rabbit_fifo: Activate next SAC deterministically #14100
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| Iterable = case is_deterministic(Vsn) of | ||
| true -> | ||
| maps:iterator(Map, ordered); | ||
| false -> | ||
| Map | ||
| end, | ||
| maps:fold(Fun, Init, Iterable). | ||
| maps:fold(Fun, Init, iterator(Map, Vsn)). |
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Note that maps:fold/3 creates an iterator with maps:iterator/1 by default, so this is equivalent to the old code which passed a map directly: https://github.com/erlang/otp/blob/d4c78e0414664564ae29d4d6a09bbaf7d2c2110b/lib/stdlib/src/maps.erl#L881-L882
This change extends the changes in 2db4843 (making `rabbit_fifo` deterministic). Determining the next active consumer uses a `maps:iterator()` which has undefined ordering. Like calls to `fold/3` or `keys/1` we should ensure ordering of the iterator so that two replicas are guaranteed to find the same consumer.
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Thank you! :)
I left this part out intentionally in #13971 because there can be only one single active consumer. So, it shouldn't matter in which order the map is iterated? And because there is a little performance benefit if the iteration order is undefined (it seems to be called on the hot path in complete_and_checkout/6), I decided against it.
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Aha I see now, this path is only used for the single-active consumer. I think I see now why @kjnilsson mentioned that using an iterator here is odd 😅. I assume it makes sense performance-wise to find the SAC this way. I'll close this out then - this is already deterministic since there can be only one SAC. Thanks @ansd! |
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Yes 🙂 Thank you though for following because it's always good to double check these different places where map traversal with undefined order in the Ra machine's |
This change extends the changes in #13971 (making
rabbit_fifodeterministic). Determining the next active consumer uses amaps:iterator()which has undefined ordering. Like calls tofold/3orkeys/1we should ensure ordering of the iterator so that two replicas are guaranteed to find the same consumer.