Mock vs stub comparison in Postman - Automation Approaches Compared

Compare behavior of mock server and stub in Postman

Preconditions (2)

Step 1: Create a stub response for GET /user/profile with fixed JSON data

Step 2: Send a request to the stub and verify the response matches the stub data

Step 3: Create a mock server in Postman with the same GET /user/profile endpoint and response

Step 4: Send a request to the mock server and verify the response matches the mock data

Step 5: Change the mock server response data and send the request again to verify dynamic response

Step 6: Attempt to change the stub response and observe that it requires manual update

✅ Expected Result: Stub returns fixed response that does not change unless manually updated. Mock server can return dynamic responses and simulate different scenarios without changing the actual API.

Automation Requirements - Postman test scripts

Assertions Needed:

Response body matches expected stub JSON

Response body matches expected mock JSON

Response status code is 200

Response changes when mock server response is updated

Best Practices:

Use pm.test and pm.expect for assertions

Use environment variables for URLs

Keep test scripts simple and readable

Use descriptive test names

Automated Solution

Postman

pm.test('Status code is 200', () => {
    pm.response.to.have.status(200);
});

const expectedStubResponse = {
    "id": 1,
    "name": "John Doe",
    "email": "[e-mail]"
};

pm.test('Response matches stub data', () => {
    const jsonData = pm.response.json();
    pm.expect(jsonData).to.eql(expectedStubResponse);
});

// For mock server test, update expectedMockResponse accordingly
const expectedMockResponse = {
    "id": 1,
    "name": "John Doe",
    "email": "[e-mail]"
};

pm.test('Response matches mock data', () => {
    const jsonData = pm.response.json();
    pm.expect(jsonData).to.eql(expectedMockResponse);
});

This Postman test script checks that the response status code is 200, which means the request was successful.

It then compares the JSON response body to the expected stub data using pm.expect().to.eql() to ensure the stub returns the fixed data.

Similarly, it compares the response to the expected mock data to verify the mock server returns the correct response.

Using environment variables for URLs allows easy switching between stub and mock endpoints.

Each test has a clear name to explain what it verifies.

Common Mistakes - 3 Pitfalls

Not verifying the response status code before checking the body

Hardcoding URLs in test scripts

Using loose equality checks instead of deep equality for JSON

Bonus Challenge

Now add data-driven testing by running the same tests with 3 different user profiles in stub and mock responses

Show Hint

Practice

1. Which statement best describes the difference between a mock and a stub in Postman testing?

easy

A. Mocks and stubs both return fixed responses without behavior simulation.

B. Mocks simulate API behavior with dynamic responses; stubs return fixed responses.

C. Stubs simulate API behavior with dynamic responses; mocks return fixed responses.

D. Mocks and stubs are the same and can be used interchangeably.

Solution

1. Step 1: Understand mock behavior in Postman

   Mocks simulate real API behavior and can return different responses based on requests, making them dynamic.

2. Step 2: Understand stub behavior in Postman

   Stubs provide fixed, predefined responses to replace real API calls, without simulating behavior.

3. Final Answer:

   Mocks simulate API behavior with dynamic responses; stubs return fixed responses. -> Option B

4. Quick Check:

   Mock = dynamic, Stub = fixed [OK]

Hint: Mocks are dynamic; stubs are fixed response tools [OK]

Common Mistakes:

• Confusing mocks as fixed response tools
• Thinking stubs simulate behavior dynamically
• Believing mocks and stubs are identical
• Assuming stubs can replace mocks fully

2. Which of the following is the correct way to create a stub in Postman?

easy

A. Create a collection with fixed example responses and disable real API calls.

B. Use Postman Mock Server with multiple dynamic response rules.

C. Write JavaScript code to simulate API logic inside Postman tests.

D. Use Postman monitors to generate live API responses.

Solution

1. Step 1: Identify stub creation method

   Stubs are created by defining fixed example responses in a Postman collection to replace real API calls.

2. Step 2: Eliminate other options

   Mocks use dynamic rules, JavaScript simulates logic but is not stub creation, and monitors check live APIs.

3. Final Answer:

   Create a collection with fixed example responses and disable real API calls. -> Option A

4. Quick Check:

   Stub = fixed examples in collection [OK]

Hint: Stubs use fixed examples, not dynamic rules [OK]

Common Mistakes:

• Confusing mocks with stubs in creation steps
• Thinking JavaScript test scripts create stubs
• Using monitors as stubs
• Assuming dynamic responses are stubs

3. Given a Postman mock server configured to return a 200 status with body {"success": true} for GET /user, what will happen if you replace it with a stub that always returns {"success": false} for the same request?

medium

A. Tests expecting success will fail because stub returns fixed failure response.

B. Tests will pass because stub simulates the same dynamic behavior as mock.

C. Tests will fail due to syntax errors in stub configuration.

D. Tests will pass because both mock and stub return 200 status.

Solution

1. Step 1: Analyze mock server response

   The mock server returns {"success": true} dynamically for GET /user, indicating success.

2. Step 2: Analyze stub response effect

   The stub always returns {"success": false}, a fixed failure response, which differs from mock.

3. Final Answer:

   Tests expecting success will fail because stub returns fixed failure response. -> Option A

4. Quick Check:

   Stub fixed failure ≠ mock success [OK]

Hint: Stub fixed response can break tests expecting mock's dynamic success [OK]

Common Mistakes:

• Assuming stub matches mock dynamic behavior
• Ignoring response body differences
• Thinking status code alone determines test pass
• Confusing syntax errors with logic errors

4. You created a Postman mock server but your tests always receive the stub's fixed response instead of the mock's dynamic response. What is the most likely cause?

medium

A. Postman does not support mock servers with dynamic responses.

B. The mock server is not enabled in Postman settings.

C. The mock server URL is incorrectly set to the stub server URL in the tests.

D. The stub server is down, causing fallback to mock responses.

Solution

1. Step 1: Identify URL configuration issue

   If tests receive stub responses instead of mock, likely the test requests use the stub server URL mistakenly.

2. Step 2: Verify other options

   Mock servers must be enabled; stub server down would not cause stub responses; Postman supports dynamic mocks.

3. Final Answer:

   The mock server URL is incorrectly set to the stub server URL in the tests. -> Option C

4. Quick Check:

   Wrong URL causes stub responses [OK]

Hint: Check URLs to avoid mixing mock and stub servers [OK]

Common Mistakes:

• Assuming mock server is disabled by default
• Believing stub server downtime causes stub responses
• Thinking Postman lacks dynamic mock support
• Ignoring URL configuration in tests

5. You want to test an API that returns different user roles based on request headers. Which approach best uses Postman mocks and stubs together to ensure reliable and flexible testing?

hard

A. Use neither mocks nor stubs; test only against the live API.

B. Use only stubs with fixed responses for all role variations to simplify tests.

C. Use only mocks with dynamic responses and avoid stubs to prevent confusion.

D. Use a mock server to simulate dynamic role responses and a stub with fixed responses for basic role tests.

Solution

1. Step 1: Identify need for dynamic and fixed responses

   Testing different user roles requires dynamic responses for flexibility and fixed responses for stable baseline tests.

2. Step 2: Combine mocks and stubs effectively

   Mocks simulate dynamic role-based responses; stubs provide fixed responses for simple, repeatable tests.

3. Final Answer:

   Use a mock server to simulate dynamic role responses and a stub with fixed responses for basic role tests. -> Option D

4. Quick Check:

   Mocks dynamic + stubs fixed = best combo [OK]

Hint: Combine mocks for dynamic and stubs for fixed tests [OK]

Common Mistakes:

• Using only stubs loses dynamic testing benefits
• Using only mocks may complicate simple tests
• Ignoring mocks and stubs reduces test reliability
• Testing only live API risks instability