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?

In the context of distributed database systems, what are the appropriate circumstances for utilising data replication as a solution to address data fragmentation? The methods of replication and fragmentation appear to be mutually exclusive. What is the function of horizontal fragmentation in contrast to vertical fragmentation? In order to persuade me to acknowledge your viewpoint, it is necessary for you to furnish an example.

In distributed database systems, which is preferable: data replication or data fragmentation? Could we mix fragmentation and replication, in your opinion? How distinct are the two types of fragmentation, vertical and horizontal? Could you provide me an illustration of your point?

How does concurrency control work in distributed systems, and why is it crucial for maintaining data consistency?