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+# ADR_0016: Scaffolding Service System Architecture
+
+## Date
+
+Decision date: 2025-07-28
+Last status update: 2025-08-06
+
+## Status
+
+- [x] Proposed
+- [ ] Accepted
+- [ ] Deprecated
+- [ ] Superseded
+
+### Implementation Status
+
+- [ ] Planned
+- [ ] In Development
+- [ ] Implemented
+- [ ] Verified
+- [ ] Discontinued
+
+## People
+
+### Author/Decision Owner (Single Point of Accountability)
+Denis Pisarev
+
+### Consulted (Subject Matter Experts)
+- Alisher Khassanov
+- Andreea Eftene
+- Sviatoslav Alekseev
+
+### Informed (Affected Parties)
+[People/teams affected by this decision who should be aware]
+
+  - [ ] [Person 1]
+  - [ ] [Person 2]
+  - [ ] [Person 3]
+
+## Decision
+
+We will build the complete scaffolding service using Bun runtime, Hono web framework, SQLite database, Server-Sent Events, Eta templating, convention-based template discovery, asynchronous job processing, IP-based rate limiting, automatic cleanup, error categorization, and structured project generation to support a web-based dApp composer platform.
+
+## Context
+
+Need to build complete scaffolding service with all components and features integrated together for generating Polkadot dApp projects.
+
+## Options
+
+1. (SELECTED) **Complete integrated system as described** 
+2. **Alternative microservices architecture with external queue**
+
+## Consequences
+
+### Option 1: Complete integrated system (SELECTED)
+
+**Selected because:**
+- Bun is 3x faster than Node.js, built-in SQLite, TypeScript support, excellent file I/O performance
+- Hono is edge-native, 10x faster than Express, built-in middleware, excellent TypeScript support, minimal footprint
+- SQLite is zero-config, perfect for single-instance services, built into Bun runtime
+- Server-Sent Events simpler than WebSockets, automatic reconnection, works through proxies, perfect for one-way status updates
+- Eta has modern syntax, 2x faster than Handlebars, async support, excellent TypeScript integration
+- Convention-based template discovery with automatic discovery, structured directories, merge points, dependency inference
+- Asynchronous processing with immediate responses, persistent state, background jobs, progress tracking
+- Rate limiting prevents abuse with 10 requests per hour per IP for generation, 50 for downloads, 1000 for status
+- Comprehensive error handling with categorization and user-friendly messages
+- Automatic resource cleanup prevents disk space leaks
+- Supports guided wizard interface for Web2 developer onboarding
+- Integrates with GitHub API for repository management
+- Provides dashboard for project management
+- Reduces dApp development setup time from weeks to minutes
+
+**Selected despite:**
+- Multiple complex integrations required
+- Many different technologies to learn and maintain
+- Complex template system with merge points and conventions
+- Rate limiting may affect legitimate users
+- Single-instance limitations for scaling
+
+### Option 2: Alternative microservices architecture with external queue
+
+**Rejected because:**
+- Multiple services require orchestration, monitoring, and inter-service communication
+- Container management, service discovery, and load balancing complexity
+- Team must build and maintain multiple codebases simultaneously
+- Inter-service calls add processing delays for job coordination
+- Multiple failure points, logging aggregation, and debugging complexity
+
+**Rejected despite:**
+- Individual services can scale independently based on load
+- Each service can use optimal technology stack for its function
+- Service failures don't cascade to entire system
+- Different teams can own different services independently
+
+## Implementation Notes
+
+**Tech Stack Components:**
+- Bun - Runtime & Package Manager
+- Hono - Web Framework  
+- Zod - Type validation
+- SQLite - Job Persistence
+- Eta - Template Engine
+- Server-Sent Events - Real-time Updates
+- PAPI - Polkadot Integration
+
+**Frontend Technologies:**
+- TypeScript
+- React
+- Vite
+- Tailwind
+- shadcn/ui
+- TanStack Query
+- Dexie
+
+**Key Modules:**
+- JobService - Job lifecycle management, SSE subscriptions, persistence, cleanup, process job queue
+- TemplateService - Eta template processing
+- TemplateDiscovery - Automatic template discovery from folder
+- GeneratorService - Code generation workflow
+- DevToolsService - Dev tools management
+- GitService - Git operations
+- Database Schema - Data persistence structure
+- Rate Limiter - API protection
+- Cleanup Service - Resource management  
+- Error Handler - Error management
+
+**API Endpoints:**
+Job Management:
+- POST /api/generate - Creates new code generation job, rate limited 10 requests per hour per IP
+- GET /api/jobs/:id/stream - Server-Sent Events stream for real-time job updates
+- GET /api/jobs/:id/status - Get current job status fallback for SSE issues
+
+File Operations:
+- GET /api/jobs/:id/download - Download generated project as ZIP file
+
+Integration Endpoints:
+- POST /api/jobs/:id/publish - Publish generated project to GitHub
+
+System Endpoints:
+- GET /api/health - Service health check endpoint
+- GET /api/metrics - Service metrics for monitoring
+
+**Key Features Summary:**
+Async Processing
+- Background job processing with real-time updates
+- Non-blocking API responses with immediate job creation
+- Progress tracking with detailed step information
+
+Error Handling
+- Comprehensive error categorization and user-friendly messages
+- Structured logging for debugging and monitoring
+- Graceful error recovery and cleanup
+
+Rate Limiting
+- IP-based rate limiting (10 jobs/hour per IP)
+- Configurable thresholds and bypass mechanisms
+- Rate limit monitoring and alerting
+
+Resource Management
+- Automatic file cleanup with configurable retention
+- Shared binary cache with version management
+- Memory usage optimization and garbage collection
+
+Real-time Updates
+- Server-Sent Events for real-time job progress
+- Automatic reconnection and connection management
+- Fallback polling endpoints for reliability
+
+Template System
+- Pre-existing base template on filesystem
+- Modular feature generators with template inheritance
+- Conditional template rendering based on configuration
+
+Idempotent requests
+
+Retryable tasks
+
+Templates versioning
+
+## Confirmation
+
+Service must handle all operations correctly including:
+- Wizard-based dApp configuration and educational scaffolding for Web2 developers
+- GitHub repository creation and management
+- Project dashboard with management capabilities
+- Template discovery, job processing, real-time updates, rate limiting, error handling, file cleanup, and project generation with all integrated components working together
+- Analytics and monitoring for QF ecosystem team insights
+- Integration with cloud hosting platforms for deployment
+
+Success Metrics:
+- Time to first working dApp under 30 minutes
+- Developer retention in ecosystem  
+- Support ticket reduction
+- Platform adoption growth
+- Developer satisfaction scores
+
+## References
+
+- [Bun - Runtime & Package Manager](https://bun.sh/)
+- [Hono - Web Framework](https://hono.dev/)
+- [Hono - Type validation](https://hono.dev/docs/guides/validation#with-zod)
+- [Zod](https://zod.dev/)
+- [SQLite](https://www.sqlite.org/)
+- [Eta - Template Engine](https://eta.js.org/)
+- [Server-Sent Events - Real-time Updates](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events)
+- [PAPI - Polkadot Integration](https://papi.how/getting-started/)
+- [GitHub API Documentation](https://docs.github.com/en/rest)
+- [Web Authentication API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Authentication_API)
+- [Polkadot Documentation](https://papermoonio.github.io/polkadot-mkdocs/)
+- [TypeScript](https://www.typescriptlang.org)
+- [React](https://react.dev)
+- [Vite](https://vitejs.dev)
+- [Tailwind](https://tailwindcss.com)
+- [shadcn/ui](https://ui.shadcn.com)
+- [TanStack Query](https://tanstack.com/query)
+- [Dexie](https://dexie.org/)
+
+## ADR Relationships
+
+[Fill in the section if applicable or leave blank for further filling.]
+
+### Supersedes
+- ADR #[X]: [Brief description of superseded decision]
+
+### Superseded By
+- ADR #[X]: [Brief description of superseding decision]
+
+### Related ADRs
+- ADR #[X]: [Brief description of relationship]
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