Replication

There are different approaches to replication. The approach that MongoDB takes is logical replication with a primary-secondary, which we will explain in more detail later in this chapter.

Logical or physical replication

With replication, we synchronize data across multiple servers, providing data availability and redundancy. Even if we lose a server due to a hardware or software failure, by using replication, we will have multiple copies that we can use to restore our data. Another advantage of replication is that we can use one of the servers as a dedicated reporting, or backup, server. 

In logical replication, we have our primary server performing operations; the secondary server tails a queue of operations from the primary and applies the same operations in the same order. Using MongoDB as an example, the operations log (oplog) keeps track of operations that have happened on the primary server and applies them in the exact same order on the secondary...