Integration Testing: "Do Components Work Together?"

The tests that catch what unit tests can't — verifying that your services, databases, and APIs actually work together correctly.

What is Integration Testing?

Integration testing verifies that different modules/services communicate correctly when combined.

Component A works alone ✅

Component B works alone ✅

But do A and B work TOGETHER? ← Integration Test

Real-World Analogy:

Like testing a new bridge between two cities:

Individual Tests:

✓ City A's road system works

✓ City B's road system works

✓ Bridge structure is sound

Integration Test:

? Can cars actually drive from City A to City B via bridge?

? Do traffic signals coordinate properly?

? Do road signs align at connection points?

Integration Testing Example: E-commerce Order

Components involved:

1. [Frontend] - User Interface

2. [Auth Service] - User authentication

3. [Product Service] - Product catalog

4. [Cart Service] - Shopping cart

5. [Payment Service] - Payment processing

6. [Inventory Service] - Stock management

7. [Email Service] - Order confirmation

Integration Test: "Place an Order"

Step 1: Frontend → Auth Service

Action: User logs in

Verify: Auth token returned

Verify: Frontend stores token ✅

Step 2: Frontend → Product Service

Action: User searches products

Verify: Product list returned with prices

Verify: Frontend displays correctly ✅

Step 3: Frontend → Cart Service (with Auth)

Action: User adds product to cart

Verify: Auth token validated

Verify: Item added to user's cart

Verify: Cart total calculated ✅

Step 4: Cart Service → Inventory Service

Action: Check stock availability

Verify: Real-time stock checked

Verify: Cart shows "In Stock" or "Low Stock" ✅

Step 5: Frontend → Payment Service

Action: User enters payment info

Verify: Payment processed

Verify: Transaction ID returned ✅

Step 6: Payment Service → Inventory Service

Action: Deduct inventory

Verify: Stock count decremented

Verify: No overselling ✅

Step 7: Order Service → Email Service

Action: Send confirmation email

Verify: Email contains order details

Verify: User receives email ✅

Integration points tested:

✓ Auth token passed between services

✓ Product IDs match across services

✓ Price consistency

✓ Transaction atomicity (all or nothing)

✓ Data format compatibility

Types of Integration Testing:

1. Big Bang Integration

Test all components together at once:

Pros: Fast to set up

Cons: Hard to debug when something fails

2. Incremental Integration

Test pairs,

then gradually add more:

Pros: Easy to isolate issues

Cons: Takes more time

3. Top-Down Integration

Start from UI, stub lower components:

Test top-level flow first, replace stubs incrementally

4. Bottom-Up Integration

Start from database, build up:

Common Integration Issues Caught:

❌ Data format mismatch

Service A sends: { "date": "2024-01-15" }

Service B expects: { "date": "01/15/2024" }

Result: Parse error! 💥

❌ Authentication token expired

Service A: Calls Service B after 1 hour

Service B: "Token expired"

Result: Request fails! 💥

❌ Race condition

Service A: Updates inventory

Service B: Reads inventory (at same time)

Result: Wrong stock count! 💥

❌ Timeout

Service A: Waits for Service B response

Service B: Takes 60 seconds

Service A: Timeout at 30 seconds

Result: Request fails! 💥

Key characteristics:

• 🔗 Component interaction: Focus on interfaces between services
• 🎭 Can use mocks/stubs: Replace some components during testing
• 🐛 Catches integration bugs: Finds issues unit tests miss
• ⚙️ API contracts: Verifies services speak same language

Key Takeaways

1. Integration tests verify that components work together correctly — testing the boundaries between services, databases, and APIs
2. Use real dependencies when possible — testcontainers and Docker make it easy to spin up real databases and services in tests
3. Integration tests are slower than unit tests but catch different bugs — connection issues, serialization problems, and query errors
4. Test at the API boundary — send real HTTP requests to your service and verify the full response