fail-fast（快速失败）

Fail Fast

Overview

When something goes wrong, fail immediately and visibly.

Don't hide errors with try/catch that returns defaults. Don't let invalid state propagate. Fail at the point of failure, not three layers later with corrupted data.

When to Use

• Writing error handling code
• Tempted to catch and return default
• Adding "defensive" null checks everywhere
• Wrapping everything in try/catch
• Returning error objects instead of throwing

The Iron Rule

NEVER hide failures. Fail loud, fail early.

No exceptions:

• Not for "the app shouldn't crash"
• Not for "return something rather than throw"
• Not for "handle errors gracefully"
• Not for "defensive programming"

Detection: The "Swallow" Smell

If errors disappear silently, you're failing slow:

// ❌ VIOLATION: Hiding failures
async function processPayment(userId: string, amount: number): Promise<PaymentResult> {
  try {
    const user = await getUser(userId);
    if (!user) return { success: false, error: 'User not found' };

    const card = await validateCard(user.cardToken);
    if (!card.valid) return { success: false, error: 'Invalid card' };

    const result = await chargeCard(card, amount);
    if (!result.success) return { success: false, error: 'Payment failed' };

    return { success: true, transactionId: result.id };
  } catch (error) {
    return { success: false, error: 'Internal error' };  // ← SWALLOWED!
  }
}

Problems:

• Caller doesn't know WHAT failed
• Stack trace is lost
• Bugs hide as "internal error"
• No visibility into actual failures

The Correct Pattern: Fail Fast

Throw at the point of failure. Let errors propagate:

// ✅ CORRECT: Fail fast
async function processPayment(userId: string, amount: number): Promise<Transaction> {
  // Validate early - fail fast on bad input
  if (!userId) throw new ValidationError('userId is required');
  if (amount <= 0) throw new ValidationError('amount must be positive');

  // Let failures propagate - don't swallow
  const user = await getUser(userId);
  if (!user) throw new NotFoundError(`User ${userId} not found`);

  const card = await validateCard(user.cardToken);
  if (!card.valid) throw new PaymentError('Card validation failed', card.errors);

  // This might throw - that's okay! Let it.
  const transaction = await chargeCard(card, amount);

  return transaction;
}

// Caller handles errors appropriately
try {
  const tx = await processPayment(userId, amount);
  res.json({ success: true, transactionId: tx.id });
} catch (error) {
  if (error instanceof ValidationError) {
    res.status(400).json({ error: error.message });
  } else if (error instanceof NotFoundError) {
    res.status(404).json({ error: error.message });
  } else if (error instanceof PaymentError) {
    res.status(402).json({ error: error.message });
  } else {
    // Unknown error - log it, return 500
    logger.error('Payment failed', error);
    res.status(500).json({ error: 'Internal server error' });
  }
}

Why Fail-Slow Is Dangerous

Fail Fast Techniques

1. Validate Early

function createUser(data: unknown): User {
  // Fail IMMEDIATELY on bad input
  if (!data || typeof data !== 'object') {
    throw new ValidationError('Invalid user data');
  }

  const { email, name } = data as Record<string, unknown>;

  if (!email || typeof email !== 'string') {
    throw new ValidationError('Email is required');
  }

  if (!name || typeof name !== 'string') {
    throw new ValidationError('Name is required');
  }

  // Only proceed with valid data
  return new User(email, name);
}

2. Assert Invariants

function withdraw(account: Account, amount: number): void {
  // Assert what must be true
  assert(amount > 0, 'Withdrawal amount must be positive');
  assert(account.balance >= amount, 'Insufficient funds');

  account.balance -= amount;

  // Post-condition check
  assert(account.balance >= 0, 'Balance went negative - invariant violated');
}

3. Use Type System

// ❌ Fail slow: null checks everywhere
function processOrder(order: Order | null): void {
  if (!order) return;  // Silent failure
  // ...
}

// ✅ Fail fast: require valid input
function processOrder(order: Order): void {
  // If order is null, TypeScript catches it
  // If it gets here with null, it will throw - good!
}

Pressure Resistance Protocol

1. "The App Shouldn't Crash"

Pressure: "Users will see errors if we throw"

Response: Users seeing a clear error is better than corrupted data or silent failure.

Action: Throw errors, catch at boundaries (API layer), return appropriate HTTP codes.

2. "Return Something Rather Than Throw"

Pressure: "Returning error objects is more functional"

Response: Error objects are fine IF callers check them. They usually don't.

Action: Throw for unexpected failures. Use Result types only if callers actually handle both cases.

3. "Handle Errors Gracefully"

Pressure: "Graceful = don't throw"

Response: Graceful = appropriate response. Swallowing is not graceful.

Action: Throw, catch at boundary, return meaningful error response.

4. "Defensive Programming"

Pressure: "Defensive code handles all cases"

Response: Defensive = validate early and fail. Not = hide failures.

Action: Validate inputs, assert invariants, throw on violations.

Red Flags - STOP and Reconsider

If you notice ANY of these, refactor:

• catch (e) {return null;}
• catch (e) {return {success: false};}
• if (!x) return; (silent early return)
• try {} catch {} (empty catch)
• Returning default values on error
• "Error: Internal error" (generic catch-all)
• Logs error but continues execution

All of these mean: Let the error propagate or throw explicitly.

Quick Reference

Common Rationalizations (All Invalid)

The Bottom Line

Fail fast. Fail loud. Fail at the source.

When errors occur: throw immediately with context. Let errors propagate to boundaries where they can be logged and translated to user-appropriate responses. Never swallow. Never return defaults to hide failure.