Home
Tags:
database
mongodb
replication
replica sets
CRUD

Reading and Writing with Replica Sets

connect to a repl set

mongo --host "m103-example/localhost:27011" -u "adminroot" -p "adminrootpw" --authenticationDatabase "admin"

see which node is primary

rs.isMaster()
  • primary shows the host:port of the primary node
  • me shows the host:port of the session im connected to
  • these should be the same!

create a new db, & add a piece of data to a new collection

use newdb
db.newcollection.insert({"new": "object"})

connect to a secondary node && review that the data has been replicated to it

# connect
mongo --host "localhost:27012" -u "rs-admin" -p "rs-admin-pw" --authenticationDatabase "admin"

NOTES

  • removed the replica-set name from the connection string, as naming the replica set would redirect to the primary automagically
  • set the port to a secondary node
  • secondary nodes require special auth
# ENABLE READ OPERATIONS on secondary node
rs.slaveOk()

show dbs

NOTE

  • mongo tries to protect reads on secondary nodes, as one way to ensure data integrity

Replica Sets Handling node crisis

# shutdown one of the secondary nodes, mocking a crisis!

# while still logged in to secondary node
db.shutdownServer()

# connect back to the host node
mongo --host "m103-example/localhost:27011" -u "adminroot" -p "adminrootpw" --authenticationDatabase "admin"

# view replica status
rs.status()

# shutdown ANOTHER secondary node, simulating a BIGGER CRISIS!!!
^C

# connect
mongo --host "localhost:27013" -u "rs-admin" -p "rs-admin-pw" --authenticationDatabase "admin"

# shut it down
db.shutdownServer()

# connect back to the host node
mongo --host "m103-example/localhost:27011" -u "adminroot" -p "adminrootpw" --authenticationDatabase "admin"
  • 1 node is left
  • 1 out of 3 is NO LONGER A MAJORITY
  • the current primary steps down to be become a secondary validate that the node that was primary is no longer

# reconnect, without the name of the replica set, to the "primary"
mongo --host "localhost:27011" -u "rs-admin" -p "rs-admin-pw" --authenticationDatabase "admin"

# check replica set status
rs.isMaster()

# see that the 'ismaster' key value is set to false
# see that the 'secondary' key/val  is set to true

Final Thoughts

  • data gets replicated to a secondar
  • reading data directly from a secondary requires a few details
    • connect to the secondary node without the replica name in the connection string
    • running rs.slaveOk() in the cli to "allow" data reads on the secondary node
  • writing to a replica set when a majority of the nodes are not available is impossible