What are virtual nodes (vnodes) in Cassandra and how do transactions work?

In Cassandra, virtual nodes (vnodes) are a mechanism that allows a single physical node to be responsible for multiple virtual partitions (token ranges) in the data ring. Instead of each node handling one large token range, vnodes split this range into many small parts distributed across nodes. This improves load balancing and simplifies cluster scaling, as adding or removing a node redistributes less data.

Regarding transactions, Cassandra does not support traditional ACID transactions like relational databases. Instead, it provides consistency at the record level using "lightweight transactions" (LWT), based on the Paxos protocol. LWTs allow conditional operations (e.g., insert a record only if it does not exist) with consistency guarantees, but with lower performance. Otherwise, operations in Cassandra are atomic at the single-row level but do not support complex transactions involving multiple rows or tables.