What is the function of virtual nodes (vnodes) in Cassandra?

The function of virtual nodes (vnodes) in Cassandra is primarily to enable more even data distribution across the cluster. By dividing each physical node into multiple virtual nodes, Cassandra improves the way data is distributed and balanced, which helps to prevent hotspots or uneven data loads. This mechanism allows for better load balancing, as data can be proportionally spread across all the vnodes, even if physical nodes have different capacities or are added or removed from the cluster.

Additionally, this even distribution of data aids in scaling the database horizontally. When new nodes are added, data can be rebalanced evenly among all nodes, minimizing disruptions and maintaining performance. It also improves fault tolerance; in the event that a physical node goes down, only a subset of the data needs to be re-replicated, as it is now part of multiple vnodes distributed across the cluster.

The other options do not accurately reflect the primary function of vnodes. Backup processes, centralizing data access, and facilitating concurrent operations are important in database management but are not the key purposes of implementing virtual nodes in Cassandra.