Replication in MongoDB

A replica set is a group of MongoDB instances that have the same dataset. A replica set has one arbiter node and multiple data-bearing nodes. In data-bearing nodes, one node is considered the primary node while the other nodes are considered the secondary nodes.

All write operations happen at the primary node. Once a write occurs at the primary node, the data is replicated across the secondary nodes internally to make copies of the data available to all nodes and to avoid data inconsistency.

If a primary node is not available for the operation, secondary nodes use election algorithms to select one of their nodes as a primary node.

A special node, called an arbiter node, is added in the replica set. This arbiter node does not store any data. The arbiter is used to maintain a quorum in the replica set by responding to a heartbeat and election request sent by the secondary nodes in replica sets. As an arbiter does not store data, it is a cost-effective resource used in the election process. If votes in the election process are even, the arbiter adds a voice to choose a primary node. The arbiter node is always the arbiter, it will not change its behavior, unlike a primary or secondary node. The primary node can step down and work as secondary node, while secondary nodes can be elected to perform as primary nodes.

Secondary nodes apply read/write operations from a primary node to secondary nodes asynchronously.