API Testing – What to Verify, Its Advantages & Difference from Web Services

🚀 API Testing – What to Verify, Its Advantages & Difference from Web Services

---

🧩 What Exactly Needs to Be Verified in API Testing?

When performing API Testing, the goal isn’t just to check if an endpoint works — it’s to validate how reliably and efficiently the API behaves under various conditions.

Here are the key elements every tester should verify:

1️⃣ Response Status Codes

Each API response carries an HTTP status code that tells you if the request was successful or failed.
Examples:

• ✅ 200 OK – Request successful

• ❌ 400 Bad Request – Invalid request

• 🔒 401 Unauthorized – Authentication required

• 🚫 404 Not Found – Resource doesn’t exist

• 💥 500 Internal Server Error – Server-side issue

---

2️⃣ Response Body Validation

Validate whether:

• The data returned matches expected values.

• Field names, data types, and structures align with the API specification.

• No missing or extra fields are returned.

Example:
If the API should return:

{
  "id": 101,
  "name": "John Doe",
  "email": "john@example.com"
}

You must confirm all keys and data types (int, string, etc.) are accurate.

---

3️⃣ Response Time & Performance

APIs must respond quickly and consistently.
⏱️ Ideally, response times should be under 1–2 seconds for most business APIs.
You can measure this in Postman or automation frameworks like RestAssured.

---

4️⃣ Authentication & Authorization

APIs often use tokens or keys to ensure secure access.
Verify that:

• Unauthorized requests are rejected.

• Valid tokens grant correct access levels.

• Session expiry or token invalidation works properly.

---

5️⃣ Error Handling

Good APIs fail gracefully.
Ensure error messages are:

• Readable and consistent.

• Contain useful information without exposing sensitive data.

Example:
❌ Avoid → "SQL Exception: syntax error near 'user_id'"
✅ Prefer → "Invalid input: user_id must be a number"

---

6️⃣ Schema & Contract Validation

Use schema validation tools to confirm the API structure remains consistent even after new releases — helping you catch breaking changes early.

---

📊 API Verification Flow (Conceptual Diagram)

[Request Sent] 
      ↓
[Server Receives Request]
      ↓
[Process Business Logic]
      ↓
[Generate Response → Validate:
  ✓ Status Code
  ✓ Response Body
  ✓ Headers
  ✓ Time
  ✓ Schema]
      ↓
[Return Response to Client]

---

⚡ Advantages of API Testing

API testing provides early and fast feedback before UI layers are even built.
Here’s why every QA professional should focus on API-level validation:

Advantage	Description
1️⃣ Faster Execution	No GUI needed — APIs run directly at the service layer.
2️⃣ Early Bug Detection	You can test logic before the UI exists, reducing rework.
3️⃣ Language Independent	APIs exchange data in JSON/XML — any client can test them.
4️⃣ Reusable Test Automation	Once automated, API tests can run in CI/CD pipelines easily.
5️⃣ Improved Coverage	You can test scenarios difficult to perform via UI.
6️⃣ Better Stability	APIs rarely change compared to frontend elements, giving stable tests.

🧠 Tip: Combine API and UI testing to create a hybrid automation framework, ensuring both layers are verified for complete application coverage.

---

🌐 Difference Between API and Web Services

Many testers use “API” and “Web Service” interchangeably — but there’s a subtle difference.

Feature	API	Web Service
Definition	Interface that allows communication between two software components.	A specific type of API that operates over the web (HTTP, SOAP, REST).
Communication Medium	Can use any protocol — HTTP, HTTPS, TCP, etc.	Works only via the web using HTTP/HTTPS.
Data Format	Can exchange data in JSON, XML, or any format.	Usually XML (SOAP) or JSON (RESTful).
Dependency	Can exist without the internet (e.g., OS APIs, Library APIs).	Requires internet/network to communicate.
Example	Java SDK API, Database API, REST API.	SOAP-based weather service, RESTful booking service.

In short:
👉 Every Web Service is an API, but not every API is a Web Service.

---

💬 Summary

✔ API testing verifies status codes, data, schema, performance, and security.
✔ It helps detect defects early, ensures data consistency, and supports faster releases.
✔ Understanding the difference between APIs and Web Services helps you design better testing strategies.