Add Application layer section

README.md

@@ -845,3 +845,40 @@ Additional benefits include:
 * [NGINX architecture](https://www.nginx.com/blog/inside-nginx-how-we-designed-for-performance-scale/)
 * [HAProxy architecture guide](http://www.haproxy.org/download/1.2/doc/architecture.txt)
 * [Wikipedia](https://en.wikipedia.org/wiki/Reverse_proxy)
+
+## Application layer
+
+<p align="center">
+  <img src="http://i.imgur.com/yB5SYwm.png">
+  <br/>
+  <i><a href=http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer>Source: Intro to architecting systems for scale</a></i>
+</p>
+
+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).
+
+### Microservices
+
+Related to this discussion are [microservices](https://en.wikipedia.org/wiki/Microservices), which can be described as a suite of independently deployable, small, modular services.  Each service runs a unique process and communicates through a well-definied, lightweight mechanism to serve a business goal. <sup><a href=https://smartbear.com/learn/api-design/what-are-microservices>1</a></sup>
+
+Pinterest, for example, could have the following microservices: user profile, follower, feed, search, photo upload, etc.
+
+### Service Discovery
+
+Systems such as [Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) can help services find each other by keeping track of registered names, addresses, ports, etc.
+
+### Disadvantage(s): application layer
+
+* Adding an application layer with loosely coupled services requires a different approach from an architectural, operations, and process viewpoint (vs a monolithic system).
+* Microservices can add complexity in terms of deployments and operations.
+
+### Source(s) and further reading
+
+* [Intro to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale)
+* [Crack the system design interview](http://www.puncsky.com/blog/2016/02/14/crack-the-system-design-interview/)
+* [Service oriented architecture](https://en.wikipedia.org/wiki/Service-oriented_architecture)
+* [Introduction to Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper)
+* [Here's what you need to know about building microservices](https://cloudncode.wordpress.com/2016/07/22/msa-getting-started/)