Technical Design Doc Creator

**Rollback Steps**:
```bashcurl -X PATCH https://api.launchdarkly.com/flags/FEATURE_X \  -H "Authorization: Bearer $API_KEY" \  -d '{"enabled": false}'
nx db:rollback billing```

**Service Implementation**:
```typescript@Injectable()export class CustomerService {  @Transactional({ connectionName: 'billing' })  async create(data: CreateCustomerDto) {    const customer = new Customer()    customer.email = data.email    return this.repository.save(customer)  }}```

# TDD - [Project/Feature Name]
| Field           | Value                        || --------------- | ---------------------------- || Tech Lead       | @Name                        || Product Manager | @Name (if applicable)        || Team            | Name1, Name2, Name3          || Epic/Ticket     | [Link to Jira/Linear]        || Figma/Design    | [Link if applicable]         || Status          | Draft / In Review / Approved || Created         | YYYY-MM-DD                   || Last Updated    | YYYY-MM-DD                   |

## Context
[2-4 paragraph description of the project]
**Background**:What is the current state? What system/feature does this relate to?
**Domain**:What business domain is this part of? (e.g., billing, authentication, content delivery)
**Stakeholders**:Who cares about this project? (users, business, compliance, etc.)

## Problem Statement & Motivation
### Problems We're Solving
- **Problem 1**: [Specific pain point with impact]  - Impact: [quantify if possible - time wasted, cost, user friction]- **Problem 2**: [Another pain point]  - Impact: [quantify if possible]
### Why Now?
- [Business driver - market expansion, competitor pressure, regulatory requirement]- [Technical driver - technical debt, scalability limits]- [User driver - customer feedback, usage patterns]
### Impact of NOT Solving
- **Business**: [revenue loss, competitive disadvantage]- **Technical**: [technical debt accumulation, system degradation]- **Users**: [poor experience, churn risk]

## Scope
### ✅ In Scope (V1 - MVP)
Explicit list of what WILL be delivered:
- Feature/capability 1- Feature/capability 2- Feature/capability 3- Integration point A- Data migration for X
### ❌ Out of Scope (V1)
Explicit list of what will NOT be included in this phase:
- Feature X (deferred to V2)- Integration Y (not needed for MVP)- Advanced analytics (future enhancement)- Multi-region support (V2)
### 🔮 Future Considerations (V2+)
What might come later:
- Feature A (user demand dependent)- Feature B (after V1 validation)

## Technical Solution
### Architecture Overview
[High-level description of the solution]
**Key Components**:
- Component A: [responsibility]- Component B: [responsibility]- Component C: [responsibility]
**Architecture Diagram**:
[Include Mermaid diagram, PlantUML, or link to diagram]
```mermaidgraph LR    A[Frontend] -->|HTTP| B[API Gateway]    B -->|GraphQL| C[Backend Service]    C -->|REST| D[External API]    C -->|Write| E[(Database)]```

## Implementation Plan
| Phase                 | Task              | Description                            | Owner   | Status | Estimate || --------------------- | ----------------- | -------------------------------------- | ------- | ------ | -------- || **Phase 1 - Setup**   | Setup credentials | Obtain API keys, configure environment | @Dev1   | TODO   | 1d       ||                       | Database setup    | Create schema, configure datasource    | @Dev1   | TODO   | 1d       || **Phase 2 - Core**    | Entities & repos  | Create TypeORM entities, repositories  | @Dev2   | TODO   | 3d       ||                       | Services          | Implement business logic services      | @Dev2   | TODO   | 4d       || **Phase 3 - APIs**    | REST endpoints    | Create controllers, DTOs               | @Dev3   | TODO   | 3d       ||                       | Integration       | Integrate with external API            | @Dev1   | TODO   | 3d       || **Phase 4 - Testing** | Unit tests        | Test services and repositories         | @Team   | TODO   | 2d       ||                       | E2E tests         | Test full flow                         | @Team   | TODO   | 3d       || **Phase 5 - Deploy**  | Staging deploy    | Deploy to staging, smoke test          | @DevOps | TODO   | 1d       ||                       | Production deploy | Phased rollout to production           | @DevOps | TODO   | 1d       |
**Total Estimate**: ~20 days (4 weeks)
**Dependencies**:
- Must complete Phase N before Phase N+1- External API access required before Phase 3- Security review required before Phase 5

## Security Considerations
### Authentication & Authorization
- **Authentication**: How users prove identity  - Example: JWT tokens, OAuth 2.0, session-based- **Authorization**: What authenticated users can access  - Example: Role-based (RBAC), Attribute-based (ABAC)  - Ensure users can only access their own resources
### Data Protection
**Encryption**:
- **At Rest**: Database encryption enabled (AES-256)- **In Transit**: TLS 1.3 for all API communication- **Secrets**: Store API keys in environment variables / secret manager (AWS Secrets Manager, HashiCorp Vault)
**PII Handling**:
- What PII is collected: [email, name, payment info]- Legal basis: [consent, contract, legitimate interest]- Retention: [how long data is kept]- Deletion: [GDPR right to be forgotten implementation]
### Compliance Requirements
| Regulation  | Requirement                        | Implementation                                    || ----------- | ---------------------------------- | ------------------------------------------------- || **GDPR**    | Data protection, right to deletion | Implement data export/deletion endpoints          || **PCI DSS** | No storage of card data            | Use Stripe tokenization, never store CVV/full PAN || **LGPD**    | Brazil data protection             | Same as GDPR compliance                           |
### Security Best Practices
- ✅ Input validation on all endpoints- ✅ SQL injection prevention (parameterized queries)- ✅ XSS prevention (sanitize user input, CSP headers)- ✅ CSRF protection (tokens for state-changing operations)- ✅ Rate limiting (e.g., 10 req/min per user, 100 req/min per IP)- ✅ Audit logging (log all sensitive operations)
### Secrets Management
**API Keys**:
- Storage: Environment variables or secret management service- Rotation: Define rotation policy (e.g., every 90 days)- Access: Backend services only, never exposed to frontend- Examples: Stripe keys, database credentials, API tokens
**Webhook Signatures**:
- Validate webhook signatures from external services- Reject requests without valid signature headers- Log invalid signature attempts for security monitoring

## Testing Strategy
| Test Type             | Scope                    | Coverage Target          | Approach             || --------------------- | ------------------------ | ------------------------ | -------------------- || **Unit Tests**        | Services, repositories   | > 80%                    | Jest with mocks      || **Integration Tests** | API endpoints + database | Critical paths           | Supertest + test DB  || **E2E Tests**         | Full user flows          | Happy path + error cases | Playwright           || **Contract Tests**    | External API integration | API contract validation  | Pact or manual mocks || **Load Tests**        | Performance under load   | Baseline performance     | k6 or Artillery      |
### Test Scenarios
**Unit Tests**:
- ✅ Service business logic (create, update, delete)- ✅ Repository query methods- ✅ Error handling (throw correct exceptions)- ✅ Edge cases (null inputs, invalid data)
**Integration Tests**:
- ✅ POST `/api/v1/resource` → creates in DB- ✅ GET `/api/v1/resource/:id` → returns correct data- ✅ DELETE `/api/v1/resource/:id` → removes from DB- ✅ Invalid input → returns 400 Bad Request- ✅ Unauthorized access → returns 401/403
**E2E Tests**:
- ✅ User creates resource → success flow- ✅ User tries to access another user's resource → denied- ✅ External API fails → graceful degradation- ✅ Database connection lost → proper error handling
**Load Tests**:
- Target: 100 req/s sustained, 500 req/s peak- Monitor: Latency (p50, p95, p99), error rate, throughput- Pass criteria: p95 < 500ms, error rate < 1%
### Test Data Management
- Use factories for test data (e.g., `@faker-js/faker`)- Seed test database with realistic data- Clean up test data after each test- Use separate test database (never use production)

## Monitoring & Observability
### Metrics to Track
| Metric                    | Type       | Alert Threshold   | Dashboard          || ------------------------- | ---------- | ----------------- | ------------------ || `api.latency`             | Latency    | p95 > 1s for 5min | DataDog / Grafana  || `api.error_rate`          | Error rate | > 1% for 5min     | DataDog / Grafana  || `external_api.latency`    | Latency    | p95 > 2s for 5min | DataDog            || `external_api.errors`     | Counter    | > 5 in 1min       | PagerDuty          || `database.query_time`     | Duration   | p95 > 100ms       | DataDog            || `webhook.processing_time` | Duration   | > 5s              | Internal Dashboard |
### Structured Logging
**Log Format** (JSON):
```json{  "level": "info",  "timestamp": "2026-02-04T10:00:00Z",  "message": "Resource created",  "context": {    "userId": "user-123",    "resourceId": "res-456",    "action": "create",    "duration_ms": 45  }}```

## Success Metrics
| Metric                  | Baseline      | Target  | Measurement        || ----------------------- | ------------- | ------- | ------------------ || API latency (p95)       | N/A (new API) | < 200ms | DataDog APM        || Error rate              | N/A           | < 0.1%  | Sentry / logs      || Conversion rate         | N/A           | > 70%   | Analytics          || User satisfaction       | N/A           | NPS > 8 | User survey        || Time to complete action | N/A           | < 30s   | Frontend analytics |
**Business Metrics**:
- Increase in [metric] by [X%]- Reduction in [cost/time] by [Y%]- User adoption: [Z%] of users using new feature within 30 days
**Technical Metrics**:
- Zero production incidents in first 30 days- Test coverage > 80%- Documentation completeness: 100% of public APIs documented

## Glossary
| Term                | Description                                                           || ------------------- | --------------------------------------------------------------------- || **Customer**        | A user who has an active subscription or has made a purchase          || **Subscription**    | Recurring payment arrangement with defined interval (monthly, annual) || **Trial**           | Free period for users to test service before payment required         || **Webhook**         | HTTP callback from external service to notify of events               || **Idempotency**     | Operation can be applied multiple times with same result              || **Circuit Breaker** | Pattern to prevent cascading failures when external service is down   |
**Acronyms**:
- **API**: Application Programming Interface- **SLA**: Service Level Agreement- **PII**: Personally Identifiable Information- **GDPR**: General Data Protection Regulation- **PCI DSS**: Payment Card Industry Data Security Standard

## Alternatives Considered
| Option                | Pros                                                     | Cons                                                                        | Why Not Chosen                                    || --------------------- | -------------------------------------------------------- | --------------------------------------------------------------------------- | ------------------------------------------------- || **Option A** (Chosen) | + Best documentation<br>+ Global support<br>+ Mature SDK | - Cost: 2.9% + $0.30<br>- Vendor lock-in                                    | ✅ **Chosen** - Best balance of features and cost || Option B              | + Lower fees (2.5%)<br>+ Brand recognition               | - Poor developer experience<br>- Limited international support              | Developer experience inferior, harder to maintain || Option C              | + Full control<br>+ No transaction fees                  | - High maintenance cost<br>- Compliance burden (PCI DSS)<br>- Security risk | Too risky and expensive to maintain in-house      || Option D              | + Cheapest option                                        | - No support<br>- Limited features<br>- Unknown reliability                 | Too risky for production payment processing       |
**Decision Criteria**:
1. Developer experience and documentation quality (weight: 40%)2. Total cost of ownership (weight: 30%)3. International support and compliance (weight: 20%)4. Reliability and uptime (weight: 10%)
**Why Option A Won**:
- Scored highest on developer experience (critical for fast iteration)- Industry-standard for startups (easier to hire developers with experience)- Built-in compliance (PCI DSS, SCA, 3D Secure) reduces risk

## Dependencies
| Dependency            | Type           | Owner       | Status           | Risk                || --------------------- | -------------- | ----------- | ---------------- | ------------------- || Stripe API            | External       | Stripe Inc. | Production-ready | Low (99.99% uptime) || Identity Module       | Internal       | Team Auth   | Production-ready | Low                 || Database (PostgreSQL) | Infrastructure | DevOps      | Ready            | Low                 || Redis (caching)       | Infrastructure | DevOps      | Needs setup      | Medium              || Feature flag service  | Internal       | Platform    | Ready            | Low                 |
**Approval Requirements**:
- [ ] Security team review (for payment/auth projects)- [ ] Compliance sign-off (for PII/payment data)- [ ] Ops team ready for monitoring setup- [ ] Product sign-off on scope
**Blockers**:
- Waiting for Stripe production keys (ETA: 2026-02-10)- Need Redis setup in staging (ETA: 2026-02-08)

## Performance Requirements
| Metric              | Requirement                   | Measurement Method || ------------------- | ----------------------------- | ------------------ || API Latency (p50)   | < 100ms                       | DataDog APM        || API Latency (p95)   | < 500ms                       | DataDog APM        || API Latency (p99)   | < 1s                          | DataDog APM        || Throughput          | 1000 req/s sustained          | Load testing (k6)  || Availability        | 99.9% (< 8.76h downtime/year) | Uptime monitoring  || Database query time | < 50ms (p95)                  | Slow query log     |
**Load Testing Plan**:
- Baseline: 100 req/s for 10 minutes- Peak: 500 req/s for 5 minutes- Spike: 1000 req/s for 1 minute
**Scalability**:
- Horizontal scaling: Add more instances (Kubernetes autoscaling)- Database: Read replicas if needed (after 10k req/s)- Caching: Redis for frequently accessed data (> 100 req/s per resource)

## Migration Plan
### Migration Strategy
**Type**: [Blue-Green / Rolling / Big Bang / Phased]
**Phases**:
| Phase             | Description                            | Users Affected | Duration | Rollback            || ----------------- | -------------------------------------- | -------------- | -------- | ------------------- || 1. Preparation    | Set up new system, run in parallel     | 0%             | 1 week   | N/A                 || 2. Shadow Mode    | New system processes but doesn't serve | 0%             | 1 week   | Instant             || 3. Pilot          | 5% of users on new system              | 5%             | 1 week   | < 5min              || 4. Ramp Up        | 50% of users on new system             | 50%            | 1 week   | < 5min              || 5. Full Migration | 100% of users on new system            | 100%           | 1 day    | < 5min              || 6. Decommission   | Turn off old system                    | 0%             | 1 week   | Restore from backup |
### Data Migration
**Source**: Old system database**Destination**: New system database**Volume**: [X million records]**Method**: [ETL script / database replication / API sync]
**Steps**:
1. Export data from old system (script: `scripts/export-old-data.ts`)2. Transform data to new schema (script: `scripts/transform-data.ts`)3. Validate data integrity (checksums, row counts)4. Load into new system (script: `scripts/load-new-data.ts`)5. Verify: Run parallel reads, compare results
**Timeline**:
- Dry run on staging: 2026-02-10- Production migration window: 2026-02-15 02:00-06:00 UTC (low traffic)
### Backward Compatibility
- Old API endpoints will remain active for 90 days- Deprecation warnings added to responses- Client libraries updated with migration guide

## Open Questions
| #   | Question                                               | Context                                        | Owner     | Status           | Decision Date || --- | ------------------------------------------------------ | ---------------------------------------------- | --------- | ---------------- | ------------- || 1   | How to handle trial expiration without payment method? | User loses access immediately or grace period? | @Product  | 🟡 In Discussion | TBD           || 2   | Allow multiple trials for same email?                  | Prevent abuse vs. legitimate use cases         | @TechLead | 🔴 Open          | TBD           || 3   | SLA for webhook processing?                            | Stripe retries for 72h, what's our target?     | @Backend  | 🔴 Open          | TBD           || 4   | Support for promo codes in V1?                         | Marketing requested, is it in scope?           | @Product  | ✅ Resolved: V2  | 2026-02-01    || 5   | Fallback if Identity Module fails?                     | Can we create subscription without user data?  | @TechLead | 🔴 Open          | TBD           |
**Status Legend**:
- 🔴 Open - needs decision- 🟡 In Discussion - actively being discussed- ✅ Resolved - decision made

## Roadmap / Timeline
| Phase                    | Deliverables                                                                      | Duration | Target Date | Status         || ------------------------ | --------------------------------------------------------------------------------- | -------- | ----------- | -------------- || **Phase 0: Setup**       | - Stripe credentials<br>- Staging environment<br>- SDK installed                  | 2 days   | 2026-02-05  | ✅ Complete    || **Phase 1: Persistence** | - Entities created<br>- Repositories implemented<br>- Migrations generated        | 3 days   | 2026-02-08  | 🟡 In Progress || **Phase 2: Services**    | - CustomerService<br>- SubscriptionService<br>- Identity integration              | 5 days   | 2026-02-15  | ⏳ Pending     || **Phase 3: APIs**        | - POST /subscriptions<br>- DELETE /subscriptions/:id<br>- GET /subscriptions      | 3 days   | 2026-02-18  | ⏳ Pending     || **Phase 4: Webhooks**    | - Webhook endpoint<br>- Signature validation<br>- Event handlers                  | 4 days   | 2026-02-22  | ⏳ Pending     || **Phase 5: Testing**     | - Unit tests (80% coverage)<br>- Integration tests<br>- E2E tests                 | 5 days   | 2026-02-27  | ⏳ Pending     || **Phase 6: Deploy**      | - Documentation<br>- Monitoring setup<br>- Staging deploy<br>- Production rollout | 3 days   | 2026-03-02  | ⏳ Pending     |
**Total Duration**: ~25 days (5 weeks)
**Milestones**:
- 🎯 M1: MVP ready for staging (2026-02-22)- 🎯 M2: Production deployment (2026-03-02)- 🎯 M3: 100% rollout complete (2026-03-09)
**Critical Path**:Phase 0 → Phase 1 → Phase 2 → Phase 3 → Phase 4 → Phase 5 → Phase 6

## Approval & Sign-off
| Role                     | Name  | Status         | Date       | Comments                          || ------------------------ | ----- | -------------- | ---------- | --------------------------------- || Tech Lead                | @Name | ✅ Approved    | 2026-02-04 | LGTM, proceed with implementation || Staff/Principal Engineer | @Name | ⏳ Pending     | -          | Requested security review first   || Product Manager          | @Name | ✅ Approved    | 2026-02-03 | Scope aligned with roadmap        || Engineering Manager      | @Name | ⏳ Pending     | -          | -                                 || Security Team            | @Name | 🔴 Not Started | -          | Required for payment integration  || Compliance/Legal         | @Name | N/A            | -          | Not required for this project     |
**Approval Criteria**:
- ✅ All mandatory sections complete- ✅ Security review passed (if applicable)- ✅ Risks identified and mitigated- ✅ Timeline realistic and agreed upon- ⏳ Test strategy approved by QA- ⏳ Monitoring plan reviewed by SRE
**Next Steps After Approval**:
1. Create Epic in Jira (link in metadata)2. Break down into User Stories3. Begin Phase 1 implementation4. Schedule kickoff meeting with team