is a good tool for monitoring what is going on inside Cassandra. However, monitoring the log file is an extremely non-scalable option. Therefore, if you are starting with fewer than five Cassandra machines, you may consider occasionally looking into their log files. The most common use of the Cassandra log is to perform the postmortem for a failure when you do not have any other monitoring and reporting mechanism in place.

The location of the log4j log can be found from Cassandra's conf/log4j-server.properties file:

# This has been altered during installation
log4j.appender.R.File=/mnt/cassandra-logs/system.log

As long as you view this file filled with lines, starting with INFO, you may think the system has been behaving alright. Lines with WARN may or may not be interesting.

For example, it is ok to have some WARN messages in the system, as follows:

INFO [MemoryMeter:1] 2013-05-24 12:31:39,099 Memtable.java (line 213) CFS(Keyspace='Keyspace1'...