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apple / swift-nio

Event-driven network application framework for high performance protocol servers & clients, non-blocking.
https://swiftpackageindex.com/apple/swift-nio/documentation
Apache License 2.0
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allegedly, DNS round-robin not working in SwiftNIO (because of getaddrinfo) #2031

Open weissi opened 2 years ago

weissi commented 2 years ago

TIL that apparently, getaddrinfo does not do DNS round-robin'ing because apparently it sorts the results.

If true that would mean that SwiftNIO programs that don't change the default resolver -- which is to say most SwiftNIO programs -- will always pick the same IP address for a given host name.

I didn't check to see if that's true but it seems likely because the Happy Eyeballs code seems to take the IP addresses in order (merging IPv6 and IPv4 addresses).

Lukasa commented 2 years ago

SwiftNIO has never really done round-robin DNS, that's correct. I think this is ultimately a dupe of #536: getaddrinfo is out of scope for us to change, and locally shuffling the DNS results does not necessarily achieve a round-robin effect. We could in principle work around this by shuffling the results of the lookups, but it's not straightforwardly clear that the increased computation effort is worthwhile.

weissi commented 2 years ago

Thanks @Lukasa, that all makes sense. I personally wouldn't dupe it because at least to me it was news that GAI doesn't RR and this might be a problem that someone is actually experiencing.

Regarding the shuffling or not: I'd personally say that shuffling it is worth the effort (especially because it can be done off-EL) but I can understand your position too.