Why testing validates contracts in Rest API - Visual Breakdown

Concept Flow - Why testing validates contracts

Define API Contract

↓

Write Tests Based on Contract

↓

Run Tests Against API Implementation

↓

Tests Pass?

No→Fix Implementation

Yes↓

Contract Validated

↓

Confidence in API Behavior

Testing checks if the API implementation follows the agreed contract by running tests based on that contract and confirming they pass.

Execution Sample

Rest API

GET /users/123 returns 200 with JSON {"id":123, "name":"value"}
Test: call GET /users/123
Assert status == 200
Assert response.id == 123

This test calls the API endpoint and checks if the response matches the contract.

Execution Table

Step	Action	Test Input	Expected Result	Actual Result	Pass/Fail
1	Send GET request to /users/123	GET /users/123	Status 200, JSON with id=123	Status 200, JSON with id=123	Pass
2	Check response status	Status 200	200	200	Pass
3	Check response body id	JSON id	123	123	Pass
4	All tests passed?	N/A	Yes	Yes	Pass

💡 All tests passed, so the API contract is validated.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
response_status	N/A	200	200	200	200
response_body.id	N/A	123	123	123	123
test_passed	false	true	true	true	true

Key Moments - 2 Insights

Why do we check both status and response body in tests?

Because the contract includes both the status code and the data format; checking both ensures the API behaves exactly as promised (see execution_table rows 2 and 3).

What happens if a test fails?

If any test fails, it means the API does not meet the contract and needs fixing before it can be trusted (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the response status after Step 1?

A 200

B 404

C 500

D 123

💡 Hint

Check the 'Actual Result' column in row 1 of the execution_table.

At which step do we verify the response body id matches the contract?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in the execution_table to find when the body id is checked.

If the response status was 404 instead of 200, what would happen in the tests?

A Tests would pass

B Tests would skip checking body

C Tests would fail

D Tests would ignore status

💡 Hint

Refer to the 'Pass/Fail' column and the explanation in key_moments about failing tests.

Concept Snapshot

Testing validates API contracts by:
- Defining expected inputs and outputs
- Writing tests that check status codes and response data
- Running tests against the API
- Passing tests confirm the API meets the contract
- Failing tests show where fixes are needed

Full Transcript

Testing validates contracts by running tests that check if the API returns the expected status codes and data. The process starts by defining the contract, then writing tests based on it. When tests run, they send requests and check responses. If all tests pass, the contract is confirmed. If any test fails, the API needs fixing. This ensures the API behaves as promised and clients can trust it.

Practice

1. Why is testing important for validating REST API contracts?

easy

A. It makes the API run faster by optimizing code.

B. It automatically generates API documentation.

C. It ensures the API follows agreed rules and behaves as expected.

D. It replaces the need for API design.

Solution

1. Step 1: Understand the purpose of API contracts

   API contracts define rules about how APIs should behave and what data they exchange.

2. Step 2: Role of testing in contracts

   Testing checks if the API follows these rules and returns expected results, catching mistakes early.

3. Final Answer:

   It ensures the API follows agreed rules and behaves as expected. -> Option C

4. Quick Check:

   Testing validates contracts = Ensures API rules followed [OK]

Hint: Testing checks if API follows rules and returns expected data [OK]

Common Mistakes:

• Thinking testing speeds up API automatically
• Confusing testing with documentation generation
• Believing testing replaces API design

2. Which of the following is the correct way to test a REST API contract for a GET request?

easy

A. Send a DELETE request and expect a 500 error.

B. Send a POST request and check if the response status is 404.

C. Send a GET request and ignore the response content.

D. Send a GET request and check if the response status is 200 and data format matches the contract.

Solution

1. Step 1: Identify correct HTTP method and expected response

   For a GET request, the API should return status 200 and data matching the contract.

2. Step 2: Match testing action to contract validation

   Sending GET and checking status 200 plus data format confirms contract adherence.

3. Final Answer:

   Send a GET request and check if the response status is 200 and data format matches the contract. -> Option D

4. Quick Check:

   GET test checks status 200 and data format [OK]

Hint: Test GET by checking status 200 and correct data format [OK]

Common Mistakes:

• Using wrong HTTP method for the test
• Ignoring response content in tests
• Expecting error codes for successful GET

3. Given this test code snippet for a REST API endpoint:

response = client.get('/users/123')
assert response.status_code == 200
assert 'id' in response.json()
assert response.json()['id'] == 123

What does this test validate about the API contract?

medium

A. The API returns status 200 and correct user id in JSON response.

B. The API returns status 404 for missing users.

C. The API accepts POST requests at /users/123.

D. The API returns an empty JSON response.

Solution

1. Step 1: Analyze the test assertions

   The test checks that the response status code is 200 and the JSON response contains an 'id' key equal to 123.

2. Step 2: Understand what this means for the API contract

   This confirms the API returns successful status and correct user data as per contract.

3. Final Answer:

   The API returns status 200 and correct user id in JSON response. -> Option A

4. Quick Check:

   Test checks status 200 and user id = 123 [OK]

Hint: Check status and key-value in JSON to validate contract [OK]

Common Mistakes:

• Confusing status 404 with 200
• Assuming POST method is tested here
• Ignoring JSON content checks

4. A test for a REST API endpoint expects a JSON response with a 'name' field. The test code is:

response = client.get('/items/5')
assert response.status_code == 200
assert 'name' in response.json()

But the test fails with a KeyError. What is the likely cause?

medium

A. The response status code is not 200.

B. The API response JSON does not include the 'name' field as per contract.

C. The test used the wrong HTTP method.

D. The client.get() method is not defined.

Solution

1. Step 1: Understand the test failure

   A KeyError means the 'name' key is missing in the JSON response.

2. Step 2: Identify contract violation

   The API response does not include the expected 'name' field, violating the contract.

3. Final Answer:

   The API response JSON does not include the 'name' field as per contract. -> Option B

4. Quick Check:

   Missing 'name' key causes KeyError [OK]

Hint: KeyError means expected JSON key missing in response [OK]

Common Mistakes:

• Assuming wrong HTTP method caused error
• Ignoring status code correctness
• Blaming client.get() method existence

5. You have a REST API contract that requires the endpoint /orders/{id} to return a JSON with fields id, status, and items (a list). Which test best validates this contract?

hard

A. Send GET /orders/10, check status 200, verify JSON has 'id', 'status', and 'items' keys, and 'items' is a list.

B. Send POST /orders with empty body and expect status 201.

C. Send GET /orders/10 and check only the status code is 404.

D. Send DELETE /orders/10 and expect status 500.

Solution

1. Step 1: Identify contract requirements

   The contract requires GET /orders/{id} to return JSON with 'id', 'status', and 'items' keys, where 'items' is a list.

2. Step 2: Match test to contract validation

   Sending GET /orders/10, checking status 200, verifying JSON has 'id', 'status', and 'items' keys, and confirming 'items' is a list fully validates the contract.

3. Final Answer:

   Send GET /orders/10, check status 200, verify JSON has 'id', 'status', and 'items' keys, and 'items' is a list. -> Option A

4. Quick Check:

   Test checks all required fields and types [OK]

Hint: Check all required fields and data types in JSON response [OK]

Common Mistakes:

• Testing wrong HTTP method
• Ignoring required JSON fields
• Not verifying data types like list