This patch adds two metrics:
As of 0.8.1, the sizes of the response queues are reported as different metrics - 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.
So this patch exposes the total number of queued responses as a metric ResponseQueueSize
In RequestChannel, create a Gauge that adds up the size of the response queues.
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.
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?)
To calculate that metric, the patch adds up the number of SelectionKeys interested in writing, across processor threads.
Another approach could be to keep all in-flight responses in a data structure (let's say a map) shared by the processor threads. A response will be added to that map when dequeued from the response queue, and removed when the write is complete. The gauge will simply report the size of that map. I decided against that second approach as it is more intrusive and requires some additional bookkeeping to gather information already available through the SelectionKey's