Learn how APIs enable different software systems to communicate. Understand REST, webhooks, authentication, and best practices for building reliable integrations.
An API (Application Programming Interface) is a set of rules that allows different software applications to communicate with each other. Think of it as a waiter in a restaurant: you (the client application) tell the waiter (API) what you want, and the waiter communicates with the kitchen (server) to get your order.
When you book a flight through a travel website, the site uses APIs to check availability with multiple airlines, retrieve pricing, and process your booking. The travel site doesn't store this information itself; it requests it from each airline's API in real-time.
Before APIs, each software application was an island. Getting data from one system to another required manual export/import, custom database connections, or screen scraping. This was slow, error-prone, and didn't work in real-time. APIs enable systems to share data instantly and automatically.
REST (Representational State Transfer) APIs use standard HTTP methods to perform operations:
// Example: Get order details from an API
GET https://api.example.com/orders/12345
// Response (JSON)
{
"id": 12345,
"customer": "ABC Company",
"status": "shipped",
"total": 1500.00
}
While REST APIs require you to ask for data (polling), webhooks push data to you when events occur. When something happens in System A, it automatically notifies System B.
A newer alternative to REST where clients specify exactly what data they need. Reduces over-fetching and under-fetching of data. Popular for complex applications with varied data requirements.
Direct connections between systems. Simple for few integrations but becomes complex as you add more systems (n systems = n*(n-1)/2 connections).
A central integration hub connects to all systems. Each system only needs one connection to the hub. The hub handles data transformation and routing.
Systems publish events to a message queue. Other systems subscribe to events they care about. Highly scalable and decoupled.
A unique identifier passed with each request. Simple but less secure. Best for server-to-server communication where the key can be kept secret.
Industry standard for authorization. Allows users to grant limited access to their data without sharing passwords. Used by Google, Facebook, and most modern APIs.
Self-contained tokens that include user identity and permissions. Commonly used with REST APIs for session management.
Always use HTTPS to encrypt data in transit. This prevents man-in-the-middle attacks and data interception.
An e-commerce site integrates with an ERP for inventory and a shipping carrier for delivery. Here's how the APIs work together:
| Integration Type | Example Services | Typical Use |
|---|---|---|
| Payment | Stripe, PayPal, Square | Process payments, handle refunds |
| Shipping | Royal Mail, DHL, FedEx | Calculate rates, create labels, track packages |
| SendGrid, Mailchimp | Send transactional and marketing emails | |
| Accounting | Xero, QuickBooks | Sync invoices, payments, expenses |
| CRM | Salesforce, HubSpot | Sync customers, track interactions |
| ERP | SAP, Entersoft, Oracle | Products, inventory, orders, invoices |
An API (Application Programming Interface) is a defined way for two software systems to communicate. It lets applications exchange data and trigger actions, for example connecting an online store to accounting software or accepting online payments.
A REST API is called by your application when it needs data (a "pull"), while a webhook pushes data to your application automatically when an event happens (a "push"). Most integrations use both: REST for requests and webhooks for real-time updates.
Through authentication such as OAuth 2.0 or API keys, encrypting data in transit, validating all inputs, applying rate limiting, and following OWASP guidelines. Credentials are stored securely and never exposed in client-side code.
Well-built integrations use versioning and monitoring so changes are detected early. Support arrangements include updating integrations when third-party APIs are deprecated or modified, minimising disruption.
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