Contract testing (Pact) in Microservices - Architecture Diagram

System Overview - Contract testing (Pact)

This system uses contract testing with Pact to ensure that microservices communicate correctly. It verifies that the service provider meets the expectations defined by the service consumer before deployment, reducing integration errors.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  +-------------------+
  |                   |
Consumer Service   Provider Service
  |                   |
  v                   v
Consumer Pact Mock  Provider Pact Verifier
  |                   |
  +---------+---------+
            |
            v
       Pact Broker
            |
            v
       CI/CD Pipeline

Components

User

user

Initiates requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to appropriate microservices

Consumer Service

service

Microservice that consumes APIs from Provider Service

Provider Service

service

Microservice that provides APIs to Consumer Service

Consumer Pact Mock

mock_service

Simulates Provider Service behavior for Consumer tests

Provider Pact Verifier

verifier

Verifies Provider Service meets Consumer's contract

Pact Broker

broker

Stores and shares contracts between Consumer and Provider

CI/CD Pipeline

pipeline

Automates contract testing and deployment

Request Flow - 14 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Consumer Service

Consumer Service → Provider Service

Provider Service → Consumer Service

Consumer Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Consumer Service → Consumer Pact Mock

Provider Pact Verifier → Provider Service

Consumer Pact Mock → Pact Broker

Provider Pact Verifier → Pact Broker

CI/CD Pipeline → Consumer Pact Mock

CI/CD Pipeline → Provider Pact Verifier

Failure Scenario

Component Fails:Pact Broker

Impact:Contracts cannot be shared or fetched, blocking contract testing and deployments

Mitigation:Use broker replication and backups; fallback to cached contracts in CI/CD pipeline

Architecture Quiz - 3 Questions

Which component stores and shares the contracts between services?

A Pact Broker

B API Gateway

C Load Balancer

D Consumer Pact Mock

Design Principle

This architecture ensures microservices communicate reliably by verifying contracts before deployment. It uses a broker to share contracts and mocks/verifiers to test both sides independently, reducing integration errors and improving deployment confidence.

Practice

1. What is the main purpose of contract testing in microservices using Pact?

easy

A. To check database schema consistency

B. To test the user interface of a microservice

C. To verify that services agree on request and response formats

D. To measure the performance of a microservice

Solution

1. Step 1: Understand contract testing role

   Contract testing ensures that two services agree on how they communicate, specifically the request and response formats.

2. Step 2: Identify Pact's function

   Pact automates contract testing by creating and verifying these agreements between microservices.

3. Final Answer:

   To verify that services agree on request and response formats -> Option C

4. Quick Check:

   Contract testing = Verify service agreements [OK]

Hint: Contract testing checks communication agreements, not UI or performance [OK]

Common Mistakes:

• Confusing contract testing with UI testing
• Thinking contract testing checks database schemas
• Assuming contract testing measures performance

2. Which of the following is the correct Pact file format used to define a contract?

easy

A. JSON

B. YAML

C. XML

D. CSV

Solution

1. Step 1: Identify Pact contract format

   Pact contracts are written in JSON format to describe interactions between services.

2. Step 2: Eliminate other formats

   YAML, XML, and CSV are not used by Pact for contract files.

3. Final Answer:

   JSON -> Option A

4. Quick Check:

   Pact contract format = JSON [OK]

Hint: Pact contracts are JSON files describing service interactions [OK]

Common Mistakes:

• Assuming Pact uses YAML or XML
• Confusing data formats with contract formats
• Thinking CSV can describe complex contracts

3. Given the following Pact interaction snippet, what is the expected response status code?

{"request": {"method": "GET", "path": "/users/123"}, "response": {"status": 200, "body": {"id": 123, "name": "Alice"}}}

medium

A. 404

B. 200

C. 500

D. 302

Solution

1. Step 1: Read the response status in the Pact snippet

   The response object shows "status": 200, indicating a successful request.

2. Step 2: Confirm status meaning

   Status 200 means OK, so the expected response code is 200.

3. Final Answer:

   200 -> Option B

4. Quick Check:

   Response status in Pact = 200 [OK]

Hint: Look for "status" field in response to find expected code [OK]

Common Mistakes:

• Confusing request method with response status
• Ignoring the status field in the response
• Choosing common error codes instead of actual status

4. A Pact test fails because the provider service returns an extra field not defined in the contract. What is the best way to fix this?

medium

A. Remove the extra field from the provider service response

B. Ignore the extra field in the provider service

C. Disable contract testing for this interaction

D. Update the contract to include the extra field

Solution

1. Step 1: Understand contract strictness

   Pact expects the provider response to match the contract exactly, including fields.

2. Step 2: Adjust contract or provider

   If the provider adds a new field, the contract must be updated to reflect this change to keep tests valid.

3. Final Answer:

   Update the contract to include the extra field -> Option D

4. Quick Check:

   Provider adds field -> update contract [OK]

Hint: Keep contract and provider response in sync to pass tests [OK]

Common Mistakes:

• Ignoring extra fields without updating contract
• Removing fields from provider causing data loss
• Disabling tests instead of fixing contract

5. You want to implement contract testing with Pact in a microservices system where multiple teams develop services independently. Which approach best ensures smooth integration?

hard

A. Each team publishes their Pact contracts to a shared broker for others to verify

B. Teams test contracts only locally without sharing

C. Use end-to-end tests only, skipping contract tests

D. Manually review API changes without automated tests

Solution

1. Step 1: Identify best practice for contract sharing

   Using a shared Pact broker allows teams to publish and verify contracts centrally, enabling independent development with integration confidence.

2. Step 2: Compare alternatives

   Local-only tests lack visibility; end-to-end tests are slower and less focused; manual reviews are error-prone.

3. Final Answer:

   Each team publishes their Pact contracts to a shared broker for others to verify -> Option A

4. Quick Check:

   Shared Pact broker = smooth integration [OK]

Hint: Use shared Pact broker for contract visibility and verification [OK]

Common Mistakes:

• Skipping contract tests for only end-to-end tests
• Not sharing contracts causing integration surprises
• Relying on manual API reviews