Bug Root Cause Finder

Systematic methods for finding the true source of bugs, not just symptoms.

Core Principle: Symptom ≠ Cause

The location where an error manifests is rarely where the bug originates.

Error Location:     OrderController::show() - NullPointerException
Symptom Location:   OrderRepository::find() - returns null
Root Cause:         OrderCreatedHandler - failed to persist order
True Root Cause:    RabbitMQ message lost due to missing ACK

Method 1: 5 Whys Technique

Ask "why" repeatedly until you reach the root cause.

Example Analysis

Bug: "Customer sees wrong order total"

1. Why? → The total displayed is $0
2. Why? → Order::getTotal() returns 0
3. Why? → OrderItem collection is empty
4. Why? → Items weren't loaded from database
5. Why? → Lazy loading failed due to closed EntityManager

Root Cause: EntityManager closed before accessing lazy-loaded collection

Fix: Eager load items in repository query, not lazy load

5 Whys Template

## 5 Whys Analysis

**Bug Description:** [What user sees]

1. Why does [symptom] occur?
   → [First-level cause]

2. Why does [first-level cause] happen?
   → [Second-level cause]

3. Why does [second-level cause] happen?
   → [Third-level cause]

4. Why does [third-level cause] happen?
   → [Fourth-level cause]

5. Why does [fourth-level cause] happen?
   → [ROOT CAUSE]

**Fix Location:** [File:line where fix should be applied]
**Fix Type:** [Category: logic/null/boundary/race/resource/exception/type/sql/infinite]

Method 2: Fault Tree Analysis

Build a tree of all possible causes for a failure.

Fault Tree Structure

[FAILURE: Order total is $0]
        │
        ├── [OR] Order has no items
        │       ├── [AND] Items not added during creation
        │       │       ├── Cart was empty
        │       │       └── Cart-to-Order mapping failed
        │       │
        │       └── [AND] Items were deleted
        │               ├── Cascade delete triggered
        │               └── Manual deletion bug
        │
        └── [OR] Items exist but total calculation wrong
                ├── Price is 0
                │       ├── Product price not set
                │       └── Currency conversion failed
                │
                └── Quantity is 0
                        ├── Validation missing
                        └── Type coercion (string "0")

Fault Tree Investigation Order

1. Start with most likely branches (based on code review)
2. Add logging/debugging to verify each branch
3. Eliminate branches systematically
4. Focus on remaining possibilities

Method 3: Git Bisect

Find the exact commit that introduced a bug.

Git Bisect Steps

# 1. Start bisect
git bisect start

# 2. Mark current (broken) as bad
git bisect bad

# 3. Mark known good commit (e.g., last release)
git bisect good v2.3.0

# 4. Git checks out middle commit - test it
# Run your reproduction test
php artisan test --filter=OrderTotalTest

# 5. Mark result
git bisect good  # if test passes
git bisect bad   # if test fails

# 6. Repeat until Git finds the culprit commit
# Git will output: "abc123 is the first bad commit"

# 7. Examine the commit
git show abc123

# 8. End bisect
git bisect reset

Automated Git Bisect

# Run automatically with test script
git bisect start HEAD v2.3.0
git bisect run php artisan test --filter=OrderTotalTest

Git Bisect Tips

• Choose good boundaries: Bad = current, Good = last known working
• Use automated testing: Faster and more reliable
• Check for flaky tests: Bisect fails with inconsistent tests
• Look at the diff: Focus on changed lines in culprit commit

Method 4: Stack Trace Parsing

Extract actionable information from error traces.

PHP Stack Trace Structure

Fatal error: Uncaught TypeError: OrderService::calculateTotal():
Argument #1 ($items) must be of type array, null given,
called in /app/src/Application/UseCase/CreateOrderUseCase.php on line 45

Stack trace:
#0 /app/src/Application/UseCase/CreateOrderUseCase.php(45): OrderService->calculateTotal(NULL)
#1 /app/src/Presentation/Api/OrderController.php(32): CreateOrderUseCase->execute(Object(CreateOrderCommand))
#2 /app/vendor/symfony/http-kernel/HttpKernel.php(163): OrderController->create(Object(Request))
#3 /app/vendor/symfony/http-kernel/HttpKernel.php(75): HttpKernel->handleRaw(Object(Request))
#4 /app/public/index.php(25): HttpKernel->handle(Object(Request))
#5 {main}

thrown in /app/src/Domain/Service/OrderService.php on line 23

Key Information to Extract

Element	Value	Meaning
Error Type	TypeError	Type mismatch bug
Message	Argument #1 must be array, null given	Null pointer issue
Thrown Location	OrderService.php:23	Where error detected
Call Location	CreateOrderUseCase.php:45	Where bad value passed
Root Investigation	CreateOrderUseCase.php:45	Start here

Stack Trace Analysis Steps

1. Read error message - What type of error?
2. Find thrown location - Where was error detected?
3. Find call location - Where was bad value passed?
4. Trace backward - How did bad value get there?
5. Find origin - Where was value first set to bad state?

Common Stack Trace Patterns

Pattern: Null from Repository

#0 Repository->find() returns null
#1 Service uses result without check
#2 Controller calls service

→ Fix in #1: Add null check

Pattern: Type Coercion

#0 Method expects int, gets string
#1 Request data not validated
#2 Controller passes raw input

→ Fix in #1 or #2: Add validation/casting

Pattern: Missing Dependency

#0 Service->method() called
#1 Container->get() fails
#2 Dependency not registered

→ Fix: Register dependency in container

Method 5: Dependency Graph Analysis

Trace data flow through the application.

Data Flow Tracing

// Trace the flow of $orderId
1. Controller receives $orderId from Request
2. UseCase receives $orderId as Command property
3. Repository uses $orderId in SQL query
4. Database returns null (ID doesn't exist)
5. UseCase passes null to Service
6. Service throws NullPointerException

Dependency Questions

• Where does the value originate?
• What transformations does it undergo?
• Where is it validated?
• Where could it become invalid?
• Who else uses this value?

Call Graph Investigation

# Find all callers of a method
grep -r "->calculateTotal(" src/

# Find all places where variable is set
grep -r "\$items\s*=" src/

# Find all null assignments
grep -r "= null" src/Domain/

Method 6: State Timeline Reconstruction

Rebuild the sequence of state changes.

Timeline Template

## State Timeline

T0: Initial state
    - Order::status = DRAFT
    - Order::items = []

T1: AddItemToOrder executed
    - Order::items = [Item(id=1)]
    - Expected: status stays DRAFT ✓

T2: SubmitOrder executed
    - Order::status = SUBMITTED
    - Expected: items preserved ✓

T3: PaymentReceived event
    - Order::status = PAID
    - BUG: items cleared unexpectedly ✗

T4: GetOrder query
    - Returns Order with empty items
    - User sees $0 total

Root Cause: PaymentReceived handler incorrectly reinitializes Order

Investigation Checklist

Before Starting

• Reproduce bug consistently
• Identify exact error message
• Note conditions when bug occurs
• Check if bug is environment-specific

During Investigation

• Parse stack trace for key locations
• Apply 5 Whys technique
• Build fault tree if multiple possible causes
• Use git bisect if recent regression
• Trace data flow from origin to error

After Finding Root Cause

• Verify fix addresses root cause, not symptom
• Check for similar bugs in related code
• Document root cause for team knowledge
• Consider if design change prevents recurrence

Quick Reference: Where to Look First

Bug Type	First Investigation Point
Null pointer	Repository/Factory that creates the null
Wrong calculation	Input values, not calculation logic
Missing data	Event handler or background job
Intermittent failure	Shared state or race condition
After deployment	Git diff between versions
Only in production	Environment config or data
Only for some users	User-specific data or permissions