Understanding Spring Boot actuator `http.server.requests` metrics MAX attribute - spring-boot

Question

can someone explain what does the MAX statistic refers to in the below response. I don't see it documented anywhere.
localhost:8081/actuator/metrics/http.server.requests?tag=uri:/myControllerMethod
Response:
{
"name":"http.server.requests",
"description":null,
"baseUnit":"milliseconds",
"measurements":[
{
"statistic":"COUNT",
"value":13
},
{
"statistic":"TOTAL_TIME",
"value":57.430899
},
{
"statistic":"MAX",
"value":0
}
],
"availableTags":[
{
"tag":"exception",
"values":[
"None"
]
},
{
"tag":"method",
"values":[
"GET"
]
},
{
"tag":"outcome",
"values":[
"SUCCESS"
]
},
{
"tag":"status",
"values":[
"200"
]
},
{
"tag":"commonTag",
"values":[
"somePrefix"
]
}
]
}

Answer 1

You can see the individual metrics by using ?tag=url:{endpoint_tag} as defined in the response of the root /actuator/metrics/http.server.requests call. The details of the measurements values are;
COUNT: Rate per second for calls.
TOTAL_TIME: The sum of the times recorded. Reported in the monitoring system's base unit of time
MAX: The maximum amount recorded. When this represents a time, it is reported in the monitoring system's base unit of time.
As given here, also here.
The discrepancies you are seeing is due to the presence of a timer. Meaning after some time currently defined MAX value for any tagged metric can be reset back to 0. Can you add some new calls to /myControllerMethod then immediately do a call to /actuator/metrics/http.server.requests to see a non-zero MAX value for given tag?
This is due to the idea behind getting MAX metric for each smaller period. When you are seeing these metrics, you will be able to get an array of MAX values rather than a single value for a long period of time.
You can get to see this in action within Micrometer source code. There is a rotate() method focused on resetting the MAX value to create above described behaviour.
You can see this is called for every poll() call, which is triggered every some period for metric gathering.

Answer 2

What does MAX represent
MAX represents the maximum time taken to execute endpoint.
Analysis for /user/asset/getAllAssets
COUNT TOTAL_TIME MAX
5 115 17
6 122 17 (Execution Time = 122 - 115 = 17)
7 131 17 (Execution Time = 131 - 122 = 17)
8 187 56 (Execution Time = 187 - 131 = 56)
9 204 56 From Now MAX will be 56 (Execution Time = 204 - 187 = 17)
Will MAX be 0 if we have less number of request (or 1 request) to the particular endpoint?
No number of request for particular endPoint does not affect the MAX (see Image from Spring Boot Admin)
When MAX will be 0
There is Timer which set the value 0. When the endpoint is not being called or executed for sometime Timer sets MAX to 0. Here approximate timer value is 2 minutes (120 seconds)
DistributionStatisticConfig has .expiry(Duration.ofMinutes(2)).
which sets some measurements to 0 if there is no request has been made in between expiry time or rotate time.
How I have determined the timer value?
For that, I have taken 6 samples (executed the same endpoint for 6 times). For that, I have determined the time difference between the time of calling the endpoint - time for when MAX set back to zero
More Details
UPDATE
Document has been updated.
NOTE:
Max for basic DistributionSummary implementations such as CumulativeDistributionSummary, StepDistributionSummary is a time
window max (TimeWindowMax).
It means that its value is the maximum value during a time window.
If the time window ends, it'll be reset to 0 and a new time window starts again.
Time window size will be the step size of the meter registry unless expiry in DistributionStatisticConfig is set to other value
explicitly.