# Interfaces 101 : Extensible API Handlers Ep. 10

- Published: April 3, 2023
- Author: Ardan Labs
- URL: https://www.ardanlabs.com/blog/2023/04/interfaces-101-extensible-api-handlers-ep-10.html

### Introduction

In episode 9, Miki discussed how a command flag can be decoded into a user defined type with the `Value` interface. As a recap, the `Value` interface consists of two methods: one for serializing the underlying concrete type and one for deserializing a string into an object. To put this interface to the test, Miki wrote a command that had a flag to specify a network address. While implementing the `Set` method of the `Value` interface, Miki demonstrated how to add validation for your program’s flags. 

![](../../../images/blog/interfaces-10.jpg?v=1)

In this video, Miki will build an HTTP server with one endpoint to display the health of his server. The first step he took was to define a HTTP handler to illustrate how a Go handler becomes a `HandlerFunc`. Next, Miki will design a function signature for middleware with the goal of abstracting input and output as much as possible. Miki will then define a logging middleware that measures the time of a request. Watch and learn how you can write a HTTP handler that is extendable with middleware.

Things you will learn in this video

- Define a route with Go’s http package.
- The ideal design for a middleware function.
- How to implement an HTTP server with composability in mind.
- Define logging middleware with Go’s STD library.

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### Video

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