Replication Overview

• What is replication?

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  • Copies of the same data on different servers

• When would you use replication?

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  • Provides redundancy
  • Safeguards data when a server goes down

• What is a primary?

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  • The primary is the mongod instance which accepts read and write requests for the data set

• What are secondaries?

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  • Secondaries store a backup copy of the data set held by the primary

• What happens when a primary fails?

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  • The secondaries elect a new primary

• When would you use an arbiter

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  • An arbiter is a mongod instance which holds no data
  • Use an arbiter if you have an even number of secondaries
  • The arbiter has "voting rights" to help decide ties

• How can "reads" take advantage of replication?

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  • Read preference modes which can be set:
  • primary, primaryPreferred
  • secondary, secondaryPreferred, nearest

• How many MongoDB instances should go into a replica set?

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  • A minimum of 3 is recommended:
    1 primary and 2 secondaries
  • There is no upper limit
  • Objective: redundancy
  • Geographic considerations may apply