This patch adds the implementation for begin and finish local sampling
of a column family.
There is a difference in the implementation of Cassandra API and Scylla.
In Cassandra and the JMX an external source start and stop the sampling.
In Scylla, a single API call start the sampling and return with the
result. In Scylla the API call always return sampling of the read and of
the writes.
To bridge the difference, the begin sampling command will use a Future
when calling the API. The finish method will wait for the future to end.
Because of the different implementation, it is possible that two
consecutive calls will be made to start sampling one for the read and
one for the write, similarly, two calls will be made to finish for read
and write.
The implementation would ignore the second call to start and will
store the result, so the second call to finish will be served from the
stored result.
Note, that the use of future is only for safety, the way we expect it to
work, the caller to the begin sampling will sleep anyhow while waiting
for the result.
To avoid breaking the MBean compatibility we piggyback the duration on
top of the sampler string.
If no duration is given, a default duration will be taken, this is also
just as a precaution, we will modify the nodetool implementation to
pass that information.
There is a known issue with cardinality, that will need to be addressed.
Also we return a value in the raw column to match what Cassandra JMX
returns, but it's a duplication of the partition key.
See scylladb/scylla#2811
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
Message-Id: <20190128143505.5241-1-amnon@scylladb.com>
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