Feature/error handling system

ERROR_HANDLING_IMPLEMENTATION.md

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+# Error Handling System Implementation
+
+## Overview
+
+This document describes the comprehensive error handling system implemented for the Tutors platform. The system provides structured error handling, automatic retry logic, user notifications, and state management to improve reliability and user experience.
+
+## Branch
+
+`feature/error-handling-system`
+
+## What Was Implemented
+
+### 1. Core Infrastructure
+
+#### Custom Error Classes (`src/lib/services/errors/tutors-error.ts`)
+- **TutorsError**: Base error class with code, severity, and context
+- **NetworkError**: HTTP and network-related failures
+- **CourseError**: Course loading and navigation errors
+- **AuthError**: Authentication and authorization issues
+- **AnalyticsError**: Non-critical tracking failures
+- **MarkdownError**: Content rendering problems
+- **ValidationError**: Input validation failures
+
+Each error class:
+- Has structured error codes for easy identification
+- Includes contextual data for debugging
+- Provides user-friendly error messages
+- Supports severity levels (info, warning, error, critical)
+
+#### Error Handler (`src/lib/services/errors/error-handler.ts`)
+- Centralized error processing
+- Configurable logging (console + remote)
+- Automatic user notifications via toasts
+- Function wrapping utilities
+- Support for async error handling with fallbacks
+
+#### Toast Notification System (`src/lib/services/errors/toast.svelte.ts`)
+- Reactive toast queue using Svelte 5 runes
+- Auto-dismiss functionality
+- Multiple toast types (success, error, warning, info)
+- Dismissible notifications
+
+#### Network Utilities (`src/lib/services/errors/network-utils.ts`)
+- `fetchWithRetry`: Automatic retry with exponential backoff
+- `fetchJsonWithRetry`: Type-safe JSON fetching with retry
+- `fetchMultipleWithRetry`: Parallel requests with individual error tracking
+- `checkUrlHealth`: Health check utility for services
+- Configurable retry strategies
+
+#### Enhanced State Management (`src/lib/services/errors/rune-with-state.ts`)
+- `runeWithState`: Rune wrapper with loading/error tracking
+- `withStateManagement`: Automatic state handling for async operations
+- Helper functions for combining multiple states
+- Type-safe state access
+
+### 2. UI Components
+
+#### Toast Component (`src/lib/ui/components/Toast.svelte`)
+- Beautiful, accessible toast notifications
+- Dark mode support
+- Smooth animations (fly transition)
+- Dismissible notifications
+
+#### ToastContainer (`src/lib/ui/components/ToastContainer.svelte`)
+- Fixed position container for toasts
+- Automatic toast lifecycle management
+- Stacks multiple notifications
+
+#### ErrorBoundary (`src/lib/ui/components/ErrorBoundary.svelte`)
+- Catches component-level errors
+- Custom fallback UI support
+- Error recovery mechanism
+- Integrates with centralized error handler
+
+### 3. Integration
+
+#### Updated Root Layout
+- Added ToastContainer to `src/routes/+layout.svelte`
+- Available globally throughout the app
+
+### 4. Documentation
+
+#### README (`src/lib/services/errors/README.md`)
+- Complete feature documentation
+- API reference for all utilities
+- Migration guide from old patterns
+- Best practices
+- Future enhancement roadmap
+
+#### Examples (`src/lib/services/errors/EXAMPLES.md`)
+- Real-world usage examples
+- Common patterns and scenarios
+- Testing examples
+- Configuration examples
+
+#### Demo Implementation (`src/lib/services/course/services/course-improved.svelte.ts`)
+- Shows how to refactor existing service
+- Demonstrates all error handling patterns
+- Production-ready example
+
+## File Structure
+
+```
+src/lib/services/errors/
+├── index.ts                    # Main exports
+├── types.ts                    # TypeScript interfaces
+├── tutors-error.ts             # Custom error classes
+├── error-handler.ts            # Centralized error handler
+├── toast.svelte.ts             # Toast notification service
+├── network-utils.ts            # Network retry utilities
+├── rune-with-state.ts          # Enhanced state management
+├── README.md                   # Complete documentation
+└── EXAMPLES.md                 # Usage examples
+
+src/lib/ui/components/
+├── Toast.svelte                # Toast UI component
+├── ToastContainer.svelte       # Toast container
+└── ErrorBoundary.svelte        # Error boundary component
+
+src/lib/services/course/services/
+└── course-improved.svelte.ts   # Example refactored service
+
+ERROR_HANDLING_IMPLEMENTATION.md # This file
+```
+
+## Key Features
+
+### 🎯 Structured Errors
+- Type-safe error handling
+- Consistent error codes
+- Rich contextual information
+- User-friendly messages
+
+### 🔄 Automatic Retry
+- Exponential backoff
+- Configurable retry logic
+- Smart retry decisions (don't retry 4xx)
+- Parallel request handling
+
+### 🔔 User Notifications
+- Beautiful toast notifications
+- Auto-dismiss
+- Dark mode support
+- Accessibility compliant
+
+### 📊 State Management
+- Loading states
+- Error states
+- Automatic state updates
+- Combinable states
+
+### 🛡️ Error Recovery
+- Error boundaries
+- Fallback values
+- Graceful degradation
+- Continue on non-critical errors
+
+### 📝 Comprehensive Logging
+- Console logging in development
+- Remote logging support (Sentry, etc.)
+- Detailed error context
+- Stack trace preservation
+
+## How to Use
+
+### Basic Error Handling
+
+```typescript
+import { CourseError, handleError } from '$lib/services/errors';
+
+try {
+  const course = await loadCourse(id);
+} catch (error) {
+  await handleError(error, { context: 'loadCourse', courseId: id });
+}
+```
+
+### Network Requests with Retry
+
+```typescript
+import { fetchJsonWithRetry } from '$lib/services/errors';
+
+const data = await fetchJsonWithRetry<Course>(url, {}, { maxRetries: 3 });
+```
+
+### State Management
+
+```typescript
+import { runeWithState, withStateManagement } from '$lib/services/errors';
+
+const courseRune = runeWithState<Course | null>(null);
+
+await withStateManagement(courseRune, async () => {
+  return await fetchCourse(id);
+});
+
+// Use in component
+{#if courseRune.loading}...{/if}
+{#if courseRune.error}...{/if}
+{#if courseRune.value}...{/if}
+```
+
+### Toast Notifications
+
+```typescript
+import { toastService } from '$lib/services/errors';
+
+toastService.success('Course loaded!');
+toastService.error('Failed to load course');
+```
+
+## Migration Path
+
+### Phase 1: Infrastructure (✅ Complete)
+- Core error classes
+- Error handler
+- Toast notifications
+- Network utilities
+- State management
+- UI components
+- Documentation
+
+### Phase 2: Service Updates (Ready to implement)
+1. Update `courseService` to use new error handling
+2. Update `analyticsService` for non-critical errors
+3. Update `markdownService` with error recovery
+4. Update `presenceService` with proper error handling
+5. Update `connectService` for auth errors
+
+### Phase 3: Component Updates
+1. Add loading states to course views
+2. Add error boundaries to major routes
+3. Implement retry UI for failed loads
+4. Add error states to forms
+
+### Phase 4: Testing
+1. Add unit tests for error utilities
+2. Add integration tests for services
+3. Add E2E tests for error scenarios
+4. Test error recovery flows
+
+### Phase 5: Monitoring
+1. Integrate with Sentry or similar
+2. Set up error dashboards
+3. Configure alerts for critical errors
+4. Implement error analytics
+
+## Benefits
+
+### For Developers
+- ✅ Type-safe error handling
+- ✅ Less boilerplate code
+- ✅ Easier debugging
+- ✅ Consistent patterns
+- ✅ Better testing
+
+### For Users
+- ✅ Better error messages
+- ✅ Automatic retry on failures
+- ✅ Clear loading states
+- ✅ Graceful degradation
+- ✅ Improved reliability
+
+### For Maintainers
+- ✅ Centralized error logging
+- ✅ Error analytics
+- ✅ Easier troubleshooting
+- ✅ Better error tracking
+- ✅ Reduced bug reports
+
+## Configuration
+
+### Development
+```typescript
+errorHandler.configure({
+  logToConsole: true,
+  logToRemote: false,
+  showToast: true
+});
+```
+
+### Production
+```typescript
+errorHandler.configure({
+  logToConsole: false,
+  logToRemote: true,
+  showToast: true,
+  remoteLogger: async (error) => {
+    await sendToSentry(error);
+  }
+});
+```
+
+## Testing
+
+The system is designed to be easily testable:
+
+```typescript
+import { describe, it, expect } from 'vitest';
+import { CourseError, toTutorsError } from '$lib/services/errors';
+
+describe('Error Handling', () => {
+  it('should convert unknown errors to TutorsError', () => {
+    const error = toTutorsError('Something went wrong');
+    expect(error).toBeInstanceOf(TutorsError);
+    expect(error.code).toBe('UNKNOWN_ERROR');
+  });
+});
+```
+
+## Performance Impact
+
+- **Minimal**: Error handling adds negligible overhead
+- **Network**: Retry logic only activates on failure
+- **Memory**: Error objects are lightweight
+- **UI**: Toasts use efficient Svelte transitions
+
+## Browser Compatibility
+
+- ✅ All modern browsers
+- ✅ ES2020+ features
+- ✅ Native fetch API
+- ✅ Promise-based
+- ✅ No polyfills required
+
+## Future Enhancements
+
+### Short Term
+- [ ] Add more error types as needed
+- [ ] Implement offline error queue
+- [ ] Add error rate limiting
+- [ ] Create error analytics dashboard
+
+### Medium Term
+- [ ] Integrate with monitoring service
+- [ ] Add i18n support for error messages
+- [ ] Implement custom retry strategies per service
+- [ ] Add error recovery UI patterns
+
+### Long Term
+- [ ] AI-powered error suggestions
+- [ ] Automatic error resolution
+- [ ] Predictive failure detection
+- [ ] User error reporting system
+
+## Notes
+
+- All code follows existing Tutors patterns
+- Uses Svelte 5 runes for reactivity
+- Fully TypeScript typed
+- No breaking changes to existing APIs
+- Backward compatible
+- Can be adopted incrementally
+
+## Next Steps
+
+1. **Review the implementation** - Check all files and documentation
+2. **Test in development** - Run the app and test error scenarios
+3. **Update one service** - Start with courseService as a proof of concept
+4. **Gather feedback** - Get team input on the patterns
+5. **Roll out incrementally** - Update services one by one
+6. **Add monitoring** - Integrate with error tracking service
+7. **Document learnings** - Update docs based on real usage
+
+## Questions or Issues?
+
+See the documentation in `src/lib/services/errors/README.md` or check the examples in `EXAMPLES.md`.
+
+## Author
+
+Implemented as part of the Tutors platform improvement initiative.
+
+## License
+
+Same as Tutors project (MIT)
+