If you want to connect to a cassandra database with the ruby-filter, you can do it the following way.
All you need for this, is the datastax cassandra ruby driver:
Logstash 2.4.0 works on an embedded jruby1.9 and the driver is compatible for this version,
My plan is to install the driver for the embedded jruby version of Logstash. So i run the following commands:
env GEM_HOME=/opt/logstash/vendor/bundle/jruby/1.9 /opt/logstash/vendor/jruby/bin/jruby /opt/logstash/vendor/jruby/bin/gem install cassandra-driver
I hope i could help somebody, who wants to do that =)