为什么创建 Redis 集群时会自动错开主从节点？( 二 ) _Redis

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从上面的代码可以看到，如果得分为 0 ，说明反亲和性已经很好，无需优化。直接跳到 cleanup 去释放 offenders 节点的内存空间。
如果得分不为 0 ，则就会设置一个最大迭代次数maxiter ，这个次数跟节点的数量成正比，然后 while 循环在有限次迭代内进行优化操作。

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这个函数的核心就在 while 循环里，我们来看一下其中的一些片段：
首先 offending_len 来存储违反规则的节点数，然后如果之前有违反规则的节点(offenders != NULL)则释放掉（zfree(offenders)）；
然后重新计算得分，如果得分为0或没有违反规则的节点，退出 while 循环；

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接着去随机选择一个违反规则的节点，尝试交换分配的 master：

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然后遍历集群中的节点，寻找能够交换 master 的 slave 。如果没有找到，那就退出循环：

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如果找到了，就开始交换并计算交换后的反亲和性得分：

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如果交换后的得分比之前的得分还大，说明白交换了，还不如不交换，就会回顾；如果交换后的得分比之前的得分小，说明交换是没毛病的：

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最后释放资源，准备下一次 while 循环：

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总结一下：
每次 while 循环会尝试随机选择一个违反反亲和性规则的从节点，并与另一个随机选中的从节点交换其主节点分配，然后重新计算交换后的反亲和性得分如果交换后的得分变大，说明交换不利于反亲和性，会回滚交换如果交换后得分变小，则保持，后面可能还需要多次交换这样，通过多次随机的交换尝试，最终可以达到更好的反亲和性分布最后则是一些收尾工作，像输出日志信息，释放内存等，这里不过多介绍：

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下面是完整代码：
static void clusterManagerOptimizeAntiAffinity(clusterManagerNodeArray *ipnodes, int ip_count) { clusterManagerNode **offenders = NULL; int score = clusterManagerGetAntiAffinityScore(ipnodes, ip_count, NULL, NULL); if (score == 0) goto cleanup; clusterManagerLogInfo(">>> Trying to optimize slaves allocation " "for anti-affinityn"); int node_len = cluster_manager.nodes->len; int maxiter = 500 * node_len; // Effort is proportional to cluster size... srand(time(NULL)); while (maxiter > 0) { int offending_len = 0; if (offenders != NULL) { zfree(offenders); offenders = NULL; } score = clusterManagerGetAntiAffinityScore(ipnodes, ip_count, &offenders, &offending_len); if (score == 0 || offending_len == 0) break; // Optimal anti affinity reached /* We'll try to randomly swap a slave's assigned master causing * an affinity problem with another random slave, to see if we * can improve the affinity. */ int rand_idx = rand() % offending_len; clusterManagerNode *first = offenders[rand_idx], *second = NULL; clusterManagerNode **other_replicas = zcalloc((node_len - 1) * sizeof(*other_replicas)); int other_replicas_count = 0; listIter li; listNode *ln; listRewind(cluster_manager.nodes, &li); while ((ln = listNext(&li)) != NULL) { clusterManagerNode *n = ln->value; if (n != first && n->replicate != NULL) other_replicas[other_replicas_count++] = n; } if (other_replicas_count == 0) { zfree(other_replicas); break; } rand_idx = rand() % other_replicas_count; second = other_replicas[rand_idx]; char *first_master = first->replicate, *second_master = second->replicate; first->replicate = second_master, first->dirty = 1; second->replicate = first_master, second->dirty = 1; int new_score = clusterManagerGetAntiAffinityScore(ipnodes, ip_count, NULL, NULL); /* If the change actually makes thing worse, revert. Otherwise * leave as it is because the best solution may need a few * combined swaps. */ if (new_score > score) { first->replicate = first_master; second->replicate = second_master; } zfree(other_replicas); maxiter--; } score = clusterManagerGetAntiAffinityScore(ipnodes, ip_count, NULL, NULL); char *msg; int perfect = (score == 0); int log_level = (perfect ? CLUSTER_MANAGER_LOG_LVL_SUCCESS : CLUSTER_MANAGER_LOG_LVL_WARN); if (perfect) msg = "[OK] Perfect anti-affinity obtAIned!"; else if (score >= 10000) msg = ("[WARNING] Some slaves are in the same host as their master"); else msg=("[WARNING] Some slaves of the same master are in the same host"); clusterManagerLog(log_level, "%sn", msg); cleanup: zfree(offenders); }