bug-root-cause-finder

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")