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Why We Test After Unit Tests: Real-World Reasons

Think of Building a House 

Unit testing is like checking individual bricks.
  • A bricklayer checks each brick: Is it solid? Is it the right shape? No cracks?
  • Each brick passes inspection 
Does this mean the house is ready to live in?
Absolutely not!

1. Bricks ≠ Walls

Even if every brick is perfect:
  • Are bricks laid straight? (Integration)
  • Is the mortar strong enough? (Interface testing)
  • Does the wall align with the blueprint? (System testing)
Real example:
A login button (unit) works perfectly when tested alone.
But does it talk correctly to the password database? Does it fail gracefully if the network drops?
Unit tests won’t catch this.

2. The "Unknown Unknowns"

Example: A car
  • Each part tested alone: engine , brakes , lights
  • But: When you drive in rain, the brakes squeal (environment issue).
  • When you use headlights + AC + radio together, the battery drains (integration load).
    Unit tests happen in a garage on a stand. Real tests happen on the road.

3. User Reality vs. Ideal Conditions

Unit tests check code in isolation with fake data.
Real-world mess:
  • User enters "O'Reilly" (with an apostrophe) in a name field → breaks SQL query.
  • User clicks "Submit" 5 times quickly → creates 5 duplicate orders.
  • App works on Wi-Fi but fails on 3G mobile data.
Unit tests won't simulate this chaos.

Real Software Example: E-commerce Checkout

Unit tests passed:
  • calculateTax() function works ✅
  • validateCreditCard() function works ✅
  • updateInventory() function works ✅
What still fails in real testing:
  1. Integration bug: calculateTax() is called twice during checkout (bug in workflow).
  2. System bug: After checkout, the "Thank You" email never sends (email service down).
  3. User bug: On iPhone Safari, the "Place Order" button is hidden below the keyboard.
  4. Performance bug: When 1000 users check out at once, the payment gateway times out.
Unit tests can't catch these — they only check each function in a lab setting.

The Big Picture

Unit testing is necessary but not sufficient.
It’s like having all healthy organs (heart, lungs, liver) but the body could still have:
  • Poor coordination (integration issues)
  • Allergic reactions (compatibility problems)
  • Bad reflexes under stress (performance issues)
We keep testing because the real world is messy, interconnected, and unpredictable — and software must survive in that world, not just in a perfect test bubble.

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2026-01-11 15:35:05
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