Add Cassandra/Scylla export plugin

[nicolargo]

Description

The goal of this feature request is to export stats to a Cassandra (or Scylla) database.

Data model

Cassandra can store up to 2 billion columns per row

Have a look on the second pattern: https://academy.datastax.com/resources/getting-started-time-series-data-modeling

Resources

Python lib: https://pypi.python.org/pypi/cassandra-driver
Python lib sample: https://github.com/nicolargo/pythonarena/blob/master/cassandra/client01.py

Data model:

• http://www.datastax.com/dev/blog/advanced-time-series-with-cassandra
• http://www.slideshare.net/patrickmcfadin/time-series-with-apache-cassandra-long-version

=> Datamodel TBD: primary key ?

For test bed:

• Scylla in a container: http://www.scylladb.com/doc/getting-started/

[nicolargo]

Testbed: https://github.com/nicolargo/pythonarena/blob/master/cassandra/testbd.ipynb

Implementation started in a dedicated branch.

Pb: 2016-06-09 14:03:51,667 -- ERROR -- Cannot export docker stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value '857c72aeaada691aa6060d27e4b083380b10b33ddc30df5b1af0d4554fd08865' is not of type float")
2016-06-09 14:03:51,670 -- ERROR -- Cannot export ip stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value '80.15.130.7' is not of type float")
2016-06-09 14:03:51,675 -- ERROR -- Cannot export network stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'docker0' is not of type float")
2016-06-09 14:03:51,676 -- ERROR -- Cannot export percpu stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'cpu_number' is not of type float")
2016-06-09 14:03:51,679 -- ERROR -- Cannot export system stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'Linux' is not of type float")
2016-06-09 14:03:51,686 -- ERROR -- Cannot export diskio stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'dm-1' is not of type float")
2016-06-09 14:03:51,687 -- ERROR -- Cannot export fs stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'mnt_point' is not of type float")
2016-06-09 14:03:51,691 -- ERROR -- Cannot export sensors stats to Cassandra (Error from server: code=2200 [Invalid query] message="Invalid map literal for stat: value 'Core 1' is not of type float")

The data model is the following:

session.execute("CREATE TABLE %s (plugin text, time timeuuid, stat map<text,float>, PRIMARY KEY (plugin, time))" % self.table)

==>

If you want untyped data, store blobs. Or store in a different column
(they're free when empty, after all). Type safety is considered a good
thing by many.

==>

String value will not be stored in the database. All numbers (including Boolean) will be stored as float.

[nicolargo]

Done in the latest commit (DEVELOP BRANCH)