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replica sets
reconfiguration

Reconfiguring a running Replica Set

Want to reconfig a running set?

Scenario

  • a running set has 3 nodes
    • primary
    • 2 secondary
  • a new goal is to add another secondary as well as an arbiter
    • arbiter is cheaper to maintain
    • maintains odd-number of voting members in the set
    • useful for an even number of voting members
    • interesting approach!

launch new nodes

# in a new term

# connect to primary node
# figure that deet out, simple

# make 2 new nodes
mongod -f done4.conf
mongod -f arbiter.conf

add and validate nodes to the repl set

# in term connected to primary node

rs.add('m103-example:27014')
rs.addArb('m103-example:28000')

rs.isMaster()

# the hosts array should have 4 nodes
# the arbiters array should have 1 node

new scenario

  • remove the arbiter
  • use a hidden node to store backups
  • kill 2 birds with one stone
  • killing the arbiter causes a problem, resulting in an even number of voting members
    • one of the secondary nodes can be set to a non-voting member
    • the same member can be hidden
rs.remove('m103-example:28000')

# review configs
rs.conf()

let cfg = rs.conf()

# chnage the vot privelege of the 4th node in the set to false
cfg.members[3].votes = 0

# hide the 4th node
cfg.members[3].hidden = true

# MUST set priority to 0 to keep things working right for hidden non-voting nodes
cfg.members[3].priority = 0;

# update the config of the replica set
rs.reconfig(cfg)
# this MIGHT trigger an election depending on the config

# review the config
rs.conf()
# should show one of the members as hidden with no voting privileges

Final thoughts

updating a replica set with rs.reconfig()

  • requires the entire config object to be passed as a param
  • does not requile any config file to be directly updated
  • does not require nodes to be restarted