# System Patterns: hosts

## System Architecture

### Layered Architecture
```
┌─────────────────────────────────────┐
│             TUI Layer               │  ← User Interface (Textual)
├─────────────────────────────────────┤
│           Manager Layer             │  ← Orchestration & Operations
├─────────────────────────────────────┤
│            Core Layer               │  ← Business Logic
├─────────────────────────────────────┤
│           System Layer              │  ← File I/O & DNS
└─────────────────────────────────────┘
```

### Component Relationships

#### TUI Layer (`src/hosts/tui/`)
- **views.py**: Main application views and widgets
- **Responsibilities**: User interaction, display logic, event handling
- **Dependencies**: Manager layer for operations

#### Manager Layer (`src/hosts/core/manager.py`)
- **Responsibilities**: Coordinate operations, maintain application state
- **Dependencies**: Core models, parser, DNS resolver
- **Patterns**: Command pattern for operations, Observer for state changes

#### Core Layer (`src/hosts/core/`)
- **models.py**: Data structures (Entry, Comment, HostsFile)
- **parser.py**: Hosts file parsing and serialization
- **dns.py**: DNS resolution and IP comparison
- **Responsibilities**: Pure business logic, no I/O dependencies

#### System Layer
- **File I/O**: Direct `/etc/hosts` file operations
- **DNS Resolution**: Network calls for hostname resolution
- **Permission Management**: Sudo privilege handling

## Key Technical Decisions

### Data Model Design
```python
@dataclass
class HostEntry:
    ip_address: str
    hostnames: list[str]
    comment: str | None = None
    is_active: bool = True
    dns_name: str | None = None  # For CNAME-like functionality
    
@dataclass
class HostsFile:
    entries: list[HostEntry]
    header_comments: list[str]
    footer_comments: list[str]
```

### State Management
- **Immutable operations**: All modifications return new state
- **Validation pipeline**: Every change goes through validation
- **Undo/Redo capability**: Maintain operation history
- **Dirty state tracking**: Know when changes need saving

### Permission Model
```python
class PermissionManager:
    def __init__(self):
        self.edit_mode = False
        self.sudo_acquired = False
    
    def enter_edit_mode(self) -> bool:
        # Request sudo permissions
        # Set edit_mode = True only if successful
    
    def exit_edit_mode(self):
        # Release sudo permissions
        # Set edit_mode = False
```

## Design Patterns in Use

### Command Pattern
```python
class Command(ABC):
    @abstractmethod
    def execute(self) -> HostsFile:
        pass
    
    @abstractmethod
    def undo(self) -> HostsFile:
        pass

class ToggleEntryCommand(Command):
    def __init__(self, entry_index: int):
        self.entry_index = entry_index
    
    def execute(self, hosts_file: HostsFile) -> HostsFile:
        # Toggle entry active state
    
    def undo(self, hosts_file: HostsFile) -> HostsFile:
        # Reverse the toggle
```

### Observer Pattern
```python
class HostsManager:
    def __init__(self):
        self.observers: list[Observer] = []
        self.state: HostsFile = HostsFile()
    
    def notify_observers(self, event: StateChangeEvent):
        for observer in self.observers:
            observer.on_state_change(event)
```

### Factory Pattern
```python
class ParserFactory:
    @staticmethod
    def create_parser(file_path: str) -> HostsParser:
        # Return appropriate parser based on file format
        return StandardHostsParser(file_path)
```

## Critical Implementation Paths

### Application Startup
1. **Initialize TUI**: Create Textual app instance
2. **Load hosts file**: Parse `/etc/hosts` in read-only mode
3. **Build UI**: Populate left pane with entries
4. **Enter main loop**: Handle user input and events

### Edit Mode Activation
1. **User triggers edit mode**: Keyboard shortcut or menu
2. **Request sudo**: Prompt for password if needed
3. **Validate permissions**: Ensure write access to `/etc/hosts`
4. **Update UI state**: Enable edit operations
5. **Maintain permissions**: Keep sudo active until exit

### Entry Modification
1. **User action**: Toggle, reorder, or edit entry
2. **Create command**: Instantiate appropriate command object
3. **Validate operation**: Check if change is valid
4. **Execute command**: Apply change to in-memory state
5. **Update UI**: Refresh display to show changes
6. **Track dirty state**: Mark file as needing save

### File Persistence
1. **User saves**: Explicit save command or auto-save
2. **Validate entire file**: Ensure no syntax errors
3. **Create backup**: Optional backup of original file
4. **Write atomically**: Use temporary file + rename
5. **Verify write**: Confirm file was written correctly

### DNS Resolution Flow
1. **User requests resolution**: For entry with DNS name
2. **Resolve hostname**: Use socket.gethostbyname()
3. **Compare IPs**: Current IP vs resolved IP
4. **Present choice**: Show user both options
5. **Update entry**: Apply user's choice
6. **Mark dirty**: Flag file for saving

## Error Handling Patterns

### Graceful Degradation
- **Permission denied**: Fall back to read-only mode
- **DNS resolution failure**: Show error but continue
- **File corruption**: Load what's possible, warn user
- **Network unavailable**: Disable DNS features

### User Feedback
- **Status bar**: Show current mode and operation status
- **Modal dialogs**: For errors requiring user attention
- **Inline validation**: Real-time feedback on input
- **Progress indicators**: For long-running operations

### Recovery Mechanisms
- **Undo operations**: Allow reverting recent changes
- **File restoration**: Restore from backup if available
- **Safe mode**: Minimal functionality if errors occur
- **Graceful exit**: Always attempt to save valid changes