Could anyone explain what is database cluster? And can we have more than one cluster for a database?
When you ask about a cluster I am assuming you are asking about the Atlas Cluster set you are connecting to for this course. In this case I it could refer to 2 things: either a replica set or a sharded cluster
A replica set or cluster is any odd number of mongod processes, 3 or greater. Likely running on different machine(servers) for high availability. And that is the main reason, to have your data replicated across different machines. Therefore if a primary node(server) goes down or is not reachable a new primary can be elected(reason for odd number) which will allow your applications to continue reading and writing to the database.
Now a shared cluster is many replica sets which are “sharing” the storage of an very large collection. The collection is split up among the replica set in groups know as shards, hence name shared cluster
If you mean replica set then, Yes when we have a sharded cluster.