SWIM: Adopt Lifeguard's buddy system

[ktoso]

Buddy System replaces SWIM’s piggyback message selector with one that prioritizes notifying a suspected member of the suspicion, to reduce the average time to refutation, which helps both the Local Health Aware Probe and Local Health Aware Suspicion components be even more effective.

https://arxiv.org/pdf/1707.00788.pdf

[avolokhov]

I suddenly realised, we already have buddy system implemented.
From the first sight it might seem that buddy system is about pinging a suspect eagerly, but it's not the case. Buddy system is a workaround to maxGossipCountPerMessage threshold. In original paper, every suspicion is disseminated at most k number of times. If suspect is pinged when this threshold is already reached, target will never know that pinging instance suspects it. But in our implementation, lastKnownStatus ensures that we always send the suspicion to target member. In fact, target takes action on lastKnownStatus only when it's a suspicion, and it only uses it to refute a suspicion.

Technically, there's no action in this issue, we have buddy system by design.
(For the reference, here's "buddy system" in memberlist. All it does is add a payload entry when the target is a suspect: https://github.com/hashicorp/memberlist/pull/85/files)

[ktoso]

I see, excellent :) Only action here then might be adding comments and documentation explaining this 👍