non_negative_difference

[phemmer]

Feature Request

Proposal: [Description of the feature]

Add a non_negative_difference function, similar to non_negative_derivative.

Current behavior: [What currently happens]

No such function.

Desired behavior: [What you would like to happen]

Such a function.

Use case: [Why is this important (helps with prioritizing requests)]

For a counter which overflows and is frequently reset, using difference results in frequent negative values.

Note, this is NOT the same as the feature requested in #7076.

[desa]

@phemmer should it report the absolute value, or 0 if the value is negative?

[phemmer]

Nither, it should drop the value entirely, just like non_negative_derivative does.

[desa]

Ah, makes sense.

[zhexuany]

@jwilder If no one takes this, I would like to devote my time on this feature. What is your opinion? Is this gonna be ok to adding in influxDB?

[jwilder]

@zhexuany Sounds good. Thanks!

[rafaelncarvalho]

It's not better to make an absolute function? This way we could write ABS(DIFFERENCE( )) on query and it could replace the non_negative_derivative too

[phemmer]

@rafaelncarvalho that's not the same. However something like max(difference(foo),0) probably would be.

[zhexuany]

I am so sorry for the delay. I planed to used my Spring Festival break to complete this.

@phemmer Plz give me some examples to precisely describe this function. My understanding is that

Say we have 5, -4, 4, 4, -1, 3, non-negative-difference will return -6. This is correct, if so I start to work on this right away.

[michalrus]

Note, this is NOT the same as the feature requested in #7076.

But it covers the same use case! Please, do it. 🐱

@zhexuany, you might want to take a look at the mentioned #7076 and #7093 for some more context on the use case.

As resetting counters are so common in systems monitoring, I have no idea how InfluxDB could’ve gone so far without this functionality. Compare → https://prometheus.io/docs/querying/functions/#increase

[phemmer]

But it covers the same use case! Please, do it. 🐱

Actually no it doesn't. Read the comments on #7076, this is a completely different feature.

[michalrus]

Ah, right, I wanted totals there. But you can get speed from totals (and I do this with my patch). Will I be able to get totals from your speed in #7393?

[zhexuany]

PR is ready. Please check: #8235.

[e-dard]

Fixed via #8235