# Comments WebSocket Notifications Guide

## Overview

The comments feature now supports real-time WebSocket notifications. When a comment is created, updated, or deleted, all subscribers to the entity's comment topic will receive instant updates.

## WebSocket Topic Format

```
/topic/comments/{entityType}/{entityId}
```

### Examples:
- Story comments: `/topic/comments/STORY/story123`
- Photo comments: `/topic/comments/PHOTO/photo456`

## Event Types

The system broadcasts three types of comment events:

1. **CREATED** - When a new comment is added
2. **UPDATED** - When an existing comment is edited
3. **DELETED** - When a comment is removed

## Event Payload Structure

```json
{
  "eventType": "CREATED|UPDATED|DELETED",
  "comment": {
    "instanceId": "comment123",
    "entityType": "STORY",
    "entityId": "story456",
    "userId": "user123",
    "userName": "John Doe",
    "userAvatarUrl": "https://...",
    "content": "This is a comment",
    "createTime": "2024-01-01T12:00:00",
    "updateTime": "2024-01-01T12:00:00",
    "isEdited": false,
    "canEdit": true,
    "canDelete": true
  },
  "commentId": "comment123",
  "entityType": "STORY",
  "entityId": "story456",
  "timestamp": "2024-01-01T12:00:00"
}
```

**Note:** For `DELETED` events, the `comment` field will be `null`, and only `commentId` will be populated.

## Frontend Integration

### 1. Subscribe to Comment Topic

```typescript
// Subscribe when viewing an entity (story or photo)
const subscription = stompClient.subscribe(
  `/topic/comments/${entityType}/${entityId}`,
  (message) => {
    const event = JSON.parse(message.body);
    handleCommentEvent(event);
  }
);

// Unsubscribe when leaving the page
subscription.unsubscribe();
```

### 2. Handle Comment Events

```typescript
function handleCommentEvent(event: CommentEventDto) {
  switch (event.eventType) {
    case 'CREATED':
      // Add new comment to the list
      addCommentToList(event.comment);
      break;
      
    case 'UPDATED':
      // Update existing comment in the list
      updateCommentInList(event.comment);
      break;
      
    case 'DELETED':
      // Remove comment from the list
      removeCommentFromList(event.commentId);
      break;
  }
}
```

## Backend Implementation Details

### Service Layer

The `CommentServiceImpl` automatically broadcasts WebSocket events after successful database operations:

- **createComment()** - Broadcasts `CREATED` event with full comment data
- **updateComment()** - Broadcasts `UPDATED` event with updated comment data
- **deleteComment()** - Broadcasts `DELETED` event with comment ID only

### Error Handling

WebSocket broadcasting failures are caught and logged but do not affect the main comment operation. This ensures that:
- Comments are always saved to the database
- WebSocket issues don't break the API
- Errors are logged for monitoring

### Transaction Safety

All comment operations are wrapped in `@Transactional` annotations, ensuring:
- Database changes are committed before WebSocket broadcast
- Rollback doesn't trigger WebSocket events
- Consistency between database and real-time updates

## Testing

### Unit Tests

The `CommentWebSocketTest` class verifies:
- WebSocket events are broadcast for all operations
- Topic format is correct
- Event payload contains expected data
- WebSocket failures don't break main operations

### Manual Testing

1. Open two browser windows with the same story/photo
2. Add a comment in window 1
3. Verify the comment appears in window 2 without refresh
4. Edit/delete the comment in window 1
5. Verify changes appear in window 2 in real-time

## Performance Considerations

- **Topic-based broadcasting**: Only users viewing the specific entity receive updates
- **Lightweight payloads**: Events contain only necessary data
- **Non-blocking**: WebSocket operations don't slow down API responses
- **Graceful degradation**: System works without WebSocket (polling fallback)

## Security

- WebSocket connections require authentication
- Users can subscribe to any public entity's comment topic
- Private entities should implement additional authorization checks
- Comment content is validated before broadcasting

## Monitoring

Key metrics to monitor:
- WebSocket connection count
- Message delivery success rate
- Broadcast latency
- Failed broadcast attempts (check logs)

## Future Enhancements

Potential improvements:
- Personal notification queue for entity owners
- Comment mention notifications (@username)
- Typing indicators
- Read receipts
- Comment reaction notifications