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Posted to dev@brpc.apache.org by "FancyJan (via GitHub)" <gi...@apache.org> on 2023/10/26 13:18:15 UTC
[I] lalb在有可用节点的情况下仍可能触发“[E112]Fail to select server...” (brpc)
FancyJan opened a new issue, #2427:
URL: https://github.com/apache/brpc/issues/2427
**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
lalb在SelectServer的循环中，如果选到不可用的节点，在一定条件下会继续尝试节点树的右子树：
[https://github.com/apache/brpc/blob/master/src/brpc/policy/locality_aware_load_balancer.cpp#L334](url)
例如某一时刻节点树只有两个节点，0号不可用1号可用。
按照现在的逻辑如果第一次选中了0，那么这次选择会失败，因为0没有可用的右子树，但是节点树里确实有可用的1号节点。如果连续两次（n=2）一开始选到0，那这一次lb会失败。
在有节点探活失败的情况下可能发生，虽然概率不大，但运气不好每次都选中不可用节点的概率确实存在，尤其是在下游节点不多的情况下。
**Describe the solution you'd like (描述你期望的解决方法)**
在单次Select循环中，如果节点不可用即[https://github.com/apache/brpc/blob/master/src/brpc/policy/locality_aware_load_balancer.cpp#L334](url)为false，
那么尝试进入左子树作为兜底，例如：
`else {
dice = butil::fast_rand_less_than(left);
index = index * 2 + 1;
if (index < n) {
continue;
}
}`
**Describe alternatives you've considered (描述你想到的折衷方案)**
lalb在节点数较少时，多尝试几轮Select循环
**Additional context/screenshots (更多上下文/截图)**
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Re: [I] lalb在有可用节点的情况下仍可能触发“[E112]Fail to select server...” (brpc)
Posted by "FancyJan (via GitHub)" <gi...@apache.org>.
FancyJan commented on issue #2427:
URL: https://github.com/apache/brpc/issues/2427#issuecomment-1781122070
近期会提交一个pr
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