An API (Application Programming Interface) is a set of rules that lets two software systems communicate with each other. It is the piece that makes it possible for the applications we use every day to exchange data and functions without having to build everything from scratch.

What is an API, with an analogy?

Imagine you are at a restaurant. You don’t go into the kitchen to prepare your dish: you tell the waiter what you want, the waiter takes your order, and brings back the result. You don’t need to know how the kitchen works; you just use the menu and trust that your order will arrive correctly.

An API plays that role between systems. One application “asks” another for something through the API —for example, “give me the weather in Lima” or “charge this payment”— and receives an orderly response, without needing to know how the system on the other side is built. The menu is the set of available operations; the waiter is the API that makes them possible.

How does an API work?

Communication through an API follows a simple pattern:

  • A request: one system asks for something, stating what it wants and with what data.
  • Processing: the system that receives the request validates it, runs the task, and prepares the result.
  • A response: it returns the requested information or confirms that the action was completed.

All of this happens in seconds and transparently for the end user, who only sees the result on their screen.

What is a REST API?

REST is the most widely used style for designing web APIs. It organizes communication around resources —entities such as a user, an order, or a product— and uses the standard internet operations to work with them: create, read, update, and delete.

Its popularity comes from being simple to understand, supported in practically any language and platform, and scaling well. That is why most of the public and business APIs that exist today are REST APIs.

What are APIs used for in a company?

APIs are the connective tissue of modern technology. Thanks to them:

  • Applications integrate with each other. A sales system talks to billing, and billing to logistics, without manual intervention.
  • Capabilities are reused. Instead of programming a payment or a notification again, you consume a service that already solves it.
  • New channels open up. A company can expose part of its data or functions to partners and allies in a controlled way.
  • Innovation accelerates. Different teams move forward in parallel as long as they respect the “contract” the API defines.

APIs in the AWS cloud

When a company modernizes its applications, APIs become a central piece, and it is best to manage them with services designed for that. AWS offers managed options to publish, secure, and scale APIs without managing servers, such as Amazon API Gateway, which acts as a front door: it controls access, throttles usage, monitors traffic, and connects the API with serverless functions or microservices.

This way, an API can go from a few users to thousands without the team having to provision infrastructure manually, paying in US dollars for actual usage.

APIs as a foundation for modernization

An architecture well designed around APIs makes systems easier to evolve, integrate, and scale. It is one of the foundations of cloud-native applications: independent components that communicate through clear APIs and can be updated without holding back the rest.

At Caleidos we support that journey within our application modernization practice, with cases in production documented in our case studies.

Frequently asked questions

What is an API in simple terms? It is a set of rules that lets two systems communicate, like a waiter who takes your order to the kitchen and brings back the response.

What is a REST API? It is the most common web API style: it organizes communication around resources and uses the standard internet operations to create, read, update, or delete them.

How are APIs managed on AWS? With managed services such as Amazon API Gateway, which publish, secure, and scale APIs without managing servers.

Want to modernize your integrations?

Let’s talk about your current systems and we’ll give you a concrete recommendation on how to organize your APIs in the cloud.