Critical Description
While doing some digging in the metrics2 system recently, we noticed that the way MutableStat values are collected (and thus MutableRate, since it is based off of MutableStat) mean that every time the value is snapshotted, all previous information is lost. So every time a JMX cache refresh occurs, it resets the MutableStat, meaning that all configured metrics sinks do not consider the previous statistics in their emitted values. The same behavior is true if you configured multiple sink periods.
MutableStat, to compute its average value, maintains a total value since last snapshot, as well as operation count since last snapshot. Upon snapshotting, the average is calculated as (total / opCount) and placed into a gauge metric, and total / operation count are cleared. So the average value represents the average since the last snapshot. If we have only a single sink period ever snapshotting, this would result in the expected behavior that the value is the average over the reporting period. However, if multiple sink periods are configured, or if the JMX cache is refreshed, this is another snapshot operation. So, for example, if you have a FileSink configured at a 60 second interval and your JMX cache refreshes itself 1 second before the FileSink period fires, the values emitted to your FileSink only represent averages over the last one second.
A few ways to solve this issue:
- Make MutableRate manage its own average refresh, similar to MutableQuantiles, which has a refresh thread and saves a snapshot of the last quantile values that it will serve up until the next refresh. Given how many MutableRate metrics there are, a thread per metric is not really feasible, but could be done on e.g. a per-source basis. This has some downsides: if multiple sinks are configured with different periods, what is the right refresh period for the MutableRate?
- Make MutableRate emit two counters, one for total and one for operation count, rather than an average gauge and an operation count counter. The average could then be calculated downstream from this information. This is cumbersome for operators and not backwards compatible. To improve on both of those downsides, we could have it keep the current behavior but additionally emit the total as a counter. The snapshotted average is probably sufficient in the common case (we've been using it for years), and when more guaranteed accuracy is required, the average could be derived from the total and operation count.
The two above suggestions will fix this for both JMX and multiple sink periods, but may be overkill. Multiple sink periods are probably not necessary though we should at least document the behavior.
Open to suggestions & input here.