backend-fundamentals

Backend Fundamentals Review

"APIs are contracts. Break them, and you break trust."

When to Apply

Activate this skill when reviewing:

• API route handlers
• Express/Fastify/Hono middleware
• Database queries and models
• Authentication/authorization logic
• Server-side business logic

Review Checklist

API Design

• RESTful: Do routes follow REST conventions? (GET for read, POST for create, etc.)
• Naming: Are endpoints nouns, not verbs? (/users not /getUsers)
• Versioning: Is API versioned for future changes? (/api/v1/)
• Status Codes: Are correct HTTP status codes returned?

Separation of Concerns

• Routes: Do routes only handle HTTP concerns (req/res)?
• Controllers: Is business logic in controllers/services, not routes?
• Services: Is data access abstracted from business logic?
• Models: Are models responsible only for data shape/validation?

Error Handling

• Try/Catch: Are async operations wrapped properly?
• Error Responses: Are errors returned with proper status codes?
• Logging: Are errors logged with context?
• No Leaks: Are internal errors hidden from clients?

Security

• Input Validation: Is ALL input validated before use?
• Authentication: Are protected routes actually protected?
• Authorization: Can users only access their own data?
• Rate Limiting: Are endpoints protected from abuse?

Common Mistakes (Anti-Patterns)

1. Fat Routes

❌ app.post('/users', async (req, res) => {
     // 100 lines of validation, business logic, DB queries
   });

✅ app.post('/users', validateUser, userController.create);

2. No Input Validation

❌ const { email } = req.body;
   await db.query(`SELECT * FROM users WHERE email = '${email}'`);

✅ const { email } = validateBody(req.body, userSchema);
   await User.findByEmail(email); // parameterized

3. Wrong Status Codes

❌ res.status(200).json({ error: 'Not found' });

✅ res.status(404).json({ error: 'User not found' });

4. Leaking Internal Errors

❌ catch (error) {
     res.status(500).json({ error: error.message, stack: error.stack });
   }

✅ catch (error) {
     logger.error('User creation failed', { error, userId });
     res.status(500).json({ error: 'Something went wrong' });
   }

Socratic Questions

Ask the junior these questions instead of giving answers:

1. Architecture: "If I wanted to switch from Express to Fastify, what would need to change?"
2. Validation: "What happens if someone sends malformed JSON?"
3. Auth: "How do you know this user owns this resource?"
4. Errors: "What does the client see when the database is down?"
5. Testing: "How would you test this endpoint in isolation?"

HTTP Status Code Reference

Architecture Layers

Request → Route → Controller → Service → Repository → Database
                     ↓
              Middleware (auth, validation, logging)

Red Flags to Call Out