You need to pull customer data from Salesforce, push orders to your shipping provider, and sync inventory with your warehouse system.
Each one has its own way of doing things. Different formats, different authentication, different everything.
You spend more time figuring out how to talk to systems than actually building what you need.
There's a common language most systems already speak.
FOUNDATIONAL - The universal language of system integration.
REST APIs use HTTP (the same protocol your browser uses) to let systems talk to each other. You make a request to a URL, you get back data. The request includes what you want to do: GET something, POST something new, PUT an update, DELETE something.
The beauty is predictability. If you know how to call one REST API, you mostly know how to call all of them. The URL tells you what resource you're working with. The method tells you what you're doing to it. The response comes back in a standard format, usually JSON.
REST APIs are the plumbing of modern software. Every SaaS tool you use, every integration you build, every automation you create probably talks REST under the hood.
REST APIs solve a universal problem: how do you let two systems exchange information without both needing to understand each other's internals?
Define resources (nouns), expose them at URLs, use HTTP methods (verbs) to act on them, return consistent responses. The contract is simple: if you follow the rules, any system can talk to any other system.
Each button simulates a real API call. Some succeed, some fail. Watch how REST APIs respond.
Calls randomly succeed or fail to show different status codes.
Click a button above to make your first API call...
Everything is addressable
Every piece of data has a URL. /customers gets all customers. /customers/123 gets customer 123. /customers/123/orders gets their orders. The URL structure tells you exactly what you're working with.
Standard operations
GET retrieves data. POST creates new data. PUT replaces existing data. PATCH updates part of it. DELETE removes it. Every API uses the same verbs, so you always know what an operation does.
Structured data exchange
Requests include headers (metadata like auth tokens) and a body (the data you're sending). Responses include status codes (200 OK, 404 Not Found, 500 Server Error) and a body (the data you asked for).
Proving who you are
Most APIs require authentication. API keys are simple but limited. OAuth tokens are more secure but complex. Bearer tokens in headers are the most common pattern. Without auth, you get 401 Unauthorized.
Someone signs up on your website. Without REST APIs, your ops team manually creates accounts in Salesforce, Stripe, Mailchimp, and Zendesk. Takes 20 minutes per customer. With APIs, it happens in 3 seconds, every time, without human error.
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Foundation layer - no upstream dependencies
You build an integration that works perfectly in testing. Deploy to production with 10x the volume. Suddenly you're getting 429 Too Many Requests and your whole automation stops. The API provider isn't broken. You're hitting them too hard.
Instead: Read the rate limit docs before you start. Build in exponential backoff and respect Retry-After headers.
Your code handles the 200 OK response beautifully. First time you get a 500 error or a timeout, the whole thing crashes. Or worse, it silently fails and you don't notice until data is missing.
Instead: Handle every status code explicitly. Log errors with enough context to debug. Implement retries for transient failures.
The API endpoint changes from api.example.com to api-v2.example.com. Your API key gets rotated. Now you're searching through code to find every place you hardcoded values.
Instead: Environment variables for credentials. Configuration files for endpoints. Make switching environments trivial.
You've learned how systems talk to each other through HTTP. The natural next step is understanding how to get data flowing automatically when things happen.