Ceilometer/AlarmEvaluatorJuno

Statistics over sliding windows
Two approaches to improve the alarm evaluator were discussed at the summit.

Increment/decrement approach


Statistics where increments/decrements are possible:
 * count,
 * sum,
 * average,
 * stddev

Statistics with special implementation:
 * min/max : needs O(log n) space (probably in DB), where n = window size
 * percentile : classical statistics query
 * cardinality : approximate algorithm, extremely efficient

Map-reduce approach covers all these statistics.