How can I plot the difference value(now) - value(24h ago) in Flux?

InfluxDB 2
, ,
1

Hello,

I have the following Flux query that plots the value read from a sensor over time:

from(bucket: "adeunis")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "mqtt_consumer")
  |> filter(fn: (r) => r["topic"] == "adeunis")
  |> filter(fn: (r) => r["_field"] == "temperatures_0_value")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")

What I would like to do is to write a query that returns the difference between the last read value and a value from yesterday (let’s say yesterday @ 8.00 PM).

Could this be possible using Flux, or should I write a script?

2

Pretty sure this recipe will work for you.

(Untested , but this should help get you started)

CurrentRange = from(bucket: "adeunis")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "mqtt_consumer")
  |> filter(fn: (r) => r["topic"] == "adeunis")
  |> filter(fn: (r) => r["_field"] == "temperatures_0_value")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")


PriorRange = from(bucket: "adeunis")
  |> range(start: v.timeRangeStart - 24h, stop: v.timeRangeStop - 24h)
  |> filter(fn: (r) => r["_measurement"] == "mqtt_consumer")
  |> filter(fn: (r) => r["topic"] == "adeunis")
  |> filter(fn: (r) => r["_field"] == "temperatures_0_value")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")


# then apply the join per the example. Something similar to this but will need tweaking probably 


join(tables: {curr : CurrentRange, prior: PriorRange}, on: ["topic"])
  |> map(fn: (r) => ({r with deviation: r._value_curr - r._value_prior}))

You will probably need to play with the join (eg on topic, and the tabling naming, and/or the map that calculates the difference

3

I will try this. Thanks!

4

Apparently, Flux has the difference() function. This, used with an agreggateWindow(every 24h), works pretty well for me.

1 Like
5

Thanks for posting the final solution you ended up with :+1:

6

What would be the final solution that uses difference() anyway?