What is an API and why a tester needs to know it
Sometimes, during an interview, they ask you: “What is an API?”
And you’re like: well, something about requests, servers, some data... But when you start working with it, you realize – the modern world simply can’t function without APIs.
What is it, actually?
API (Application Programming Interface) – is a way for two or more programs to talk to each other.
Imagine: you order a pizza through an app. You tap “Pepperoni,” and your browser or phone “talks” to a server hundreds of kilometers away and “asks”:
GET /menu/pizza/pepperoni
The server replies: “Here’s the price, ingredients, delivery time.”
That’s the API at work – it sends data between the client (your phone) and the server.
Why testers need it
If a website or app doesn’t work – the user just sees “Oops, an error.” But a tester needs to know exactly where something went wrong. And often, the reason is the API.
When testing APIs, we check:
- Whether the server returns the correct data.
- Whether errors are handled properly (for example, when a user is not authorized).
- Whether the system can handle the load.
- And whether everything matches the documentation.
Real-life example
Let’s say we have a request:
POST /users
which should create a new user.
We send:
{
"name": "Alex",
"email": "[email protected]"
}
If the API works correctly, we get:
{
"id": 123,
"name": "Alex",
"email": "[email protected]"
}
But if the server suddenly returns “500 Internal Server Error,” the tester already knows – there’s a problem in the backend logic.
And this must be reported to the developers, with a clear description: “When creating a user without a password, the server crashes.”
Tools to start with
For beginners, I always recommend two simple tools:
- Postman – to manually send requests and see what comes back.
- Swagger – a handy way to view API documentation right in your browser and test requests online.
If you want to go further – automation. There are libraries like RestAssured (for Java) or Requests (for Python) that let you run hundreds of tests at once. But the main thing is to understand the logic, not just click buttons.
Common mistakes
- Testing only “happy” cases.
What if the user sends an invalid email? - Ignoring response codes.
200 – OK, 400 – bad request, 401 – unauthorized, 500 – server error. It’s basic, but often forgotten. - Not reading the documentation.
And it might say that the “email” field is required!
How to grow further
If you’re just starting – don’t be afraid to experiment. Take any public API (for example, OpenWeather or JSONPlaceholder), play with the requests, see how it reacts.
And most importantly – don’t memorize, understand why a request returns a specific response.
🔹 Next step:
If you want to systematically improve your skills, learn API test automation, and work with real cases – join the SkillsUp courses.
We don’t just show buttons – we teach you to think like a tester.