Hi, I’m new to InfluxDB and looking to use it in a realtime monitoring set up.
I am writing data to an InfluxDB measurement that has a tag to identify the instance that the data point it for and its value. Different instances send a data point to the same measurement at times but with the same sample frequency. The value being sent is strictly increasing.
Example data could be:
T, id, value
1, a, 100
2, b, 50
3, a, 150
4, b, 125
5, a, 155
6, b, 180
I’d like to query the sum of values across instances and I would expect a strictly-increasing time series as the result, e.g. for the above sample data:
T, sum(value)
1, 100
2, 150
3, 200
4, 275
5, 280
6, 335
The issue I’m having is that for sufficiently small intervals the resulting time series is not strictly increasing and appears to use 0 instead of the previous value for a tag when the tag doesn’t have a data point in the interval. This ends up with a series that looks like:
T, sum(value)
1, 100
2, 50
3, 150
4, 125
5, 155
6, 180
Concretely, if T above is in minutes, the issue I’m describing can be observed with this query:
SELECT sum(“value”) AS “sum_value” FROM “testdb”.“autogen”.“test_metric” WHERE time > :dashboardTime: AND time < :upperDashboardTime: GROUP BY time(1m) FILL(previous)
Changing the FILL policy seems to have no effect, but previous sounded like the most likely to have the behaviour I want. Changing the time grouping to 2m gives the time series I expect, but with lower resolution than I want.
Is there a way to write a query that carries forward values for tags across intervals?
I can think of a couple of work-arounds but they feel a bit hacky to me:
- Send values more frequently so that I can use the resolution I want in the above query, though in my real-world app this is already a lot of data and I’m not wanting to unnecessarily increase it.
- Write an intermediary service that synchronises the data points and sends the current value for all instances when any of the instances sends a new data point.
Thanks in advance for the advice 
