• In replica set Primary is the only member that receives write operations.
• In replica set secondary member is to log and apply the operations to their data.
• MongoDB applies write operations and records the operation on primary’s oplog.
• The primary member accepts all write operations in the following three-members replica set.
• The secondary member replicates the primary’s oplog to apply their data sets.
• In replica sets all members can accept to read captions.
• In replica sets, read preference describes how MongoDB clients route read operations to the members.
• An election determines the new primary, If the current primary becomes unavailable.
• Replica setsuse elections to determine which set member will become primary.
• The primary becomes unavailable, in the following 3-member replica set.
• This triggers an election which selects one of the remaining secondaries as the new primary.

Replica set Secondary Member:

• In replica sets secondary maintains a copy of the primary’s data set.
• A secondary applies operations from the primary’s oplog to its own data set in an asynchronous process, to replicate data.
• A replica set can have one or more secondaries.
• In replica sets, the following three-member has two secondary members.
• The secondaries replicate the primary’s oplog and apply the operations to their data sets.
• In replica set client cannot write data, but they can read data from secondary members.
• A secondary can become a primary.
• The replica set holds an election to choose which of the secondaries becomes the new primary, If the current primary becomes unavailable.