A replica can be specified as a group of mongo instances that host the same data set. In a replica set, one node is primary, and another is secondary. All data is replicated from primary to secondary nodes.
In MongoDB, a replica set is a group of MongoDB servers that maintain the same data set, providing high availability and fault tolerance. Replica sets are the primary mechanism for implementing redundancy and ensuring data availability in MongoDB.
Key features of a MongoDB replica set include:
- Primary Node: One node in the replica set serves as the primary node, which receives all write operations. The primary node is responsible for replicating the data to secondary nodes.
- Secondary Nodes: These are replica nodes that replicate data from the primary node. They can be used for read operations and automatically take over if the primary node fails.
- Automatic Failover: If the primary node becomes unavailable (due to network issues, hardware failure, etc.), a replica set can automatically elect a new primary node from the available secondaries to ensure continuous operation.
- Data Redundancy: By maintaining multiple copies of data across different nodes, replica sets provide data redundancy and protect against data loss.
- Arbiter: An arbiter is an optional node that participates in replica set elections but doesn’t replicate data. It helps in achieving an odd number of voting members, which is useful for reaching a majority in the election process.
The use of replica sets enhances the availability, reliability, and scalability of MongoDB deployments, making it a fundamental feature for building robust and fault-tolerant applications.