结论：
这种情况下复制节点（即从节点）无法提升为主节点，复制节点会一直尝试和主节点建立连接，直接成功。主节点恢复后，复制节点仍然保持为复制节点，并不会成为主节点。
复制节点无法提升为主节点的原因是复制节点未发起成为主节点的选举。
复制节点日志：
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14304:S 26 Mar 2019 15:42:01.158 * Connecting to MASTER 10.49.126.98:4076 14304:S 26 Mar 2019 15:42:01.158 * MASTER <-> REPLICA sync started 14304:S 26 Mar 2019 15:42:01.158 # Error condition on socket for SYNC: Connection refused 14304:S 26 Mar 2019 15:42:02.161 * Connecting to MASTER 10.49.126.98:4076 14304:S 26 Mar 2019 15:42:02.161 * MASTER <-> REPLICA sync started 14304:S 26 Mar 2019 15:42:02.161 # Error condition on socket for SYNC: Connection refused 14304:S 26 Mar 2019 15:42:03.167 * Connecting to MASTER 10.49.126.98:4076 14304:S 26 Mar 2019 15:42:03.167 * MASTER <-> REPLICA sync started 14304:S 26 Mar 2019 15:42:03.167 * Non blocking connect for SYNC fired the event. 主节点正在从磁盘加载数据集（-LOADING）到内存 14304:S 26 Mar 2019 15:42:03.173 # Error reply to PING from master: '-LOADING Redis is loading the dataset in memory' 14304:S 26 Mar 2019 15:42:03.770 * Clear FAIL state for node c67dc9e02e25f2e6321df8ac2eb4d99789917783: is reachable again and nobody is serving its slots after some time. 集群状态恢复正常（之前因为其中一个master故障转为fail状态） 14304:S 26 Mar 2019 15:42:03.770 # Cluster state changed: ok 14304:S 26 Mar 2019 15:42:04.169 * Connecting to MASTER 10.49.126.98:4076 14304:S 26 Mar 2019 15:42:04.169 * MASTER <-> REPLICA sync started 14304:S 26 Mar 2019 15:42:04.169 * Non blocking connect for SYNC fired the event. 14304:S 26 Mar 2019 15:42:04.169 * Master replied to PING, replication can continue... 14304:S 26 Mar 2019 15:42:04.169 * Trying a partial resynchronization (request 725b1fbcfc073eec81837cb0f1fd786c995f4d46:1). 复制节点全量复制主节点数据 14304:S 26 Mar 2019 15:42:04.174 * Full resync from master: 68ef812d5b3dc70adca8c6ed0f306249725df91f:0 因为是全量复制，所以原来的状态没用了（Discarding） 14304:S 26 Mar 2019 15:42:04.174 * Discarding previously cached master state. 14304:S 26 Mar 2019 15:42:04.275 * MASTER <-> REPLICA sync: receiving 106404 bytes from master 14304:S 26 Mar 2019 15:42:04.275 * MASTER <-> REPLICA sync: Flushing old data 14304:S 26 Mar 2019 15:42:04.275 * MASTER <-> REPLICA sync: Loading DB in memory 14304:S 26 Mar 2019 15:42:04.292 * MASTER <-> REPLICA sync: Finished with success 复制节点开始重新AOF文件 14304:S 26 Mar 2019 15:42:04.293 * Background append only file rewriting started by pid 21172 14304:S 26 Mar 2019 15:42:04.325 * AOF rewrite child asks to stop sending diffs. 21172:C 26 Mar 2019 15:42:04.326 * Parent agreed to stop sending diffs. Finalizing AOF... 21172:C 26 Mar 2019 15:42:04.326 * Concatenating 0.00 MB of AOF diff received from parent. 21172:C 26 Mar 2019 15:42:04.326 * SYNC append only file rewrite performed 21172:C 26 Mar 2019 15:42:04.326 * AOF rewrite: 0 MB of memory used by copy-on-write 14304:S 26 Mar 2019 15:42:04.370 * Background AOF rewrite terminated with success 14304:S 26 Mar 2019 15:42:04.370 * Residual parent diff successfully flushed to the rewritten AOF (0.00 MB) 14304:S 26 Mar 2019 15:42:04.370 * Background AOF rewrite finished successfully |
在主节点未恢复之前，从节点无法提供读写服务，即使设置了READONLY：
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127.0.0.1:4071> get k4156 (error) CLUSTERDOWN The cluster is down 127.0.0.1:4071> readonly OK 127.0.0.1:4071> get k4156 (error) CLUSTERDOWN The cluster is down |
虽然执行SCAN可看到数据：
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127.0.0.1:4071> scan 0 1) "10752" 2) 1) "k5948" 2) "k4156" 3) "k12819" 4) "k24497" 5) "k5926" 6) "k10947" 7) "k7653" 8) "k21631" 9) "k6672" 10) "k2687" 11) "k29036" |
如果主节点永久无法恢复，那么怎么恢复集群？
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127.0.0.1:4071> CLUSTER FAILOVER (error) ERR Master is down or failed, please use CLUSTER FAILOVER FORCE |
也就是这种情况下，只能强制恢复（丢失数据和数据不一风险），这个时候复制节点日志变化如下：
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1021:S 26 Mar 2019 16:02:04.994 * Connecting to MASTER 10.49.126.98:4076 1021:S 26 Mar 2019 16:02:04.994 * MASTER <-> REPLICA sync started 未强制恢复之前 1021:S 26 Mar 2019 16:02:04.995 # Error condition on socket for SYNC: Connection refused 强制恢复 1021:S 26 Mar 2019 16:02:05.842 # Forced failover user request accepted. 准备发起选举（随机延迟后发起） 1021:S 26 Mar 2019 16:02:05.896 # Start of election delayed for 0 milliseconds (rank #0, offset 0). 1021:S 26 Mar 2019 16:02:05.996 * Connecting to MASTER 10.49.126.98:4076 1021:S 26 Mar 2019 16:02:05.996 * MASTER <-> REPLICA sync started 正式发起选举（任期号为34） 1021:S 26 Mar 2019 16:02:05.996 # Starting a failover election for epoch 34. 1021:S 26 Mar 2019 16:02:06.004 # Error condition on socket for SYNC: Connection refused 不出意料地赢得选举 1021:S 26 Mar 2019 16:02:06.020 # Failover election won: I'm the new master. 1021:S 26 Mar 2019 16:02:06.020 # configEpoch set to 34 after successful failover 1021:M 26 Mar 2019 16:02:06.020 # Setting secondary replication ID to 7b83d297fa53f119c79021661fff533eafabc222, valid up to offset: 1. New replication ID is c5011813ad8fda9ef68da648f2fdfc27eae2afd3 自己已为主，不需要“cached master”了 1021:M 26 Mar 2019 16:02:06.020 * Discarding previously cached master state. 集群状态又恢复正常 1021:M 26 Mar 2019 16:02:06.021 # Cluster state changed: ok |
同时段集群其它主节点日志：
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30651:M 26 Mar 2019 15:31:45.438 * Marking node c67dc9e02e25f2e6321df8ac2eb4d99789917783 as failing (quorum reached). 集群状态变标记为fail 30651:M 26 Mar 2019 15:31:45.438 # Cluster state changed: fail 30651:M 26 Mar 2019 15:34:03.022 * Clear FAIL state for node f805e652ff8abe151393430cb3bcbf514b8a7399: replica is reachable again. 30651:M 26 Mar 2019 15:35:45.005 * 10 changes in 300 seconds. Saving... 30651:M 26 Mar 2019 15:35:45.006 * Background saving started by pid 28683 28683:C 26 Mar 2019 15:35:45.016 * DB saved on disk 28683:C 26 Mar 2019 15:35:45.018 * RDB: 0 MB of memory used by copy-on-write 30651:M 26 Mar 2019 15:35:45.106 * Background saving terminated with success 30651:M 26 Mar 2019 15:42:03.769 * Clear FAIL state for node c67dc9e02e25f2e6321df8ac2eb4d99789917783: is reachable again and nobody is serving its slots after some time. 集群状态恢复正常 30651:M 26 Mar 2019 15:42:03.769 # Cluster state changed: ok |
同时段集群其它复制节点日志：
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31463:S 26 Mar 2019 15:31:45.438 * FAIL message received from 29fcce29837d3e5266b6178a15aecfa938ff241a about c67dc9e02e25f2e6321df8ac2eb4d99789917783 集群状态变标记为fail 31463:S 26 Mar 2019 15:31:45.439 # Cluster state changed: fail 31463:S 26 Mar 2019 15:34:03.023 * Clear FAIL state for node f805e652ff8abe151393430cb3bcbf514b8a7399: replica is reachable again. 31463:S 26 Mar 2019 15:35:45.100 * 10 changes in 300 seconds. Saving... 31463:S 26 Mar 2019 15:35:45.101 * Background saving started by pid 28695 28695:C 26 Mar 2019 15:35:45.116 * DB saved on disk 28695:C 26 Mar 2019 15:35:45.118 * RDB: 0 MB of memory used by copy-on-write 31463:S 26 Mar 2019 15:35:45.201 * Background saving terminated with success 31463:S 26 Mar 2019 15:42:03.769 * Clear FAIL state for node c67dc9e02e25f2e6321df8ac2eb4d99789917783: is reachable again and nobody is serving its slots after some time. 集群状态恢复正常 31463:S 26 Mar 2019 15:42:03.769 # Cluster state changed: ok |