srdo commented on code in PR #17139:
URL: https://github.com/apache/kafka/pull/17139#discussion_r1890122551
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clients/src/test/java/org/apache/kafka/clients/consumer/KafkaConsumerTest.java:
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@@ -3098,31 +3098,53 @@ public void testPollIdleRatio(GroupProtocol
groupProtocol) {
assertEquals(Double.NaN,
consumer.metrics().get(pollIdleRatio).metricValue());
// 1st poll
- // Spend 50ms in poll so value = 1.0
+ // Spend 50ms in poll. value=NaN because "the fraction of time spent
inside poll" is undefined until the polling interval has an end point,
+ // which is only known once the second poll starts.
+ // This also means the metric will always exclude the latest poll,
since we don't know how much time is spent outside poll for that interval
+ // until poll is called again
Review Comment:
I can change this, but the consequence is that we will exclude the first
poll rather than the last poll.
The issue is essentially that this metric is measuring a property of the
interval between two polls, which means you're going to have to either exclude
either the first or the last poll depending on whether you choose to start
intervals at the start or end of the poll.
If you define the poll interval as being from start to start (as I did
here), that means the last poll is excluded because until the poll after that
begins, the interval for the last poll has no endpoint.
If you define the poll interval as being from end to end (as you suggest),
the first poll is excluded, because there is no starting point for that
interval.
Here's a small illustration:
Picking the start time of the poll call as the boundary of the interval, the
poll intervals for 3 sequential completed polls look like this:
```
[start1, start2], [start2, start3], [start3, ?].
```
By using these boundaries, each poll interval starts when poll is called,
and ends when the subsequent poll is called. The time you spend outside of poll
in an interval is the time _after_ poll returns.
The third interval has an unknown boundary because that boundary point will
be the start time of the fourth poll, and that poll hasn't started yet. This
means we have to wait to record that last interval until then, which means the
last poll is not included in the metric.
If we instead pick the end time as the boundary, we get these intervals:
```
[?, end1], [end1, end2], [end2, end3]
```
By using these boundaries, each poll interval starts when the previous poll
returns, and ends when poll returns. The time you spend outside of poll in a
specific interval is the time _before_ you called poll.
The first interval has an unknown boundary, because there is no "previous
poll" to get an end point from, and so that interval will be excluded from the
metric.
I don't know which is preferable, they seem pretty equal to me. Do you have
an opinion @ahuang98?
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