Passing calculated values to random_int does not work as expected

[sagikazarmark]

I'm trying to pick a random element from a list. Here is what I tried so far:

let regions = ["us-east-1", "us-west-1", "us-east-2", "us-west-2"]

let region = $regions.index(random_int(max: $regions.length()))

However, this always returns the last item in the list. Checking the output of the call to random_int reveals that it always outputs 4.

Changing $regions.length() to a static number works as expected.

Changing the code to the following also works fine:

let regions = ["us-east-1", "us-west-1", "us-east-2", "us-west-2"]

let region = $regions.index(random_int() % $regions.length())

Not sure if this is a bug or a known limitations. I also wasn't sure how to search the issue tracker for potentially existing issues, my attempts didn't reveal any.

[mihaitodor]

Hey @sagikazarmark that's because the random_int() function has a seed parameter which is set to 0 by default (docs here). Try random_int(seed:timestamp_unix_nano(), max: $regions.length()).

PS: Converting this into a discussion as mentioned in #2026.

[sagikazarmark]

@mihaitodor appreciate the quick answer.

Can you tell me why Whis works?

let region = $regions.index(random_int(max: 4))

It always returns a random number. Also when no params are passed to the function. It only fails in the first case.

[mihaitodor]

Indeed, you're right! I should've thought more about this before answering. Glad @Jeffail chimed in ❤️

[Jeffail]

This is actually kind of a bug, or at least a limitation of bloblang. The problem with your mapping is that the argument for max is dynamic as the evaluation of the variable $regions is redone for each invocation. Since the arguments for the random_int function are dynamic it means the random number generator is recreated for each invocation, so since the seed is static the generator is always created brand new for each mapping execution and that's why you get the same value multiple times.

We could expand the static evaluation of mappings in order to detect static variables, which in the example you posted would fix things as the arguments would no longer be dynamic. However, that wouldn't solve the greater issue of when those arguments aren't capable of being statically evaluated.

The real bug here is that the random number generator is being recreated (and therefore reset) and it's not obvious to you as a mapping author or the config executor why that is happening, or even that it's happening at all.

We could potentially maintain a cache of generators that have been created, since by their nature they're meant to be random I think it'd be reasonable to have a shared cache. As a short term work-around you can implement a max the old fashioned way with random_int() % $regions.length(). Either way the current behaviour is a bug (at the very least in documentation) so I'm going to reopen the original issue.

[sagikazarmark]

Thanks @Jeffail ! I'm comfortable with the old-fashioned solution, I just wanted to raise this with you in case it was a bug (and apparently it is). But I can live with the current solution.

Thanks again for the detailed reply!

[Jeffail]

Can't reopen it now that it's been converted so I've opened #2112