Contract testing in Rest API

Introduction

Contract testing helps make sure two parts of a system agree on how they talk to each other. It checks that the data sent and received matches what both sides expect.

When you have a frontend app talking to a backend API and want to avoid surprises.

When different teams build different services that need to work together smoothly.

Before releasing updates to an API to make sure clients won't break.

When you want to catch communication errors early without full end-to-end tests.

Syntax

Rest API

contract_test {
  provider: "API Service",
  consumer: "Frontend App",
  interactions: [
    {
      request: {
        method: "GET",
        path: "/users/123"
      },
      response: {
        status: 200,
        body: {
          id: 123,
          name: "Alice"
        }
      }
    }
  ]
}

This is a simplified example of a contract test definition.

It describes what the consumer expects from the provider for a specific request.

Examples

This example shows a POST request contract where the consumer expects a new order ID in response.

Rest API

contract_test {
  provider: "Order API",
  consumer: "Mobile App",
  interactions: [
    {
      request: { method: "POST", path: "/orders" },
      response: { status: 201, body: { orderId: 789 } }
    }
  ]
}

This example shows a DELETE request contract where the consumer expects a 204 No Content response.

Rest API

contract_test {
  provider: "User Service",
  consumer: "Web Client",
  interactions: [
    {
      request: { method: "DELETE", path: "/users/456" },
      response: { status: 204 }
    }
  ]
}

Sample Program

This simple Python program tests if the API returns the expected user data for user 123. It checks the status code and required fields.

Rest API

import requests

def test_get_user_contract():
    url = "https://api.example.com/users/123"
    response = requests.get(url)
    assert response.status_code == 200
    data = response.json()
    assert "id" in data and data["id"] == 123
    assert "name" in data

if __name__ == "__main__":
    test_get_user_contract()
    print("Contract test passed!")

Important Notes

Contract tests focus on the interface, not the internal logic of services.

They help catch mismatches early, saving time and bugs in production.

Use contract testing alongside other tests for best results.

Summary

Contract testing ensures two systems agree on data exchange.

It is useful when different teams or services communicate.

It helps catch errors early and keeps integrations stable.

Practice

1. What is the main purpose of contract testing in REST APIs?

easy

A. To check the API's user interface design

B. To test the speed of the API response

C. To ensure two systems agree on the data they exchange

D. To monitor server hardware performance

Solution

1. Step 1: Understand contract testing definition

   Contract testing checks if two systems agree on the data format and behavior during communication.

2. Step 2: Compare options to definition

   Only To ensure two systems agree on the data they exchange matches this purpose; others relate to speed, UI, or hardware, which are unrelated.

3. Final Answer:

   To ensure two systems agree on the data they exchange -> Option C

4. Quick Check:

   Contract testing = data agreement [OK]

Hint: Contract testing checks data agreement between systems [OK]

Common Mistakes:

• Confusing contract testing with performance testing
• Thinking contract testing checks UI or hardware
• Assuming contract testing is about security

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

easy

A. Test the API's front-end layout on different browsers

B. Measure the time taken for the API to respond

C. Verify the API server's CPU usage during requests

D. Check if the API returns the expected status code and response body

Solution

1. Step 1: Identify contract test elements

   Contract tests verify the API response matches the agreed format, including status codes and body content.

2. Step 2: Match options to contract test definition

   Check if the API returns the expected status code and response body matches this exactly; others relate to performance, server metrics, or UI, which are not contract tests.

3. Final Answer:

   Check if the API returns the expected status code and response body -> Option D

4. Quick Check:

   Contract test = expected response check [OK]

Hint: Contract tests check expected status and response body [OK]

Common Mistakes:

• Confusing contract tests with performance or UI tests
• Ignoring response body content in contract tests
• Testing server hardware instead of API contract

3. Given this contract test snippet in JavaScript using Pact: <pre>provider.addInteraction({ state: 'user exists', uponReceiving: 'a request for user data', withRequest: { method: 'GET', path: '/user/123' }, willRespondWith: { status: 200, body: { id: 123, name: 'Alice' } } }); What will happen if the provider returns status 404 instead of 200 during the test?

medium

A. The test will pass because the body matches the contract

B. The test will fail because the status code doesn't match the contract

C. The test will ignore the status code and only check the path

D. The test will pass if the response time is fast enough

Solution

1. Step 1: Understand the contract test expectation

   The contract expects a 200 status code with a specific body when requesting '/user/123'.

2. Step 2: Analyze the provider's actual response

   The provider returns 404, which does not match the expected 200 status code.

3. Final Answer:

   The test will fail because the status code doesn't match the contract -> Option B

4. Quick Check:

   Status mismatch causes test failure [OK]

Hint: Mismatch in status code causes contract test failure [OK]

Common Mistakes:

• Assuming body match overrides status code mismatch
• Ignoring status code in contract tests
• Thinking response time affects contract test result

4. You wrote a contract test that expects a JSON response with a field email. The test fails because the provider returns emailAddress instead. What is the best way to fix this?

medium

A. Change the provider to return email as expected by the contract

B. Update the contract to expect emailAddress instead of email

C. Ignore the field mismatch and pass the test anyway

D. Remove the email field from the contract test

Solution

1. Step 1: Identify contract responsibility

   The contract defines the expected data format; the provider must follow it.

2. Step 2: Choose the correct fix

   Changing the provider to return email matches the contract, ensuring stable integration.

3. Final Answer:

   Change the provider to return email as expected by the contract -> Option A

4. Quick Check:

   Provider must match contract fields [OK]

Hint: Provider must match contract fields exactly [OK]

Common Mistakes:

• Ignoring field name mismatches
• Changing contract to fit provider without agreement
• Removing important fields from contract

5. You have two microservices: Service A (consumer) and Service B (provider). Service A expects Service B's API to return a list of products with fields id, name, and price. You want to write a contract test that allows price to be optional (sometimes missing). Which approach best achieves this?

hard

A. Define the contract so price is optional using a matcher that allows missing fields

B. Require price always in the contract and fail if missing

C. Remove price from the contract entirely

D. Ignore the contract and test only after deployment

Solution

1. Step 1: Understand optional fields in contract testing

   Contract tests can use matchers to allow fields to be optional or have flexible types.

2. Step 2: Choose the approach that supports optional price

   Defining price as optional with a matcher allows the provider to omit it without failing the test.

3. Final Answer:

   Define the contract so price is optional using a matcher that allows missing fields -> Option A

4. Quick Check:

   Use matchers for optional fields in contracts [OK]

Hint: Use matchers to allow optional fields in contract tests [OK]

Common Mistakes:

• Forcing optional fields to be always present
• Removing important fields from contract
• Skipping contract tests and testing only post-deployment