Redis集群節點的選舉（實驗）
當master掛掉之後，就會在該集群中的slave中選取一個來代替mater角色，
從而保證redis集群slot的完整性。
如果其中一個mster和它的slave都掛掉後，會導致slot不完整，整個集群都會掛掉。
集群節點信息：
192.168.2.200:6379> cluster nodes
3ff3a74f9dc41f8bc635ab845ad76bf77ffb0f69 192.168.2.201:6379 master - 0 1527145806504 5 connected 10923-16383
2faf68564a70372cfc06c1afff197019cc6a39f3 192.168.2.201:6380
227f51028bbe827f27b4e40ed7a08fcc7d8df969 192.168.2.200:6380 slave 098e7eb756b6047fde988ab3c0b7189e1724ecf5 0 1527145804485 4 connected
5844b4272c39456b0fdf73e384ff8c479547de47 192.168.2.200:6379 myself,master - 0 0 3 connected 5461-10922
7119dec91b086ca8fe69f7878fa42b1accd75f0f 192.168.2.100:6380
098e7eb756b6047fde988ab3c0b7189e1724ecf5 192.168.2.100:6379 master - 0 1527145803476 1 connected 0-5460
新建3個key：
192.168.2.200:6379> set name zhangsan
OK
192.168.2.200:6379> set age 26
-> Redirected to slot [741] located at 192.168.2.100:6379
OK
192.168.2.100:6379> set home beijing
-> Redirected to slot [10814] located at 192.168.2.200:6379
OK
模擬192.168.2.100:6379掛掉：
# ps -ef |grep redis
root 19023 1 0 15:05 ? 00:00:01 redis-server 192.168.2.100:6379 [cluster]
root 19030 1 0 15:05 ? 00:00:01 redis-server 192.168.2.100:6380 [cluster]
root 19127 2912 0 15:13 pts/0 00:00:00 grep --color=auto redis
# kill 19023
查看集群節點狀態：（可以看到集群中的一個slave的角色變成了master）
# redis-trib.rb check 192.168.2.200:6379
>>> Performing Cluster Check (using node 192.168.2.200:6379)
M: 5844b4272c39456b0fdf73e384ff8c479547de47 192.168.2.200:6379
slots:5461-10922 (5462 slots) master
1 additional replica(s)
M: 3ff3a74f9dc41f8bc635ab845ad76bf77ffb0f69 192.168.2.201:6379
slots:10923-16383 (5461 slots) master
1 additional replica(s)
S: 2faf68564a70372cfc06c1afff197019cc6a39f3 192.168.2.201:6380
slots: (0 slots) slave
replicates 3ff3a74f9dc41f8bc635ab845ad76bf77ffb0f69
M: 227f51028bbe827f27b4e40ed7a08fcc7d8df969 192.168.2.200:6380
slots:0-5460 (5461 slots) master
0 additional replica(s)
S: 7119dec91b086ca8fe69f7878fa42b1accd75f0f 192.168.2.100:6380
slots: (0 slots) slave
replicates 5844b4272c39456b0fdf73e384ff8c479547de47
[OK] All nodes agree about slots configuration.
>>> Check for open slots...
>>> Check slots coverage...
[OK] All 16384 slots covered.
查看數據是否存在：
# redis-cli -h 192.168.2.200 -p 6380 -c
192.168.2.200:6380> keys *
1) "age"
模擬恢復故障節點：
# redis-server redis.conf
查看集群節點狀態：（故障節點恢復後，是變成master還是slave？）
# redis-trib.rb check 192.168.2.200:6379
>>> Performing Cluster Check (using node 192.168.2.200:6379)
M: 5844b4272c39456b0fdf73e384ff8c479547de47 192.168.2.200:6379
slots:5461-10922 (5462 slots) master
1 additional replica(s)
M: 3ff3a74f9dc41f8bc635ab845ad76bf77ffb0f69 192.168.2.201:6379
slots:10923-16383 (5461 slots) master
1 additional replica(s)
S: 2faf68564a70372cfc06c1afff197019cc6a39f3 192.168.2.201:6380
slots: (0 slots) slave
replicates 3ff3a74f9dc41f8bc635ab845ad76bf77ffb0f69
M: 227f51028bbe827f27b4e40ed7a08fcc7d8df969 192.168.2.200:6380
slots:0-5460 (5461 slots) master
1 additional replica(s)
S: 7119dec91b086ca8fe69f7878fa42b1accd75f0f 192.168.2.100:6380
slots: (0 slots) slave
replicates 5844b4272c39456b0fdf73e384ff8c479547de47
S: 098e7eb756b6047fde988ab3c0b7189e1724ecf5 192.168.2.100:6379
slots: (0 slots) slave
replicates 227f51028bbe827f27b4e40ed7a08fcc7d8df969
[OK] All nodes agree about slots configuration.
>>> Check for open slots...
>>> Check slots coverage...
[OK] All 16384 slots covered.
總結：master：192.168.2.100:6379 宕機後，
slave：192.168.2.200:6380 代替了其位置，變成了master角色。
當192.168.2.100:6379故障恢復後，並沒有恢復到master的角色，
而是充當了slave角色。
Redis集群節點的選舉（實驗）