divijvaidya commented on code in PR #12045:
URL: https://github.com/apache/kafka/pull/12045#discussion_r862906685
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clients/src/main/java/org/apache/kafka/common/metrics/stats/Rate.java:
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@@ -68,28 +63,61 @@ public double measure(MetricConfig config, long now) {
}
public long windowSize(MetricConfig config, long now) {
- // purge old samples before we compute the window size
+ // Purge obsolete samples. Obsolete samples are the ones which are not
relevant to the current calculation
+ // because their creation time is outside (before) the duration of
time window used to calculate rate.
stat.purgeObsoleteSamples(config, now);
/*
* Here we check the total amount of time elapsed since the oldest
non-obsolete window.
- * This give the total windowSize of the batch which is the time used
for Rate computation.
- * However, there is an issue if we do not have sufficient data for
e.g. if only 1 second has elapsed in a 30 second
- * window, the measured rate will be very high.
- * Hence we assume that the elapsed time is always N-1 complete
windows plus whatever fraction of the final window is complete.
+ * This gives the duration of computation time window which used to
calculate Rate.
+ *
+ * For scenarios when rate computation is performed after at least
`config.samples` have completed,
+ * the duration of computation time window is determined by:
+ * window duration = (now - start time of oldest non-obsolete
window)
+ *
+ * ## Special case: First ever window
+ * A special scenario occurs when rate calculation is performed before
at least `config.samples` have completed
+ * (e.g. if only 1 second has elapsed in a 30 second). In such a
scenario, window duration would be equal to the
+ * time elapsed in the current window (since oldest non-obsolete
window is current window). This leads to the
+ * following values for rate. Consider the following example:
+ * config.timeWindowMs() = 1s
+ * config.samples() = 2
+ * Record events (E) at timestamps:
+ * E1 = CurrentTimeStamp (T1)
+ * E2 = T1 + 30ms
+ * E2 = T1 + 60ms
Review Comment:
Fixed. Thanks for point thing this out.
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