Web 429 implement centralized error handler service for user friendly api error messaging

src/app/core/error-handler/README.md

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+# Error Handler Service
+
+## Overview
+
+The `ErrorHandlerService` is a centralized error-handling service for the Mifos X Web App. It provides consistent, user-friendly error messaging across the application by converting HTTP errors into meaningful messages and displaying them through Material Design snackbars.
+
+## How It Works
+
+### Trigger Mechanism
+
+The `ErrorHandlerService` is **manually triggered** by components and services using RxJS's `catchError` operator. It works **alongside** (not replacing) the existing `ErrorHandlerInterceptor`:
+
+#### Existing Interceptor (`error-handler.interceptor.ts`)
+
+- Automatically intercepts **all** HTTP errors globally
+- Shows generic error dialog for unhandled errors
+- Cannot provide context-specific messages
+- Located at: `src/app/core/http/error-handler.interceptor.ts`
+
+#### ErrorHandlerService (This Service)
+
+- **Manually invoked** per API call
+- Provides **context-specific** error messages
+- Displays user-friendly snackbar notifications
+- Extracts Fineract-specific error details
+- Gives developers fine-grained control
+
+### When to Use
+
+Use `ErrorHandlerService` when you need:
+
+- Custom error messages for specific operations
+- Context-aware error handling (e.g., "User not found", "Loan approval failed")
+- Snackbar notifications instead of modal dialogs
+- To extract and display Fineract API error messages
+
+The interceptor still catches errors you don't explicitly handle, providing a safety net.
+
+## Problem Solved
+
+**Before:** The app handled API errors inconsistently:
+
+- Some errors were shown directly from the server
+- Others were not displayed clearly to users
+- Error handling code was duplicated across components
+- Poor user experience with technical error messages
+
+**After:** Centralized error handling with:
+
+- Consistent error messaging
+- User-friendly error descriptions
+- Reusable service across all components
+- Better UX with appropriate snackbar notifications
+
+## Features
+
+### 1. HTTP Error Handling
+
+- Automatically converts HTTP status codes into user-friendly messages
+- Extracts Fineract-specific error messages
+- Handles network/connection errors
+- Supports contextual error messages
+
+### 2. Supported Error Codes
+
+| Status Code   | Title               | Behavior                |
+| ------------- | ------------------- | ----------------------- |
+| 400           | Invalid Request     | Shows validation errors |
+| 401           | Unauthorized        | Session expired message |
+| 403           | Access Denied       | Permission error        |
+| 404           | Not Found           | Resource not found      |
+| 409           | Conflict            | Duplicate resource      |
+| 500           | Server Error        | Generic server error    |
+| 503           | Service Unavailable | Service down message    |
+| Network Error | Connection Error    | Connection issues       |
+
+### 3. Notification Types
+
+- **Error Notifications**: Red snackbar, top-center, 5 seconds
+- **Success Notifications**: Green snackbar, bottom-center, 3 seconds
+- **Info Notifications**: Blue snackbar, bottom-center, 4 seconds
+
+## Usage
+
+### Basic Error Handling
+
+```typescript
+import { ErrorHandlerService } from '@core/error-handler/error-handler.service';
+import { catchError } from 'rxjs/operators';
+
+export class MyComponent {
+  constructor(private errorHandler: ErrorHandlerService) {}
+
+  loadData() {
+    this.dataService
+      .getData()
+      .pipe(catchError((error) => this.errorHandler.handleError(error)))
+      .subscribe((data) => {
+        // Handle success
+      });
+  }
+}
+```
+
+### Error Handling with Context
+
+```typescript
+loadUser(userId: string) {
+  this.userService.getUser(userId).pipe(
+    catchError(error => this.errorHandler.handleError(error, 'User'))
+  ).subscribe(user => {
+    // Handle success
+  });
+}
+// Shows: "Not Found: User not found." for 404 errors
+```
+
+### Success Messages
+
+```typescript
+saveData() {
+  this.dataService.save(data).subscribe(
+    response => {
+      this.errorHandler.showSuccess('Data saved successfully!');
+    },
+    error => {
+      this.errorHandler.handleError(error);
+    }
+  );
+}
+```
+
+### Info Messages
+
+```typescript
+loadData() {
+  this.errorHandler.showInfo('Loading data, please wait...');
+  this.dataService.getData().subscribe(/* ... */);
+}
+```
+
+## Advanced Usage
+
+### Custom Error Handling
+
+```typescript
+import { HttpErrorResponse } from '@angular/common/http';
+
+processData() {
+  this.dataService.process().subscribe(
+    response => {
+      this.errorHandler.showSuccess('Processing complete!');
+    },
+    (error: HttpErrorResponse) => {
+      if (error.status === 409) {
+        // Custom handling for conflicts
+        this.handleConflict(error);
+      } else {
+        this.errorHandler.handleError(error, 'Data processing');
+      }
+    }
+  );
+}
+```
+
+### Integration with Form Validation
+
+```typescript
+submitForm() {
+  if (this.form.invalid) {
+    this.errorHandler.showInfo('Please fill all required fields');
+    return;
+  }
+
+  this.formService.submit(this.form.value).pipe(
+    catchError(error => this.errorHandler.handleError(error, 'Form submission'))
+  ).subscribe(response => {
+    this.errorHandler.showSuccess('Form submitted successfully!');
+  });
+}
+```
+
+## Styling
+
+The service uses three CSS classes defined in `error-handler.component.scss`:
+
+```scss
+.error-snackbar   // Red background (#f44336) for errors
+.success-snackbar // Green background (#4caf50) for success
+.info-snackbar    // Blue background (#2196f3) for info
+```
+
+These styles are automatically applied based on the notification type. The styles are imported globally in `main.scss` so they work across the entire application.
+
+## API Reference
+
+### Methods
+
+#### `handleError(error: HttpErrorResponse, context?: string): Observable<never>`
+
+Handles HTTP errors and shows user-friendly messages.
+
+**Parameters:**
+
+- `error`: The HTTP error response
+- `context` (optional): Additional context for more specific messages
+
+**Returns:** Observable that throws the original error
+
+**Example:**
+
+```typescript
+this.errorHandler.handleError(error, 'Client');
+```
+
+---
+
+#### `showSuccess(message: string, action?: string): void`
+
+Shows a success message to the user.
+
+**Parameters:**
+
+- `message`: The success message to display
+- `action` (optional): Button text (defaults to 'OK')
+
+**Example:**
+
+```typescript
+this.errorHandler.showSuccess('Client created successfully!', 'View');
+```
+
+---
+
+#### `showInfo(message: string, action?: string): void`
+
+Shows an informational message to the user.
+
+**Parameters:**
+
+- `message`: The info message to display
+- `action` (optional): Button text (defaults to 'OK')
+
+**Example:**
+
+```typescript
+this.errorHandler.showInfo('Loading data...', 'Dismiss');
+```
+
+## Fineract Integration
+
+The service automatically extracts Fineract-specific error messages:
+
+```typescript
+// Fineract error structure
+{
+  "errors": [{
+    "defaultUserMessage": "Client with same name already exists"
+  }],
+  "defaultUserMessage": "Validation error"
+}
+```
+
+The service prioritizes:
+
+1. `errors[0].defaultUserMessage`
+2. `defaultUserMessage`
+3. Generic fallback message
+
+## Migration Guide
+
+### Before (Old Approach)
+
+```typescript
+// Inconsistent error handling
+this.clientService.getClient(id).subscribe(
+  (data) => {
+    /* success */
+  },
+  (error) => {
+    console.error(error);
+    alert('Error loading client'); // Poor UX
+  }
+);
+```
+
+### After (Centralized Approach)
+
+```typescript
+// Consistent error handling
+this.clientService
+  .getClient(id)
+  .pipe(catchError((error) => this.errorHandler.handleError(error, 'Client')))
+  .subscribe((data) => {
+    // Handle success
+  });
+```
+
+## Best Practices
+
+1. **Always provide context** for better error messages:
+
+   ```typescript
+   catchError((error) => this.errorHandler.handleError(error, 'Loan Application'));
+   ```
+
+2. **Use appropriate notification types**:
+   - Errors: Use `handleError()` for API failures
+   - Success: Use `showSuccess()` for successful operations
+   - Info: Use `showInfo()` for informational messages
+
+3. **Don't duplicate error handling**: Let the service handle standard errors
+
+4. **Custom handling when needed**: Handle specific error cases before using the service
+
+5. **Combine with loading states**:
+   ```typescript
+   this.isLoading = true;
+   this.service
+     .getData()
+     .pipe(
+       finalize(() => (this.isLoading = false)),
+       catchError((error) => this.errorHandler.handleError(error))
+     )
+     .subscribe();
+   ```
+
+## Testing
+
+```typescript
+import { TestBed } from '@angular/core/testing';
+import { MatSnackBar } from '@angular/material/snack-bar';
+import { Router } from '@angular/router';
+import { ErrorHandlerService } from './error-handler.service';
+
+describe('ErrorHandlerService', () => {
+  let service: ErrorHandlerService;
+  let snackBar: jasmine.SpyObj<MatSnackBar>;
+
+  beforeEach(() => {
+    const snackBarSpy = jasmine.createSpyObj('MatSnackBar', ['open']);
+    const routerSpy = jasmine.createSpyObj('Router', ['navigate']);
+
+    TestBed.configureTestingModule({
+      providers: [
+        ErrorHandlerService,
+        { provide: MatSnackBar, useValue: snackBarSpy },
+        { provide: Router, useValue: routerSpy }]
+    });
+
+    service = TestBed.inject(ErrorHandlerService);
+    snackBar = TestBed.inject(MatSnackBar) as jasmine.SpyObj<MatSnackBar>;
+  });
+
+  it('should show success message', () => {
+    service.showSuccess('Test success');
+    expect(snackBar.open).toHaveBeenCalledWith(
+      'Test success',
+      'OK',
+      jasmine.objectContaining({
+        panelClass: ['success-snackbar']
+      })
+    );
+  });
+});
+```
+
+## Related Issues
+
+- **WEB-429**: Implement centralized error handler service for user-friendly API error messaging
+
+## Contributing
+
+When modifying this service:
+
+1. Ensure all HTTP status codes are handled appropriately
+2. Update the documentation with new features
+3. Maintain consistent snackbar positioning and duration
+4. Test with actual Fineract API responses
+5. Follow Angular and TypeScript best practices
+
+## License
+
+Licensed under the Apache License, Version 2.0. See the LICENSE file for details.