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MongoDB for Oracle DBA’s Part 10 : Switchover & Role Transition between Primary and Secondary

In MongoDB as like Oracle Dataguard switchover is possible to but in different way.

You can ask Primary to stepdown and let nodes elect a new primary in replicaset. This can be done using rs.stepDown() method.

Before stepping down primary, one must know the below

  • The procedure blocks all writes to primary while it runs
  • Terminates all sessions and running jobs, index rebuilts etc
  • Disconnects all connections
  • If the wait period to elect a new primary exceeds then primary will not step down.

Let's take a look of the same,

  1. I have opened three sessions and in one sessions querying a table which is running (the right top most one in screenshot)
  2. In another session tail the log file so can understand what happens when a step down is initiated
  3. In the session that left side , initiated the stepdown of primary , immediately disconnects the current session and also the session that running a query on the database (top left side)
  4. In the log it shows the primary is transitioning to secondary
  5. as per rs.status() the 47018 (original primary) becomes secondary.

Screenshot

 

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