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mongodb集群采用的replication set模式，至少3个节点以上。有个问题，当mongodb主节点down的时候，如何让ceilometer自动连接重新选举出来的主节点？
这里有两种解法，至于采用哪种，看需求。
1、采用vip来定位mongodb主节点
使用keepalived来起vip，这里有两个vip，vrrp_instance VI_1是专门给mongodb用的vip，vrrp_instance VI_2是OpenStack HA用的vip
vrrp script chk_mongo_primary脚本专门来检测该节点是不是mongo master节点，如果是的话权重+2，而且vrrp_instance VI_1的vip是可以抢占的；
当mongodb主节点发生切换，vrrp_instance VI_1的vip也会跟着切换。
keepalived.conf配置文件
! Configuration File for keepalived vrrp_script chk_haproxy { script "killall -0 haproxy" interval 2 weight 2 } # Define the script used to check if mongod is running vrrp_script chk_mongod { script "killall -0 mongod" interval 2 # every two seconds weight 2 } # Define the script to see if the local node is the primary vrrp_script chk_mongo_primary { script "mongo --eval '(!!db.runCommand("ismaster")["ismaster"])?quit(0):quit(1)'" interval 2 # every two seconds weight 2 } # Configuation for the virtual interface vrrp_instance VI_1 { interface br-ex state node MASTER # SLAVE on the other nodes priority 101 # 100 on other nodes virtual_router_id 55 authentication { auth_type PASS auth_pass secret # Set this to some secret phrase } # The virtual ip address shared between the two nodes virtual_ipaddress { 172.16.140.251/24 } # Use the script above to check if we should fail over track_script { chk_mongod chk_mongo_primary } } vrrp_instance VI_2 { interface br-ex virtual_router_id 51 state BACKUP priority 200 # if use it,the openstack api do not response normally # use_vmac virtualmac # advert_int 1 dont_track_primary nopreempt authentication { auth_type PASS auth_pass password } virtual_ipaddress { 172.16.140.250/24 } track_script { chk_haproxy } notify /usr/local/bin/keepalivednotify.sh }2、使用haproxy的tcp-check来检测
haproyx mongodb配置
listen mongodb_cluster bind openstack_vip:27017 option tcpka option tcplog option tcp-check # MongoDB Wire Protocol tcp-check send-binary 3a000000 # Message Length (58) tcp-check send-binary EEEEEEEE # Request ID (random value) tcp-check send-binary 00000000 # Response To (nothing) tcp-check send-binary d4070000 # OpCode (Query) tcp-check send-binary 00000000 # Query Flags tcp-check send-binary 61646d696e2e # fullCollectionName (admin.$cmd) tcp-check send-binary 24636d6400 # continued tcp-check send-binary 00000000 # NumToSkip tcp-check send-binary FFFFFFFF # NumToReturn # Start of Document tcp-check send-binary 13000000 # Document Length (19) tcp-check send-binary 10 # Type (Int32) tcp-check send-binary 69736d617374657200 # ismaster: tcp-check send-binary 01000000 # Value : 1 tcp-check send-binary 00 # Term tcp-check expect binary 69736d61737465720001 #ismaster True server controller1 controller1:27017 check inter 2000 rise 2 fall 3 server controller2 controller2:27017 check inter 2000 rise 2 fall 3这里有个问题，在haproxy监控页面上看到的非master节点都是down的，因为tcp-check的缘故。
上面send-binary为什么那么写？(用tcpdum抓包出来，拿到wireshark分析)
详情参考这里：https://blog.danman.eu/mongodb-haproxy/
http://serverfault.com/questions/625492/how-to-construct-a-mongodb-health-check-in-haproxy
最后发现mongodb的uri本身支持multi host，形如：mongodb://ceilometer:ceilometer@controller2,controller1,controller3/ceilometer?readPreference=primaryPreferred&replicaSet=rs0
详情参考mongodb官方：https://docs.mongodb.com/manual/reference/connection-string/
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