Some things simply cannot be cached. Ad hoc reports, up-to-the-second changes, and sites where access patterns are spread across a large number of unrelated pages are all difficult to optimize using caching. For workloads like this, you can deploy a bigger database server, but eventually even those run into limits.
Therefore, many application architectures include database replication, where there is more than one database server serving out requests.
7.1 Types of Database Replication
There are many types of database replication depending on your needs. ...