impossible to get a deterministic output for selectors or aggregations

[rdissauer]

System info:
Linux 4.9.0-8-amd64 #1 SMP Debian 4.9.110-3+deb9u6 (2018-10-08) x86_64 GNU/Linux
InfluxDB v1.6.4 (git: 1.6 c75cdfd)

Steps to reproduce:

1. use telegraf to send procstat to influxdb
2. write query to select and group by stats from procstat
   for example:
   SELECT max("pid") FROM "procstat" WHERE ("host" = 'AT1LKAM01S' AND "systemd_unit" = 'kamailio.service') AND time >= now() - 1m group by time(20s) fill(0)

Expected behavior:
time max
2018-10-21T09:42:40Z 20671
2018-10-21T09:43:00Z 20671
2018-10-21T09:43:20Z 20671
2018-10-21T09:43:40Z 20671

Actual behavior:
time max
2018-10-21T09:42:40Z 20671
2018-10-21T09:43:00Z 20671
2018-10-21T09:43:20Z 20671
2018-10-21T09:43:40Z 0

same behaviour for max(), last(), mean(), ...
I also tried various combinations of time frame and group by time() values.

This makes it impossible to do a monitoring based on actual value of time series!

[dgnorton]

@rdissauer looks like there hadn't been any values written into the 2018-10-21T09:43:20Z - 2018-10-21T09:43:40Z window of time that satisfied the host and systemd_unit tag values requested when that query ran. Do you know for sure that there were values already in that window?

[rdissauer]

@dgnorton You're right, the sliding max(), mean(),... functions show a value where there is none in the db because of the fill() function at the end.
What I want to achieve is some monitoring of a Process where the value can be >0 or null (nothing in db). As long as the process is running, telegraf writes pid values to influx but if the process dies there is no more value in db.
The thing is that with last() function I cannot use fill(0) to always get a value and with max(), mean(),... in combination with group by time(20s) and fill(0) I get a value but depending on the sliding group by window I always get 0 which leads to alertings!

Hope I could clarify my issue a bit. Thanks for you help!

[stale]

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