# Application layer > From [The System Design Primer](https://github.com/donnemartin/system-design-primer) by Donne Martin, licensed under CC BY 4.0, arranged into chapters for this site. Part of "Building blocks". Web page: https://system.tangentbyte.com/learn/application-layer ![Diagram](https://system.tangentbyte.com/repo/images/yB5SYwm.png)\ _[Source: Intro to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer)_ 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](https://system.tangentbyte.com/learn/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-defined, lightweight mechanism to serve a business goal. \1](https://smartbear.com/learn/api-design/what-are-microservices%3E1) Pinterest, for example, could have the following microservices: user profile, follower, feed, search, photo upload, etc. ### Service Discovery Systems such as [Consul](https://www.consul.io/docs/index.html), [Etcd](https://coreos.com/etcd/docs/latest), and [Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) can help services find each other by keeping track of registered names, addresses, and ports. [Health checks](https://www.consul.io/intro/getting-started/checks.html) help verify service integrity and are often done using an [HTTP](https://system.tangentbyte.com/learn/communication#hypertext-transfer-protocol-http) endpoint. Both Consul and Etcd have a built in [key-value store](https://system.tangentbyte.com/learn/database#key-value-store) that can be useful for storing config values and other shared data. ### 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/2016-02-13-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/)