[jira] [Created] (KAFKA-1593) new metrics: ResponseQueueSize and BeingSentResponses

["Alexis Midon (JIRA)" <ji...@apache.org>]

Alexis Midon created KAFKA-1593:
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             Summary: new metrics: ResponseQueueSize and BeingSentResponses
                 Key: KAFKA-1593
                 URL: https://issues.apache.org/jira/browse/KAFKA-1593
             Project: Kafka
          Issue Type: New Feature
          Components: core
            Reporter: Alexis Midon
            Priority: Minor



This patch adds two metrics:

h3. ResponseQueueSize
As of 0.8.1, the sizes of the response queues are [reported as different metrics|https://github.com/apache/kafka/blob/0.8.1/core/src/main/scala/kafka/network/RequestChannel.scala#L127-L134] - one per processor thread. This is not very ideal for different reasons:
* charts have to sum the different metrics
* the metrics collection system might not support 'wild card queries' like {{sum:kafka.network.RequestChannel.Processor_*_ResponseQueueSize}} in which case monitoring now depends on the number of configured network threads
* monitoring the response by thread is not very valuable. However the global number of responses is useful.

* proposal*
So this patch exposes the total number of queued responses as a metric {{ResponseQueueSize}}

*implementation*
In {{RequestChannel}}, create a Gauge that adds up the size of the response queues.


*BeingSentResponses
As of 0.8.1, the processor threads will poll responses from the queues and attach them to the SelectionKey as fast as possible. The consequence of that is that the response queues are not a good indicator of the number of "in-flight" responses. The {{ServerSocketChannel}} acting as another queue of response to be sent.
The current metrics don't reflect the size of this "buffer", which is an issue.

*proposal*
This patch adds a gauge that keeps track of the number of responses being handled by the {{ServerSocketChannel}}.
That new metric is named "BeingSentResponses" (who said naming was hard?)

*implementation*
To calculate that metric, the patch adds up the number of SelectionKeys interested in writing across processor threads.

An another approach could be to keep all in-flight responses in a data structure (let's say a map) shared by the processor thread. A response will be added to that map when dequeued from the response queue, and removed when the write is complete. I decided agains that second approach as it is more intrusive and requires some additional bookkeeping to gather information already available through the {{SelectionKey}}'s





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