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.