rrdcached is a daemon that receives updates to existing RRD files, accumulates them and, if enough have been received or a defined time has passed, writes the updates to the RRD file. A flush command may be used to force writing of values to disk, so that graphing facilities and similar can work with up-to-date data.

rrdcached uses the line protocol to communicate with client. When we start rrdcached , it will listen on port 42217 and create a socket file.  We can telnet to port 42217  or use ncat to create a connection to socket file.

After we connected successfully, we can issue some commands to see what rrdcached is doing.