What is two-phase commit (2PC) protocol in the context of distributed databases? Describe its operation and discuss its limitations.

When should distributed database systems employ data replication to address data fragmentation if it is a problem? It seems that the processes of fragmentation and replication are incompatible. What distinguishes vertical fragmentation from horizontal fragmentation? Give me an example so I can better grasp your position.

Discuss two-phase commit (2PC) and three-phase commit (3PC) protocols in the context of distributed transactions. How do they ensure data consistency?

What are the challenges and strategies for achieving fault tolerance in distributed database systems? How do distributed consensus algorithms like Paxos and Raft contribute to fault tolerance?