How to: Application layer
Source: Intro to architecting systems for scale
Separating out the web layer from the application layer (also known as platform layer) allows you to scale and configure both layers independently.  Adding a new API results in adding application servers without necessarily adding additional web servers.  The **single responsibility principle** advocates for small and autonomous services that work together.  Small teams with small services can plan more aggressively for rapid growth.

Workers in the application layer also help enable [asynchronism](#asynchronism).