Logical replication theory and concepts

It's a layman introduction to the concepts and theory behind logical replication in PostgreSQL.

We will discuss following topics:

1. Physical WAL streaming as a way to replicate physical view of database
2. Logical change streaming as a way to replicate logical view of database
3. Role of reorder buffer, WAL sender on the origin database
4. Role of logical receiver and change application on the target database
5. Theory behind initial data synchronization
6. Reasons behind logical replication being a delicate feature to handle

These simple concepts will help users make educated decision about using logical replication in their applications and set correct expectations from the feature.