What is an API Call? Understanding API Requests and Responses

If you’ve ever wondered how your weather app gets data, how your social feed updates instantly, or how payment systems connect behind the scenes, the answer is API calls. Application Programming Interfaces (APIs) let different software systems exchange information. When you make an API call, your app sends a request to another system asking for data or triggering an action, and then waits for a response.

This guide explains what API calls are, how they work, the main types you can make, and how to test them in Postman.

What is an API call?

An API call is a request sent from a client (such as a web browser, mobile app, or another server) to an API endpoint to perform an operation. The API processes the request and returns a response, which usually contains data or confirmation that the action was completed successfully.

Think of it like using an ATM:

1. You (the client) request $100, this is the API call.

2. The ATM (the API) communicates with your bank’s system (the server).

3. The bank verifies your account and processes the transaction.

4. The ATM gives you cash (the API response).

APIs provide functionality that applications can reuse without reinventing the wheel, such as user authentication, payments, or weather data retrieval.

Key components of an API call

Every API call includes several essential elements:

• HTTP method: The type of operation you want to perform (GET, POST, PUT, PATCH, DELETE)

• Endpoint URL: The address of the resource you’re requesting

• Headers: Metadata about the request, including authentication and content type

• Request body: The data you’re sending with POST, PUT, or PATCH requests

• Query parameters: Optional filters appended to the URL

How API calls work

When you make an API call, here’s what happens behind the scenes:

1. Client initiates a request: Your application makes an HTTP request to an API endpoint.

2. Request travels over the network: The request moves through the internet to reach the server.

3. Server processes the request: The API validates input and performs logic or functionality.

4. Database interaction: If needed, the server queries or updates the database.

5. Response generated: The API provides a structured response, often in JSON.

6. Response returned: The client receives the response and processes it.

Example: Making a simple GET request

Here’s what a basic API call looks like to retrieve user information:

GET /api/users/12345 HTTP/1.1
Host: api.example.com
Authorization: Bearer your_token_here
Accept: application/json

Here’s an example of how the server might respond:

HTTP/1.1 200 OK
Content-Type: application/json

{
  "id": "12345",
  "name": "Poe St. Mann",
  "email": "[email protected]",
  "role": "Developer"
}

Types of API calls

Different HTTP methods serve different purposes in RESTful APIs.

GET → Retrieve data

Fetch data without changing anything on the server.

GET /api/products?category=electronics&limit=10

Use cases:

• Fetching a list of products

• Retrieving user profile information

• Getting search results

• Loading blog articles

GET calls are read-only and should not modify functionality on the server.

POST → Create new resources

Send data to the server to create something new.

POST /api/orders
Content-Type: application/json

{
  "product_id": "789",
  "quantity": 2
}

Use cases:

• Creating a new user account

• Submitting a form

• Uploading a file

• Processing a payment

POST calls enable you to add new data.

PUT → Update or replace resources

Update an entire resource or create it if it does not exist.

PUT /api/users/12345
Content-Type: application/json

{
  "name": "Patty Postmanaut",
  "role": "Senior Developer"
}

Use cases:

• Updating a complete user profile

• Replacing configuration settings

• Overwriting a document

PATCH → Partial updates

Modify only specific fields of a resource.

PATCH /api/users/12345
Content-Type: application/json

{
  "role": "Senior Developer"
}

Use cases:

• Updating just an email address

• Changing a single setting

• Modifying specific product details

DELETE → Remove resources

Remove a specific resource from the server.

PATCH /api/users/12345
Content-Type: application/json

{
  "role": "Senior Developer"
}

Example use cases:

• Deleting a user account

• Removing an item from a cart

• Canceling an order

Understanding API responses

When you make an API call, you always receive a response containing key information.

Status codes

Status codes tell you the outcome of your API call:

2xx Success

• 200 OK → Request succeeded

• 201 Created → New resource created

• 204 No Content → Success with no response body

4xx Client errors

• 400 Bad Request → Invalid request format

• 401 Unauthorized → Authentication required

• 403 Forbidden → Permission denied

• 404 Not Found → Resource doesn’t exist

5xx Server errors

• 500 Internal Server Error → Server-side problem

• 503 Service Unavailable → Server temporarily down

Response headers

Headers contain metadata about the response:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 156
Cache-Control: max-age=3600
X-RateLimit-Remaining: 99

Response body

The response body contains the actual data returned by the API:

{
  "success": true,
  "data": {
    "id": "12345",
    "name": "Poe St. Mann"
  },
  "message": "User retrieved successfully"
}

Real-world API call examples

These examples show some ways that applications use API functionality to interact with other services.

Social media post

When you post a status update:

POST /api/posts
Authorization: Bearer token123
Content-Type: application/json

{
  "text": "Just learned about API calls!",
  "visibility": "public"
}

Weather data retrieval

When a weather app checks current conditions:

GET /api/weather?city=London&units=metric
API-Key: your_api_key

Response:

{
  "temperature": 18,
  "condition": "Partly Cloudy",
  "humidity": 65,
  "wind_speed": 12
}

Payment processing

When completing an online purchase:

POST /api/payments
Authorization: Bearer token123
Content-Type: application/json

{
  "amount": 99.99,
  "currency": "USD",
  "payment_method": "credit_card",
  "card_token": "tok_visa_4242"
}

Authentication methods

Most APIs require authentication to verify the caller’s identity and control access.

API keys

Simple string tokens passed in headers or query parameters:

GET /api/data
X-API-Key: sk_live_abc123xyz789

Bearer tokens

JWT or OAuth tokens included in the Authorization header:

POST /api/resource
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

Basic authentication

Username and password encoded in Base64:

GET /api/users
Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ=

Always keep credentials secure. Regardless of the authentication method, never hardcode them in public code or client-side apps.

Making API calls in Postman

Postman makes it easy to test and debug API calls without writing code.

To make an API call:

1. Click New → HTTP Request.

2. Select the HTTP method and enter the endpoint URL.

3. Add necessary headers, such as Content-Type: application/json.

4. Add a body for POST, PUT, or PATCH requests.

5. Click Send to see the response.

You can also use variables for reusable values like base URLs and tokens:

{{base_url}}/api/users/{{user_id}}

This improves flexibility when testing different environments or API functionality.

Best practices for API calls

• Use the correct HTTP method. Follow REST conventions for predictable behavior. Don’t use GET for operations that modify data or POST when you should use PUT or DELETE.

• Handle errors. Always check response status codes and handle errors appropriately:

if (response.status === 200) {
  // Success → process the data
} else if (response.status === 404) {
  // Resource not found
} else if (response.status === 500) {
  // Server error → retry or show error message
}

• Include proper headers. Always specify Content-Type and Accept to indicate your preferred response format:

  Content-Type: application/json
  Accept: application/json

• Respect rate limits. APIs often limit how many calls you can make per minute or hour. Monitor headers such as X-RateLimit-Limit and use appropriate throttling.

• Use HTTPS. Always make API calls over HTTPS to encrypt data in transit and protect sensitive information like authentication tokens.

• Cache GET requests. Cache GET responses when the data doesn’t change frequently to reduce API calls and improve performance.

• Implement retry logic. Network issues happen. Implement exponential backoff for failed requests, especially for critical operations.

Common mistakes to avoid

Exposing API keys in client-side code

Never hardcode API keys in mobile apps or JavaScript that runs in browsers. Use server-side proxies or environment variables instead.

Not validating input

Always validate data before making API calls to catch errors early and prevent unnecessary requests.

Ignoring response pagination

When fetching large datasets, APIs typically paginate results. Always handle pagination properly:

GET /api/products?page=1&limit=20

Making too many sequential calls

Batch operations when possible instead of making individual API calls in loops. Many APIs offer batch endpoints for this purpose.

Forgetting timeouts

Set appropriate timeout values to prevent your application from hanging indefinitely on slow or unresponsive APIs.

Monitoring and debugging

Use logging

Log all API calls, including timestamps, endpoints, parameters, and response codes, to troubleshoot issues.

Check API documentation

Always refer to the official API documentation for correct endpoints, required parameters, and expected response formats.

Test in Postman first

Before integrating API calls into your application, test them in Postman to verify they work as expected.

Monitor API health

Track metrics like response times, error rates, and success rates to identify performance issues or outages.

Quick Reference

Question	Answer
What is an API call?	A request from a client to an API endpoint to perform an operation or retrieve data.
Are API calls and API requests the same?	Yes, the terms are often used interchangeably.
Are API calls free?	It depends on the API. Many offer free tiers with rate limits.
What makes a call successful?	Correct HTTP method, endpoint, headers, authentication, and well-formatted data.