What is Replication in MongoDB? Explain with an example.?

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In MongoDB, replication is the process of synchronizing data across multiple servers or nodes in a database cluster. It provides high availability and data redundancy by maintaining multiple copies of the data. Replication ensures that if one server fails, another server can take over and continue serving the application without any downtime.

In a MongoDB replica set, there are typically multiple nodes: one primary node and one or more secondary nodes. The primary node receives all write operations and replicates the data changes to the secondary nodes. If the primary node fails, one of the secondary nodes automatically becomes the new primary, ensuring continuous operation of the database.

Let's walk through an example to illustrate how replication works in MongoDB:

1. Setting up a Replica Set:

  - Assume we have three MongoDB nodes: Node1, Node2, and Node3.

  - Node1 is initially configured as the primary node, and Node2 and Node3 are configured as secondary nodes.

  - Node1 receives all write operations, and the data changes are replicated to Node2 and Node3.

2. Write Operations:

  - Let's say a new document is inserted into the database.

  - The client application sends the write operation to Node1 (the primary node).

  - Node1 processes the write operation and stores the new document locally.

  - Node1 then replicates the data change to Node2 and Node3, ensuring they have the same data.

3. Read Operations:

  - Read operations can be performed on either the primary or secondary nodes.

  - If a read operation is sent to the primary node (Node1), it processes the operation and returns the result.

  - If a read operation is sent to a secondary node (Node2 or Node3), the secondary node checks if it is in sync with the primary.

  - If the secondary node is up-to-date, it can serve the read operation.

  - If the secondary node is lagging behind, it requests the necessary data from the primary node before executing the read operation.

4. Failover and Promotion:

  - If Node1 (the primary node) fails or becomes unavailable, the replica set detects the failure.

  - One of the secondary nodes, such as Node2 or Node3, is automatically elected as the new primary.

  - The newly elected primary node takes over the write operations and ensures data consistency across the replica set.

By utilizing replication, MongoDB provides fault tolerance and high availability. It also allows for scaling read operations by distributing the load across multiple nodes. Replication is a key feature in MongoDB that ensures data durability and enables continuous operation of applications even in the face of failures.