目录
目录 1
1. 前言 1
2. slave发起选举 2
3. master响应选举 5
4. 选举示例 5
5. 哈希槽传播方式 6
6. 一次主从切换记录1 6
6.1. 相关参数 6
6.2. 时间点记录 6
6.3. 其它master日志 6
6.4. 其它master日志 7
6.5. slave日志 7
7. 一次主从切换记录2 8
7.1. 相关参数 8
7.2. 时间点记录 8
7.3. 其它master日志 8
7.4. 其它master日志 9
7.5. slave日志 9
8. slave延迟发起选举代码 9
1. 前言
Redis官方原文：https://redis.io/topics/cluster-spec。另外，从Redis-5.0开始，slave已改叫replica，配置项和部分文档及变量已做改名。
Redis集群的主从切换采取选举机制，要求少数服从多数，而参与选举的只能为master，所以只有多数master存活动时才能进行，选举由slave发起。
Redis用了和Raft算法term（任期）类似的的概念，在Redis中叫作epoch（纪元），epoch是一个无符号的64整数，一个节点的epoch从0开始。
如果一个节点接收到的epoch比自己的大，则将自已的epoch更新接收到的epoch（假定为信任网络，无拜占庭将军问题）。
每个master都会在ping和pong消息中广播自己的epoch和所负责的slots位图，slave发起选举时，创建一个新的epoch（增一），epoch的值会持久化到文件nodes.conf中，如（最新epoch值为27，最近一次投票给了27）：
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vars currentEpoch 27 lastVoteEpoch 27 |
2. slave发起选举
只有master为fail状态，slave才会发起选举。但并不是master为fail时立即发起选举，而是延迟下列随机时长，以避免多个slaves同时发起选举（至少延迟0.5秒后才会发起选举）：
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500 milliseconds + random delay between 0 and 500 milliseconds + SLAVE_RANK * 1000 milliseconds |
一个slave发起选举的条件：
1) 它的master为fail状态（非pfail状态）；
2) 它的master至少负责了一个slot；
3) slave和master的复制连接断开时间不超过给定的值（值可配置，目的是确保slave上的数据足够完整，所以运维时不能任由一个slave长时间不可用，需要通过监控将异常的slave及时恢复）。
因过长时间不可用而不能自动切换的slave日志：
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slave过长时间不可用，导致无法自动切换为master 12961:S 06 Jan 2019 19:00:21.969 # Currently unable to failover: Disconnected from master for longer than allowed. Please check the 'cluster-replica-validity-factor' configuration option. |
相关的源代码：
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/* This function is called if we are a slave node and our master serving * a non-zero amount of hash slots is in FAIL state. * * The gaol of this function is: * 1) To check if we are able to perform a failover, is our data updated? * 2) Try to get elected by masters. * 3) Perform the failover informing all the other nodes. */ void clusterHandleSlaveFailover(void) { mstime_t data_age; // 与master断开的时长，单位毫秒 mstime_t auth_age = mstime() - server.cluster->failover_auth_time; int needed_quorum = (server.cluster->size / 2) + 1; int manual_failover = server.cluster->mf_end != 0 && server.cluster->mf_can_start; auth_timeout = server.cluster_node_timeout*2; if (auth_timeout < 2000) auth_timeout = 2000; auth_retry_time = auth_timeout*2; 。。。。。。 /* Set data_age to the number of seconds we are disconnected from * the master. */ if (server.repl_state == REPL_STATE_CONNECTED) { data_age = (mstime_t)(server.unixtime - server.master->lastinteraction) * 1000; } else { data_age = (mstime_t)(server.unixtime - server.repl_down_since) * 1000; } /* Remove the node timeout from the data age as it is fine that we are * disconnected from our master at least for the time it was down to be * flagged as FAIL, that's the baseline. */ if (data_age > server.cluster_node_timeout) data_age -= server.cluster_node_timeout; /* Check if our data is recent enough according to the slave validity * factor configured by the user. * * Check bypassed for manual failovers. */ if (server.cluster_slave_validity_factor && data_age > (((mstime_t)server.repl_ping_slave_period * 1000) + (server.cluster_node_timeout * server.cluster_slave_validity_factor))) { // slave不可用时间过长，导致不能自动切换为master if (!manual_failover) { // 人工切换除外 clusterLogCantFailover(CLUSTER_CANT_FAILOVER_DATA_AGE); return; } } 。。。。。。 /* Ask for votes if needed. */ // failover_auth_sent标记是否已发送过投票消息 if (server.cluster->failover_auth_sent == 0) { server.cluster->currentEpoch++; server.cluster->failover_auth_epoch = server.cluster->currentEpoch; serverLog(LL_WARNING,"Starting a failover election for epoch %llu.", (unsigned long long) server.cluster->currentEpoch); // 给所有节点（包括slaves）发送投票消息FAILOVE_AUTH_REQUEST（请求投票成为master消息），但注意只有master响应该消息 clusterRequestFailoverAuth(); server.cluster->failover_auth_sent = 1; clusterDoBeforeSleep(CLUSTER_TODO_SAVE_CONFIG| CLUSTER_TODO_UPDATE_STATE| CLUSTER_TODO_FSYNC_CONFIG); return; /* Wait for replies. */ } /* Check if we reached the quorum. */ if (server.cluster->failover_auth_count >= needed_quorum) { /* We have the quorum, we can finally failover the master. */ serverLog(LL_WARNING, "Failover election won: I'm the new master."); /* Update my configEpoch to the epoch of the election. */ if (myself->configEpoch < server.cluster->failover_auth_epoch) { myself->configEpoch = server.cluster->failover_auth_epoch; serverLog(LL_WARNING, "configEpoch set to %llu after successful failover", (unsigned long long) myself->configEpoch); } /* Take responsibility for the cluster slots. */ clusterFailoverReplaceYourMaster(); } else { clusterLogCantFailover(CLUSTER_CANT_FAILOVER_WAITING_VOTES); } } |
从上段代码，还可以看到配置项cluster-slave-validity-factor影响slave是否能够切换为master。
发起选举前，slave先给自己的epoch（即currentEpoch）增一，然后请求其它master给自己投票。slave是通过广播FAILOVER_AUTH_REQUEST包给集中的每一个masters。
slave发起投票后，会等待至少两倍NODE_TIMEOUT时长接收投票结果，不管NODE_TIMEOUT何值，也至少会等待2秒。
master接收投票后给slave响应FAILOVER_AUTH_ACK，并且在（NODE_TIMEOUT*2）时间内不会给同一master的其它slave投票。
如果slave收到FAILOVER_AUTH_ACK响应的epoch值小于自己的epoch，则会直接丢弃。一旦slave收到多数master的FAILOVER_AUTH_ACK，则声明自己赢得了选举。
如果slave在两倍的NODE_TIMEOUT时间内（至少2秒）未赢得选举，则放弃本次选举，然后在四倍NODE_TIMEOUT时间（至少4秒）后重新发起选举。
只所以强制延迟至少0.5秒选举，是为确保master的fail状态在整个集群内传开，否则可能只有小部分master知晓，而master只会给处于fail状态的master的slaves投票。如果一个slave的master状态不是fail，则其它master不会给它投票，Redis通过八卦协议（即Gossip协议，也叫谣言协议）传播fail。而在固定延迟上再加一个随机延迟，是为了避免多个slaves同时发起选举。
slave的SLAVE_RANK是一个与master复制数有关的值，具有最新复制时SLAVE_RANK值为0，第二则为1，以此类推。这样可让具有最全数据的slave优先发起选举。当具有更高SLAVE_RANK值的slave如果没有当选，则其它slaves会很快发起选举（至少4秒后）。
在slave赢得选举后，会向集群内的所有节点广播pong，以尽快完成重新配置（体现在node.conf的更新）。当前未能到达的节点，最终也会完成重新配置。
其它节点会发现有两个相同的master负责相同的slots，这时就看哪个master的epoch值更大。
slave成为master后，并不立即服务，而是留了一个时间差。
3. master响应选举
master收到slave的投票请求FAILOVER_AUTH_REQUEST后，只有满足下列条件时，才会响应投票：
1) 对一个epoch，只投票一次；
2) 会拒绝所有更小epoch的投票请求；
3) 不会给小于lastVoteEpoch的epoch投票；
4) master只给master状态为fail的slave投票；
5) 如果slave请求的currentEpoch小于master的currentEpoch，则master忽略该请求，但下列情况例外：
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① 假设master的currentEpoch值为5，lastVoteEpoch值为1（当有选举失败会出现这个情况，亦即currentEpoch值增加了，但因为选举失败，lastVoteEpoch值未变）； ② slave的currentEpoch值为3； ③ slave增一，使用值为4的epoch发起选举，这个时候master会响应epoch值为5，不巧这个响应延迟了； ④ slave重新发起选举，这个时候选举用的epoch值为5（每次发起选举epoch值均需增一），凑巧这个时候原来延迟的响应达到了，这个时候原来延迟的响应被slave认为有效。 |
在master投票后，会用请求中的epoch更新本地的lastVoteEpoch，并持久化到node.conf文件中。master不会参与选择最优的slave，由于最优的slave有最好的SLAVE_RANK，因此最优的slave可相对更快发起选举。
4. 选举示例
假设一个master有A、B和C三个slaves节点，当这个master不可达时：
1) 假设slave A赢得选举成为master；
2) slave A因为网络分区不再可用；
3) slave B赢得选举；
4) slave B因为网络分区不再可用；
5) 网络分区修复，slave A又可用。
B挂了，A又可用。同一时刻，slave C发起选举，试图替代B成为master。由于slave C的master已不可用，所以它能够选举成为master，并将configEpoch值增一。而A将不能成为master，因为C已成为master，并且C的epoch值更大。
5. 哈希槽传播方式
有两种哈希槽（hash slot）传播途径：
1) 心跳消息（Heartbeat messages）。节点在发送ping和pong消息时，总是携带了它所负责（或它的master所负责）的哈希槽信息；
2) 更新消息（UPDATE messages）。由于心跳包还包含了epoch信息，当消息接收者发现心跳包携带的信息陈旧时，会响应更新的信息，这样强迫发送者更新哈希槽。
6. 一次主从切换记录1
测试集群运行在同一个物理机上，cluster-node-timeout值比repl-timeout值大。
6.1. 相关参数
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cluster-slave-validity-factor值为1 cluster-node-timeout值为30000 repl-ping-slave-period值为1 repl-timeout值为10 |
6.2. 时间点记录
master为FAIL之时的1秒左右时间内，即为主从切换之时。
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master A标记fail时间：20:12:55.467 master B标记fail时间：20:12:55.467 master A投票时间：20:12:56.164 master B投票时间：20:12:56.164 slave发起选举时间：20:12:56.160 slave准备发起选举时间：20:12:55.558（延迟579毫秒） slave发现和master心跳超时时间：20:12:32.810（在这之后24秒才发生主从切换） slave收到其它master发来的自己的master为fail时间：20:12:55.467 切换前服务最后一次正常时间：（服务异常约发生在秒）20:12:22/279275 切换后服务恢复正常时间：20:12:59/278149 服务不可用时长：约37秒 |
6.3. 其它master日志
该master ID为c67dc9e02e25f2e6321df8ac2eb4d99789917783。
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30613:M 04 Jan 2019 20:12:55.467 * FAIL message received from bfad383775421b1090eaa7e0b2dcfb3b38455079 about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 // 从其它master收到44eb43e50c101c5f44f48295c42dda878b6cb3e9已fail消息 30613:M 04 Jan 2019 20:12:55.467 # Cluster state changed: fail 30613:M 04 Jan 2019 20:12:56.164 # Failover auth granted to 0ae8b5400d566907a3d8b425d983ac3b7cbd8412 for epoch 30 // 对选举投票 30613:M 04 Jan 2019 20:12:56.204 # Cluster state changed: ok 30613:M 04 Jan 2019 20:12:56.708 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 |
6.4. 其它master日志
该master ID为bfad383775421b1090eaa7e0b2dcfb3b38455079。
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30614:M 04 Jan 2019 20:12:55.467 * Marking node 44eb43e50c101c5f44f48295c42dda878b6cb3e9 as failing (quorum reached). // 标记44eb43e50c101c5f44f48295c42dda878b6cb3e9为已fail 30614:M 04 Jan 2019 20:12:56.164 # Failover auth granted to 0ae8b5400d566907a3d8b425d983ac3b7cbd8412 for epoch 30 // 对选举投票 30614:M 04 Jan 2019 20:12:56.709 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 |
6.5. slave日志
slave的master ID为44eb43e50c101c5f44f48295c42dda878b6cb3e9，slave自己的ID为0ae8b5400d566907a3d8b425d983ac3b7cbd8412。
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30651:S 04 Jan 2019 20:12:32.810 # MASTER timeout: no data nor PING received... // 发现master超时，master异常10秒后发现，原因是repl-timeout的值为10 30651:S 04 Jan 2019 20:12:32.810 # Connection with master lost. 30651:S 04 Jan 2019 20:12:32.810 * Caching the disconnected master state. 30651:S 04 Jan 2019 20:12:32.810 * Connecting to MASTER 1.9.16.9:4073 30651:S 04 Jan 2019 20:12:32.810 * MASTER <-> REPLICA sync started 30651:S 04 Jan 2019 20:12:32.810 * Non blocking connect for SYNC fired the event. 30651:S 04 Jan 2019 20:12:43.834 # Timeout connecting to the MASTER... 30651:S 04 Jan 2019 20:12:43.834 * Connecting to MASTER 1.9.16.9:4073 30651:S 04 Jan 2019 20:12:43.834 * MASTER <-> REPLICA sync started 30651:S 04 Jan 2019 20:12:43.834 * Non blocking connect for SYNC fired the event. 30651:S 04 Jan 2019 20:12:54.856 # Timeout connecting to the MASTER... 30651:S 04 Jan 2019 20:12:54.856 * Connecting to MASTER 1.9.16.9:4073 30651:S 04 Jan 2019 20:12:54.856 * MASTER <-> REPLICA sync started 30651:S 04 Jan 2019 20:12:54.856 * Non blocking connect for SYNC fired the event. 30651:S 04 Jan 2019 20:12:55.467 * FAIL message received from bfad383775421b1090eaa7e0b2dcfb3b38455079 about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 // 从其它master收到自己的master的FAIL消息 30651:S 04 Jan 2019 20:12:55.467 # Cluster state changed: fail 30651:S 04 Jan 2019 20:12:55.558 # Start of election delayed for 579 milliseconds (rank #0, offset 227360). // 准备发起选举，延迟579毫秒，其中500毫秒为固定延迟，279秒为随机延迟，因为RANK值为0，所以RANK延迟为0毫秒 30651:S 04 Jan 2019 20:12:56.160 # Starting a failover election for epoch 30. // 发起选举 30651:S 04 Jan 2019 20:12:56.180 # Failover election won: I'm the new master. // 赢得选举 30651:S 04 Jan 2019 20:12:56.180 # configEpoch set to 30 after successful failover 30651:M 04 Jan 2019 20:12:56.180 # Setting secondary replication ID to 154a9c2319403d610808477dcda3d4bede0f374c, valid up to offset: 227361. New replication ID is 927fb64a420236ee46d39389611ab2d8f6530b6a 30651:M 04 Jan 2019 20:12:56.181 * Discarding previously cached master state. 30651:M 04 Jan 2019 20:12:56.181 # Cluster state changed: ok 30651:M 04 Jan 2019 20:12:56.708 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 // 忽略来自非集群成员1.9.16.9:4077的消息 |
7. 一次主从切换记录2
测试集群运行在同一个物理机上，cluster-node-timeout值比repl-timeout值小。
7.1. 相关参数
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cluster-slave-validity-factor值为1 cluster-node-timeout值为10000 repl-ping-slave-period值为1 repl-timeout值为30 |
7.2. 时间点记录
master为FAIL之时的1秒左右时间内，即为主从切换之时。
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master A标记fail时间：20:37:10.398 master B标记fail时间：20:37:10.398 master A投票时间：20:37:11.084 master B投票时间：20:37:11.085 slave发起选举时间：20:37:11.077 slave准备发起选举时间：20:37:10.475（延迟539毫秒） slave发现和master心跳超时时间：没有发生，因为slave在超时之前已成为master slave收到其它master发来的自己的master为fail时间：20:37:10.398 切换前服务最后一次正常时间：20:36:55/266889（服务异常约发生在56秒） 切换后服务恢复正常时间：20:37:12/265802 服务不可用时长：约17秒 |
7.3. 其它master日志
该master ID为c67dc9e02e25f2e6321df8ac2eb4d99789917783。
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30613:M 04 Jan 2019 20:37:10.398 * Marking node 44eb43e50c101c5f44f48295c42dda878b6cb3e9 as failing (quorum reached). 30613:M 04 Jan 2019 20:37:10.398 # Cluster state changed: fail 30613:M 04 Jan 2019 20:37:11.084 # Failover auth granted to 0ae8b5400d566907a3d8b425d983ac3b7cbd8412 for epoch 32 30613:M 04 Jan 2019 20:37:11.124 # Cluster state changed: ok 30613:M 04 Jan 2019 20:37:17.560 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 |
7.4. 其它master日志
该master ID为bfad383775421b1090eaa7e0b2dcfb3b38455079。
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30614:M 04 Jan 2019 20:37:10.398 * Marking node 44eb43e50c101c5f44f48295c42dda878b6cb3e9 as failing (quorum reached). 30614:M 04 Jan 2019 20:37:11.085 # Failover auth granted to 0ae8b5400d566907a3d8b425d983ac3b7cbd8412 for epoch 32 30614:M 04 Jan 2019 20:37:17.560 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 |
7.5. slave日志
slave的master ID为44eb43e50c101c5f44f48295c42dda878b6cb3e9，slave自己的ID为0ae8b5400d566907a3d8b425d983ac3b7cbd8412。
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30651:S 04 Jan 2019 20:37:10.398 * FAIL message received from c67dc9e02e25f2e6321df8ac2eb4d99789917783 about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 30651:S 04 Jan 2019 20:37:10.398 # Cluster state changed: fail 30651:S 04 Jan 2019 20:37:10.475 # Start of election delayed for 539 milliseconds (rank #0, offset 228620). 30651:S 04 Jan 2019 20:37:11.077 # Starting a failover election for epoch 32. 30651:S 04 Jan 2019 20:37:11.100 # Failover election won: I'm the new master. 30651:S 04 Jan 2019 20:37:11.100 # configEpoch set to 32 after successful failover 30651:M 04 Jan 2019 20:37:11.100 # Setting secondary replication ID to 0cf19d01597610c7933b7ed67c999a631655eafc, valid up to offset: 228621. New replication ID is 53daa7fa265d982aebd3c18c07ed5f178fc3f70b 30651:M 04 Jan 2019 20:37:11.101 # Connection with master lost. 30651:M 04 Jan 2019 20:37:11.101 * Caching the disconnected master state. 30651:M 04 Jan 2019 20:37:11.101 * Discarding previously cached master state. 30651:M 04 Jan 2019 20:37:11.101 # Cluster state changed: ok 30651:M 04 Jan 2019 20:37:17.560 * Ignoring FAIL message from unknown node 082c079149a9915612d21cca8e08c831a4edeade about 44eb43e50c101c5f44f48295c42dda878b6cb3e9 |
8. slave延迟发起选举代码
// 摘自Redis-5.0.3
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// cluster.c /* This function is called if we are a slave node and our master serving * a non-zero amount of hash slots is in FAIL state. * * The gaol of this function is: * 1) To check if we are able to perform a failover, is our data updated? * 2) Try to get elected by masters. * 3) Perform the failover informing all the other nodes. */ void clusterHandleSlaveFailover(void) { 。。。。。。 /* Check if our data is recent enough according to the slave validity * factor configured by the user. * * Check bypassed for manual failovers. */ if (server.cluster_slave_validity_factor && data_age > (((mstime_t)server.repl_ping_slave_period * 1000) + (server.cluster_node_timeout * server.cluster_slave_validity_factor))) { if (!manual_failover) { clusterLogCantFailover(CLUSTER_CANT_FAILOVER_DATA_AGE); return; } } /* If the previous failover attempt timedout and the retry time has * elapsed, we can setup a new one. */ if (auth_age > auth_retry_time) { server.cluster->failover_auth_time = mstime() + 500 + /* Fixed delay of 500 milliseconds, let FAIL msg propagate. */ random() % 500; /* Random delay between 0 and 500 milliseconds. */ server.cluster->failover_auth_count = 0; server.cluster->failover_auth_sent = 0; server.cluster->failover_auth_rank = clusterGetSlaveRank(); /* We add another delay that is proportional to the slave rank. * Specifically 1 second * rank. This way slaves that have a probably * less updated replication offset, are penalized. */ server.cluster->failover_auth_time += server.cluster->failover_auth_rank * 1000; /* However if this is a manual failover, no delay is needed. */ if (server.cluster->mf_end) { server.cluster->failover_auth_time = mstime(); server.cluster->failover_auth_rank = 0; } serverLog(LL_WARNING, "Start of election delayed for %lld milliseconds " "(rank #%d, offset %lld).", server.cluster->failover_auth_time - mstime(), server.cluster->failover_auth_rank, replicationGetSlaveOffset()); /* Now that we have a scheduled election, broadcast our offset * to all the other slaves so that they'll updated their offsets * if our offset is better. */ clusterBroadcastPong(CLUSTER_BROADCAST_LOCAL_SLAVES); return; } 。。。。。。 } |