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Design Document: {Title}

Feature ID: {FeatureId} Status: Draft Created: {CreatedDate} Last Updated: {CreatedDate} Authors: [To be filled by AI from discussion participants] Reviewers: [To be filled from discussion]

Executive Summary

[1-2 paragraph overview of the feature and this design's approach. What problem does this solve? What's the proposed solution at a high level?]

1. Context & Problem Statement

Background

[What is the current state? What pain points or opportunities led to this feature request?]

Business Goals

  • [Goal 1: e.g., Reduce user onboarding time by 50%]
  • [Goal 2: e.g., Support 10,000 concurrent users]
  • [Goal 3: e.g., Enable third-party integrations]

Success Metrics

Metric Target Measurement Method
[e.g., Page load time] [< 2s] [Performance testing]
[e.g., User satisfaction] [> 4.5/5] [Post-feature survey]

2. Requirements

Functional Requirements

Must Have (MVP):

  • [FR-1: User can create account with email/password]
  • [FR-2: User can reset password via email]
  • [FR-3: User can log in with Google OAuth]

Should Have (Nice to Have):

  • [FR-4: Remember me functionality]
  • [FR-5: Two-factor authentication]

Won't Have (Out of Scope):

  • [FR-6: Social login with Facebook/Twitter (deferred to v2)]
  • [FR-7: Biometric authentication (platform limitations)]

Non-Functional Requirements

Performance:

  • Response time: < 200ms for API calls
  • Throughput: Support 1,000 requests/second
  • Database queries: < 50ms p95

Security:

  • Password hashing: bcrypt with salt
  • Token expiration: 1 hour for access, 7 days for refresh
  • Rate limiting: 10 failed login attempts = 15min lockout

Scalability:

  • Horizontal scaling: Support 10+ app instances
  • Database: Read replicas for query performance
  • Caching: Redis for session storage

Reliability:

  • Uptime: 99.9% SLA
  • Data durability: Daily backups with 30-day retention
  • Graceful degradation: Fallback to email-only if OAuth fails

3. Options Considered

Option 1: [Name, e.g., "In-house Authentication System"]

Approach: [Description of this option]

Pros:

  • ✅ [Pro 1]
  • ✅ [Pro 2]

Cons:

  • ❌ [Con 1]
  • ❌ [Con 2]

Cost/Complexity:

  • Development: [X person-weeks]
  • Maintenance: [Y hours/month]
  • Infrastructure: [Z $/month]

Risk Assessment:

  • [Risk 1: Security vulnerabilities - HIGH]
  • [Risk 2: Development timeline - MEDIUM]

Option 2: [Name, e.g., "Third-Party Auth Service (Auth0)"]

Approach: [Description]

Pros:

  • ✅ [Pro 1]
  • ✅ [Pro 2]

Cons:

  • ❌ [Con 1]
  • ❌ [Con 2]

Cost/Complexity:

  • Development: [X person-weeks]
  • Maintenance: [Y hours/month]
  • Infrastructure: [Z $/month]

Risk Assessment:

  • [Risk 1]
  • [Risk 2]

Option 3: [Name, if applicable]

[Repeat structure from Option 1/2]

4. Decision & Rationale

Selected Approach: [Option Name]

Primary Reasons:

  1. [Reason 1: Aligns with technical stack]
  2. [Reason 2: Lowest total cost of ownership]
  3. [Reason 3: Fastest time to market]

Trade-offs Accepted:

  • [Trade-off 1: Higher infrastructure costs vs. lower dev time]
  • [Trade-off 2: Vendor lock-in vs. managed service reliability]

Alternatives Rejected:

  • Option X rejected because: [reason]
  • Option Y rejected because: [reason]

Key Assumptions:

  • [Assumption 1: User growth will remain under 100K for next 12 months]
  • [Assumption 2: OAuth providers maintain 99.9% uptime]

When to Revisit:

  • If user base exceeds 500K (cost model changes)
  • If OAuth vendor has >2 major outages in 6 months
  • After 1 year in production (reevaluate build vs. buy)

5. Architecture

System Architecture Diagram

[Insert diagram here - can be ASCII art, PlantUML, or image link]

Example:
┌─────────────┐      ┌──────────────┐      ┌─────────────┐
│   Browser   │─────▶│  API Gateway │─────▶│   Auth      │
│             │◀─────│              │◀─────│   Service   │
└─────────────┘      └──────────────┘      └─────────────┘
                             │                      │
                             │                      ▼
                             │              ┌──────────────┐
                             │              │   User DB    │
                             │              │  (Postgres)  │
                             │              └──────────────┘
                             ▼
                     ┌──────────────┐
                     │  Session     │
                     │  Store       │
                     │  (Redis)     │
                     └──────────────┘

Component Breakdown

Component 1: [Name, e.g., "Authentication Service"]

  • Responsibility: Handle user login, registration, token issuance
  • Technology: Node.js (Express), PassportJS
  • Interfaces:
    • REST API: /auth/login, /auth/register, /auth/refresh
    • Events: user.logged_in, user.registered
  • Dependencies: User DB, Session Store, Email Service

Component 2: [Name] [Repeat structure]

Data Models

User Table:

CREATE TABLE users (
  id            UUID PRIMARY KEY,
  email         VARCHAR(255) UNIQUE NOT NULL,
  password_hash VARCHAR(255),
  oauth_provider VARCHAR(50),
  oauth_id      VARCHAR(255),
  created_at    TIMESTAMP DEFAULT NOW(),
  updated_at    TIMESTAMP DEFAULT NOW(),
  INDEX idx_email (email),
  UNIQUE idx_oauth (oauth_provider, oauth_id)
);

Session Table:

CREATE TABLE sessions (
  id              UUID PRIMARY KEY,
  user_id         UUID REFERENCES users(id),
  access_token    VARCHAR(500) NOT NULL,
  refresh_token   VARCHAR(500),
  expires_at      TIMESTAMP NOT NULL,
  created_at      TIMESTAMP DEFAULT NOW()
);

API Contracts

POST /auth/register

Request:
{
  "email": "user@example.com",
  "password": "SecurePass123!",
  "name": "John Doe"
}

Response (201 Created):
{
  "user_id": "uuid",
  "access_token": "jwt",
  "refresh_token": "jwt",
  "expires_in": 3600
}

Errors:
400 - Invalid email format
409 - Email already registered
422 - Password too weak

POST /auth/login [Similar structure]

6. Implementation Plan

Phases

Phase 1: Core Authentication (Week 1-2)

  • Set up database schema
  • Implement email/password registration
  • Implement login endpoint
  • Add password hashing (bcrypt)
  • Write unit tests

Phase 2: OAuth Integration (Week 3)

  • Integrate Google OAuth
  • Add OAuth callback handling
  • Link OAuth accounts to existing users
  • Test OAuth flow

Phase 3: Security Hardening (Week 4)

  • Add rate limiting
  • Implement token refresh
  • Add password reset flow
  • Security audit

Phase 4: Testing & Deployment (Week 5)

  • End-to-end testing
  • Load testing
  • Documentation
  • Production deployment

Dependencies

  • External:

    • Google OAuth credentials (waiting on: Platform team)
    • Email service API key (waiting on: DevOps)

  • Internal:

    • User profile service (blocks: User settings feature)
    • Session management (required by: All authenticated endpoints)

Resource Requirements

  • Development: 1 backend engineer (full-time, 5 weeks)
  • Design: 0.5 designer (mockups, 1 week)
  • QA: 0.5 QA engineer (testing, 1 week)
  • Infrastructure: $200/month (database + Redis)

7. Risks & Mitigations

Risk Probability Impact Mitigation
OAuth provider downtime Medium High Fallback to email auth; cache OAuth tokens
Password breach Low Critical Bcrypt + salt; enforce strong passwords; rate limit
Database bottleneck Medium High Add read replicas; implement caching
Token theft Medium High Short expiration; secure httpOnly cookies

Security Considerations

Threat Model:

  • Threat 1: Brute force attacks → Mitigation: Rate limiting, CAPTCHA after 3 failures
  • Threat 2: SQL injection → Mitigation: Parameterized queries, ORM usage
  • Threat 3: XSS in user data → Mitigation: Input validation, output encoding

Compliance:

  • GDPR: User data deletion within 30 days of request
  • CCPA: User data export API endpoint
  • SOC 2: Audit logging for all auth events

8. Testing Strategy

Unit Tests

  • Password hashing/validation
  • Token generation/validation
  • Input validation logic
  • Coverage target: 90%

Integration Tests

  • Registration flow (email + OAuth)
  • Login flow (email + OAuth)
  • Token refresh flow
  • Password reset flow

End-to-End Tests

  • New user signup journey
  • Returning user login journey
  • OAuth account linking
  • Session expiration handling

Performance Tests

  • Concurrent logins: 1,000 users/second
  • Database query performance: < 50ms p95
  • API response time: < 200ms p95

9. Monitoring & Observability

Metrics to Track

Business Metrics:

  • Daily active users (DAU)
  • Registration conversion rate
  • OAuth vs. email signup ratio

Technical Metrics:

  • API response time (p50, p95, p99)
  • Error rate by endpoint
  • Database connection pool utilization
  • Cache hit rate

Alerts

  • Error rate > 5% for 5 minutes → Page on-call engineer
  • Response time p95 > 500ms → Slack warning
  • Failed login attempts > 100/min → Slack + investigate

Dashboards

  • Real-time: Login success/failure rates, active sessions
  • Daily: User growth, OAuth provider breakdown
  • Weekly: Performance trends, error analysis

10. Documentation & Training

User-Facing Documentation

  • Registration guide
  • Password reset guide
  • OAuth connection guide
  • Security best practices

Developer Documentation

  • API reference (OpenAPI spec)
  • Local development setup
  • Testing guide
  • Deployment runbook

Training Materials

  • Team demo of authentication flow
  • Security review session
  • Runbook walkthrough for on-call engineers

11. Rollout Plan

Feature Flags

feature_flags:
  auth_email_registration: true
  auth_google_oauth: false  # Enable after testing
  auth_password_reset: false  # Enable in phase 2

Rollout Stages

  1. Internal Alpha (Week 1)

    • Deploy to staging
    • Team testing (10 users)
    • Fix critical bugs

  2. Beta (Week 2)

    • Deploy to 10% of production traffic
    • Monitor error rates
    • Collect user feedback

  3. General Availability (Week 3)

    • Ramp to 50%, then 100%
    • Enable OAuth
    • Sunset old authentication system (Week 4)

Rollback Plan

  • Trigger: Error rate > 10% or critical security issue
  • Procedure:
    1. Disable feature flag
    2. Route traffic to old system
    3. Incident post-mortem within 24 hours
  • RTO: 5 minutes (time to disable flag)
  • RPO: 0 (no data loss)

12. Future Enhancements

Deferred to v2

  • Two-factor authentication (SMS, TOTP)
  • Social login (Facebook, Twitter, GitHub)
  • Biometric authentication
  • SSO for enterprise customers

Technical Debt Accepted

  • [Debt 1: Monolithic auth service - plan to split into microservices after 100K users]
  • [Debt 2: In-memory session cache - migrate to distributed cache under high load]

13. Acceptance Criteria

This design is considered complete and ready for implementation when:

  • All stakeholders have reviewed and approved
  • Security team has completed threat model review
  • At least 2 technical reviewers have signed off
  • All "Must Have" functional requirements are addressed
  • Performance targets are achievable (validated by load test plan)
  • Rollback plan is documented and tested
  • Cost estimate approved by finance

14. Appendices

Appendix A: Research & References

  • [Link to competitive analysis]
  • [Link to user research findings]
  • [Link to technology evaluation matrix]

Appendix B: Meeting Notes

Design Review 1 (YYYY-MM-DD):

  • Attendees: [Names]
  • Decisions: [Key decisions]
  • Action items: [Follow-ups]

Appendix C: Change Log

Date Author Change
{CreatedDate} AI_Claude Initial draft from feature discussion

Document Status: 🟡 Draft - Awaiting Review Next Review Date: [YYYY-MM-DD] Related Documents:

  • Feature Discussion: ../discussions/design.discussion.md
  • Implementation Plan: ../implementation/plan.md (created after approval)