API (Application Programming Interface) là gì?

An API specifies exactly how software components should interact: what requests can be made, what data format they must use, what responses will be returned, and how errors are handled. When you log into a website using 'Sign in with Google,' Google's OAuth API is handling authentication. When your accounting software syncs with your bank, it's using the bank's API. When an e-commerce site shows real-time shipping rates at checkout, it's calling a carrier API. APIs are the connective tissue of modern software — they allow businesses to build on top of existing platforms rather than rebuilding everything from scratch.

The most common API architecture today is REST (Representational State Transfer), which uses standard HTTP methods (GET, POST, PUT, DELETE) and returns data in JSON format. GraphQL is an alternative that lets clients request exactly the data they need. SOAP is an older XML-based protocol still common in enterprise and financial systems. Webhooks are a related mechanism where one system pushes real-time notifications to another when an event occurs (e.g., notifying your CRM every time a payment processes).

From a business perspective, APIs create value in two directions. Consuming third-party APIs allows businesses to add capabilities (payment processing, mapping, email delivery, SMS, AI) without building them from scratch. Publishing APIs allows platforms to become ecosystems — letting partners, developers, and customers integrate with their product, extending its value and creating switching costs. Companies like Stripe, Twilio, and Plaid have built multi-billion-dollar businesses whose entire product is an API.