Hypothesis Testing

Observations:
- API returns 500 error on POST /orders
- Happens only when cart has > 10 items
- Started after deployment on 2024-01-15
- Works fine in staging environment
- Error logs show "connection refused" to inventory service

Hypothesis: Connection pool exhausted for large orders

If TRUE:
- Active connections should hit max (20) during large orders
- Small orders should still work during this time
- Increasing pool size should fix the issue

If FALSE:
- Connection count stays well below max
- Small orders also fail during the issue
- Pool size change has no effect

Test Plan for Connection Pool Hypothesis:

1. Add connection pool monitoring
   - Log active connections before/after each request
   - Expected if true: Count reaches 20 during failures

2. Artificial stress test
   - Send 5 large orders simultaneously
   - Expected if true: Failures start when pool exhausted

3. Increase pool size to 50
   - Repeat stress test
   - Expected if true: Failures stop or threshold moves

4. Control test with small orders
   - Send 20 small orders simultaneously
   - Expected if true: No failures (faster processing)

Results:
- Connection count reached 20/20 during failures ✓
- Small orders succeeded during same period ✓
- Pool size increase to 50 → failures stopped ✓

Conclusion: Hypothesis CONFIRMED
Connection pool exhaustion is the proximate cause.

New question: Why do large orders exhaust the pool?
New hypothesis: Large orders make multiple inventory calls per item

## Bug: [Description]

### Hypothesis 1: [Theory]
**Status:** Testing | Confirmed | Rejected
**Probability:** High | Medium | Low

**Evidence For:**
- [Evidence 1]
- [Evidence 2]

**Evidence Against:**
- [Evidence 1]

**Test Plan:**
1. [Test 1] - Expected result if true
2. [Test 2] - Expected result if false

**Test Results:**
- [Result 1]: [Supports/Contradicts]
- [Result 2]: [Supports/Contradicts]

**Conclusion:** [Confirmed/Rejected] because [reasoning]

---

### Hypothesis 2: [Next Theory]
...

// Add timing instrumentation
const start = performance.now();
await suspectedSlowOperation();
const duration = performance.now() - start;
console.log(`Operation took ${duration}ms`);
// Hypothesis confirmed if duration > expected

// Validate data at key points
function processWithValidation(data) {
  console.assert(data.id != null, 'Missing id');
  console.assert(data.items?.length > 0, 'Empty items');
  console.assert(typeof data.total === 'number', 'Invalid total');
  // If assertions fail, data hypothesis likely true
}

// Snapshot state before and after
const stateBefore = JSON.stringify(currentState);
suspectedStateMutation();
const stateAfter = JSON.stringify(currentState);
if (stateBefore !== stateAfter) {
  console.log('State changed:', diff(stateBefore, stateAfter));
}

Is the hypothesis testable?
├── NO → Refine it to be more specific
└── YES → Can I test it without side effects?
    ├── NO → Design a safe test (staging, logs-only)
    └── YES → Run the test
        └── Results conclusive?
            ├── NO → Design a better test
            └── YES → Hypothesis confirmed or rejected?
                ├── CONFIRMED → Root cause found?
                │   ├── YES → Fix and verify
                │   └── NO → Form next hypothesis (why?)
                └── REJECTED → Form next hypothesis