Design First

Design First Methodology

You are following a design-first approach. Before writing any code, design the solution.

Core Principle

Think first, code second.

When to Use Design First

Apply this methodology when:

• Building a new feature or component
• Making significant architectural changes
• The task involves multiple components or systems
• Requirements are complex or ambiguous
• Multiple valid approaches exist

Skip for trivial changes (typos, simple bug fixes, config changes).

Design Process

Phase 1: Understand the Problem

Before designing, ensure clarity on:

Requirements Checklist:

• What is the user/business need?
• What are the inputs and outputs?
• What are the constraints (performance, security, compatibility)?
• What are the edge cases?
• What are the non-requirements (out of scope)?

Questions to Ask:

• What exactly should this do?
• What should it NOT do?
• How will users interact with it?
• How will it integrate with existing systems?
• What happens when things go wrong?

Phase 2: Explore Options

Generate multiple approaches before choosing:

## Option A: [Approach Name]

**Description:** [Brief explanation]

**Pros:**
- [Advantage 1]
- [Advantage 2]

**Cons:**
- [Disadvantage 1]
- [Disadvantage 2]

**Complexity:** Low/Medium/High

---

## Option B: [Approach Name]
...

Evaluate options against:

• Requirements fit
• Implementation complexity
• Maintenance burden
• Performance characteristics
• Team familiarity

Phase 3: Design the Solution

Create a design document covering:

System Overview

┌─────────────┐    ┌─────────────┐    ┌─────────────┐
│   Client    │───▶│   Service   │───▶│  Database   │
└─────────────┘    └─────────────┘    └─────────────┘

Data Model

interface Order {
  id: string;
  customerId: string;
  items: OrderItem[];
  status: OrderStatus;
  createdAt: Date;
  total: Money;
}

API/Interface Design

// Public interface
interface OrderService {
  createOrder(customerId: string, items: OrderItem[]): Promise<Order>;
  getOrder(orderId: string): Promise<Order | null>;
  cancelOrder(orderId: string): Promise<void>;
}

Key Algorithms/Logic

Order Total Calculation:
1. Sum item prices (price × quantity)
2. Apply discounts (percentage-based first, then fixed)
3. Calculate tax (rate based on customer location)
4. Add shipping (free over threshold, otherwise flat rate)

Error Handling

• What errors can occur?
• How should they be handled?
• What should users see?

Phase 4: Validate the Design

Before implementing, validate:

Self-Review:

• Does it meet all requirements?
• Are there simpler alternatives?
• What could go wrong?
• Is it testable?

External Validation:

• Rubber duck explanation (explain to yourself/others)
• Quick review with teammate
• Check against similar patterns in codebase

Design Document Template

Use DESIGN_TEMPLATE.md as the standard artifact for each feature. It covers:

• Overview — one-paragraph summary of what is being built and why
• Requirements — functional and non-functional (performance, security)
• Architecture — component diagram and explanation
• Data Model — entity definitions and relationships
• API Design — interface definitions
• Key Decisions — decision table with options considered, choice made, and rationale
• Implementation Plan — ordered steps
• Testing Strategy — unit and integration test scope
• Open Questions — unresolved items

Create this file at the start of each design session and keep it updated as the design evolves.

Design Levels

High-Level Design (Architecture)

• System components and their interactions
• Data flow between systems
• Technology choices
• Deployment architecture

Mid-Level Design (Module/Component)

• Class/module structure
• Interfaces and contracts
• State management
• Error handling strategy

Low-Level Design (Implementation)

• Algorithm details
• Data structures
• Method signatures
• Edge case handling

Anti-Patterns to Avoid

Integration with Implementation

After design is approved:

1. Review the design one more time before coding
2. Break into tasks using task-decomposition skill
3. Implement incrementally - verify design assumptions as you code
4. Update design if you discover issues during implementation

The design document is living — update it as you learn.

Signals You Need More Design

• "I'm not sure where to start"
• "This is getting complicated"
• "I keep refactoring"
• "The requirements are unclear"
• "Multiple approaches seem valid"

Stop and design before proceeding.

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