# Design Document Standards

> **Language**: English | [繁體中文](../locales/zh-TW/core/design-document-standards.md)

**Version**: 1.0.0
**Last Updated**: 2026-04-01
**Applicability**: All software projects
**Scope**: universal
**Industry Standards**: C4 Model, ADR (Architecture Decision Records), IEEE 1016
**References**: [c4model.com](https://c4model.com/), [adr.github.io](https://adr.github.io/)

---

## Overview

This document defines standards for creating, maintaining, and reviewing design documents in software projects. It covers High-Level Design (HLD) and Low-Level Design (LLD) structures, document lifecycle management, architecture diagram conventions using the C4 Model, design decision recording, and systematic design review.

Good design documents serve as both communication tools and living artifacts that evolve with the system.

---

## High-Level Design (HLD)

A High-Level Design document captures the system-wide architecture and strategic decisions. Every HLD SHALL include the following 6 sections:

### 1. Overview

**Purpose**: Define the problem statement, project goals, and success criteria.

**Content Guidance**:
- Problem being solved and its business context
- In-scope and out-of-scope boundaries
- Key success metrics
- Target audience for this document

### 2. Architecture

**Purpose**: Describe the system architecture decisions and component relationships.

**Content Guidance**:
- Architecture style (microservices, monolith, serverless, etc.)
- Key components and their responsibilities
- Technology stack selection with rationale
- Architecture diagrams (see C4 Model section)

### 3. Data Flow

**Purpose**: Describe how data moves through the system.

**Content Guidance**:
- Data flow diagrams showing inputs, transformations, and outputs
- Data storage locations and persistence strategy
- Data synchronization mechanisms (if applicable)
- Data volume and throughput estimates

### 4. API Surface

**Purpose**: Define the external interfaces exposed by the system.

**Content Guidance**:
- API endpoints or interface contracts
- Authentication and authorization model
- Versioning strategy
- Rate limiting and quota policies

### 5. Non-Functional Requirements

**Purpose**: Define quality attributes and constraints.

**Content Guidance**:
- Performance targets (latency, throughput)
- Scalability requirements (horizontal, vertical)
- Availability and reliability targets (SLA/SLO)
- Security requirements and compliance constraints

### 6. Milestones

**Purpose**: Define the implementation timeline and delivery phases.

**Content Guidance**:
- Phase breakdown with deliverables
- Dependencies between phases
- Risk factors and mitigation plans
- Go/no-go criteria for each milestone

---

## Low-Level Design (LLD)

A Low-Level Design document captures the detailed implementation design for a specific component or module. Every LLD SHALL include the following 5 sections:

### 1. Component Design

**Purpose**: Define the internal structure of the component.

**Content Guidance**:
- Class/module hierarchy and relationships
- Interface definitions and contracts
- Design patterns applied and rationale
- Dependency injection and configuration

### 2. Data Model

**Purpose**: Define the data schema and relationships.

**Content Guidance**:
- Entity-relationship diagrams
- Database schema definitions (tables, columns, types, constraints)
- Index strategy and query optimization considerations
- Data migration strategy (if modifying existing schema)

### 3. Algorithm Details

**Purpose**: Describe key algorithms and their complexity.

**Content Guidance**:
- Algorithm pseudocode or step-by-step description
- Time and space complexity analysis (Big-O notation)
- Edge cases and boundary conditions
- Performance benchmarks or expectations

### 4. Error Handling

**Purpose**: Define error scenarios and recovery strategies.

**Content Guidance**:
- Error classification (transient, permanent, partial)
- Retry policies and circuit breaker configuration
- Fallback behavior and graceful degradation
- Error propagation and user-facing error messages

### 5. Testing Strategy

**Purpose**: Define the test approach for the component.

**Content Guidance**:
- Unit test scope and key test cases
- Integration test boundaries
- Mock/stub strategy for external dependencies
- Performance test scenarios (if applicable)

---

## Document Lifecycle

Design documents SHALL follow a managed lifecycle with clear states and transitions.

### Lifecycle States

```
Draft → In Review → Approved → Implemented → Archived
```

| State | Definition | Entry Condition | Exit Condition | Responsible |
|-------|-----------|-----------------|----------------|-------------|
| **Draft** | Initial creation and iteration | Author starts writing | Author submits for review | Author |
| **In Review** | Under peer review and feedback | Submitted for review | All reviewers approve or reject | Reviewers |
| **Approved** | Accepted for implementation | Review approved | Implementation begins | Tech Lead / Architect |
| **Implemented** | Design realized in code | Code matches design | System is stable in production | Development Team |
| **Archived** | No longer active; kept for reference | System decommissioned or design superseded | N/A | Author / Tech Lead |

### Transition Rules

- **Draft → In Review**: Author confirms document is complete enough for meaningful review
- **In Review → Draft**: Reviewers request significant changes; author revises
- **In Review → Approved**: All required reviewers approve; no blocking issues remain
- **Approved → Implemented**: Implementation matches the approved design
- **Implemented → Archived**: System is decommissioned or a new design supersedes this one
- **Any state → Archived**: Document is abandoned (must document reason)

---

## Architecture Diagrams

### C4 Model

Design documents SHALL adopt the C4 Model for architecture diagrams, providing four levels of abstraction.

| Level | Name | Scope | Applicable Scenario | Required Elements |
|-------|------|-------|---------------------|-------------------|
| **L1** | **Context** | System and external actors | Stakeholder communication, project kickoff | System boundary, users, external systems, relationships |
| **L2** | **Container** | Applications and data stores | Technical overview, deployment planning | Applications, databases, message queues, protocols |
| **L3** | **Component** | Internal components | Developer onboarding, detailed design | Classes/modules, interfaces, dependencies |
| **L4** | **Code** | Class/module level detail | Complex algorithm documentation | UML class diagrams, sequence diagrams |

### Diagram Guidelines

- **Start from L1**: Always begin with a Context diagram before diving deeper
- **Not all levels required**: Use L1 and L2 for most projects; L3 and L4 only when complexity warrants
- **Keep diagrams current**: Update diagrams when architecture changes
- **Use consistent notation**: Follow C4 Model conventions for shapes, colors, and labels

---

## Design Decision Records

Every significant design decision SHALL be recorded with the following format.

### Required Fields

| Field | Purpose | Content |
|-------|---------|---------|
| **Options** | Available alternatives considered | List each option with brief description, pros, and cons |
| **Decision** | The chosen option | State which option was selected |
| **Rationale** | Reasoning behind the decision | Explain why this option was chosen over alternatives |

### Supplementary Fields

| Field | Purpose | Content |
|-------|---------|---------|
| **Constraints** | Limitations that influenced the decision | Technical, business, regulatory, or resource constraints |
| **Consequences** | Expected outcomes of this decision | Both positive and negative consequences; follow-up actions needed |
| **Status** | Current state of the decision | Proposed, Accepted, Deprecated, Superseded |
| **Date** | When the decision was made | ISO 8601 date format |

### Example Format

```markdown
## DD-001: Database Selection

**Status**: Accepted
**Date**: 2026-04-01

### Options
1. **PostgreSQL** — Mature RDBMS, strong JSON support. Pros: ecosystem, reliability. Cons: scaling complexity.
2. **MongoDB** — Document store, flexible schema. Pros: developer velocity. Cons: transaction limitations.
3. **DynamoDB** — Managed NoSQL. Pros: zero ops, auto-scaling. Cons: vendor lock-in, cost at scale.

### Decision
PostgreSQL

### Rationale
Our data model is relational with well-defined schemas. PostgreSQL provides ACID transactions required by our financial domain, strong JSON support for semi-structured data, and the team has extensive experience.

### Constraints
- Must support ACID transactions (regulatory requirement)
- Team has limited NoSQL experience
- Budget does not allow managed database services initially

### Consequences
- (+) Leverages existing team expertise
- (+) Strong ecosystem of tools and extensions
- (-) Requires manual scaling configuration for high traffic
- (-) Need to invest in connection pooling (PgBouncer)
```

---

## Design Review Checklist

Design documents SHALL be reviewed using the following checklist covering 5 dimensions. Each dimension includes specific check items.

### 1. Correctness

- [ ] Design meets all stated functional requirements
- [ ] Edge cases and boundary conditions are addressed
- [ ] Data flow is complete — no orphaned inputs or outputs
- [ ] Error scenarios are handled appropriately

### 2. Scalability

- [ ] Design supports expected load growth (10x, 100x)
- [ ] Bottlenecks are identified with mitigation plans
- [ ] Horizontal scaling strategy is defined (if applicable)
- [ ] Data partitioning or sharding strategy is considered

### 3. Security

- [ ] Authentication and authorization are properly designed
- [ ] Data encryption at rest and in transit is specified
- [ ] Input validation and sanitization are addressed
- [ ] Compliance requirements are met (GDPR, SOC2, etc.)

### 4. Maintainability

- [ ] Components have clear boundaries and responsibilities
- [ ] Dependencies are minimized and well-managed
- [ ] Configuration is externalized (not hardcoded)
- [ ] Logging and debugging support is designed in

### 5. Operability

- [ ] Deployment strategy is defined (blue-green, canary, etc.)
- [ ] Monitoring and alerting are planned
- [ ] Backup and disaster recovery procedures are specified
- [ ] Rollback strategy is documented

---

## References

- Brown, S. (2018). *The C4 Model for Visualising Software Architecture*. [c4model.com](https://c4model.com/)
- Nygard, M. (2011). *Documenting Architecture Decisions*. [cognitect.com](https://www.cognitect.com/blog/2011/11/15/documenting-architecture-decisions)
- IEEE 1016-2009. *IEEE Standard for Information Technology — Systems Design — Software Design Descriptions*.
- Clements, P. et al. (2010). *Documenting Software Architectures: Views and Beyond*. Addison-Wesley.

---

## License

This document is licensed under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
