An application server is the program that runs your business logic and handles requests for a web application. It sits between a web server that terminates HTTP connections and backend services like databases or queues. When a client calls an endpoint, the web server forwards the request to the application server, which executes code, talks to data stores, and builds a response. Frameworks like Express for Node.js or Django for Python are commonly used to implement application servers. They usually run on one or more virtual machines or in containers.
Why it matters
Understanding the role of an application server helps you reason about where different responsibilities live in your system. It is where you enforce authentication, authorization, validation, and most business rules. When performance issues appear, you often look at the application server first to analyze CPU usage, memory, and slow database calls.
Examples
A Next.js backend route running in a Node environment is acting as an application server. A Java Spring Boot service behind a load balancer in the cloud is another example. You can see how requests flow through these layers in the lessons How Servers Send Responses and The Journey of a Request.