# Plugin Development Guide

This guide explains how to create custom plugins for the Heartbeat monitoring system.

## Table of Contents

- [Plugin Architecture](#plugin-architecture)
- [Plugin Types](#plugin-types)
- [Creating a Plugin](#creating-a-plugin)
- [Plugin Lifecycle](#plugin-lifecycle)
- [Server-initiated InfoPlugin refresh](#server-initiated-infoplugin-refresh)
- [Configuration](#configuration)
- [Best Practices](#best-practices)
- [Examples](#examples)
- [Testing](#testing)

## Plugin Architecture

Heartbeat's plugin system is designed to be simple yet powerful. Plugins are Python classes that inherit from one of the base plugin types and implement a few key methods.

### Key Concepts

- **Plugin Registry**: Central registry that manages all loaded plugins
- **Plugin Loader**: Automatically discovers and loads plugins from the `hbd/plugins/` directory
- **Plugin Types**: InfoPlugin (static data) and MonitorPlugin (periodic metrics)
- **Async/Await**: All plugin methods are async for non-blocking operation

## Plugin Types

### InfoPlugin

InfoPlugins collect static information that doesn't change frequently (OS version, hardware specs, etc.).

- **Runs once** at startup (interval = 0)
- **Cached** - data is collected once and reused
- **Lightweight** - no periodic overhead

**Use InfoPlugin for:**
- Operating system details
- Hardware information
- Software versions
- Configuration data
- Static inventory

### MonitorPlugin

MonitorPlugins collect metrics that change over time (CPU usage, memory, network traffic).

- **Runs periodically** based on configured interval
- **Scheduled** - collected at regular intervals
- **Dynamic** - captures changing system state

**Use MonitorPlugin for:**
- Resource usage (CPU, memory, disk, network)
- Performance metrics
- Counters and gauges
- Time-series data

## Creating a Plugin

### Step 1: Choose Plugin Type

Decide whether your plugin collects static information (InfoPlugin) or dynamic metrics (MonitorPlugin).

### Step 2: Create Plugin File

Create a new Python file in `hbd/plugins/` directory:

```python
"""
My awesome plugin for Heartbeat.

Brief description of what this plugin does.
"""

import logging
from typing import Dict, Any, Optional

# Import psutil or other dependencies if needed
try:
    import psutil
except ImportError:
    psutil = None

from hbd.plugin import MonitorPlugin  # or InfoPlugin

logger = logging.getLogger(__name__)


class MyAwesomePlugin(MonitorPlugin):  # or InfoPlugin
    """
    One-line description of the plugin.
    
    Collects:
    - List of metrics/data collected
    - Another metric
    
    Configuration:
        interval: Collection interval in seconds (default: 60)
        option1: Description of option1 (default: value)
        option2: Description of option2 (default: value)
    """
    
    name = "my_awesome_plugin"  # Unique plugin name
    interval = 60  # For MonitorPlugin, use 0 for InfoPlugin
    
    def __init__(self, config: Optional[Dict[str, Any]] = None):
        """Initialize the plugin with optional configuration."""
        super().__init__(config)
        
        # Extract configuration options
        self.option1 = self.config.get('option1', 'default_value')
        self.option2 = self.config.get('option2', True)
        
        # Check dependencies
        if psutil is None:
            raise ImportError("psutil is required for my_awesome_plugin")
    
    async def initialize(self):
        """
        Initialize the plugin.
        
        This is called once when the plugin is loaded.
        Use this to verify dependencies, establish connections, etc.
        
        Returns:
            True if initialization successful, False otherwise
        """
        logger.info(f"My awesome plugin initialized (option1: {self.option1})")
        return True
    
    async def collect(self) -> Dict[str, Any]:
        """
        Collect data.
        
        This is called periodically (MonitorPlugin) or once (InfoPlugin).
        
        Returns:
            Dictionary of collected data (will be sent to server)
        """
        try:
            data = await self._collect_metrics()
            logger.debug(f"Collected {len(data)} metrics")
            return data
        except Exception as e:
            logger.error(f"Error collecting data: {e}")
            return {"error": str(e)}
    
    async def _collect_metrics(self) -> Dict[str, Any]:
        """Internal method to collect actual metrics."""
        metrics = {}
        
        # Collect your data here
        metrics['metric1'] = self._get_metric1()
        metrics['metric2'] = self._get_metric2()
        
        return metrics
    
    def _get_metric1(self):
        """Helper method for metric collection."""
        # Implementation here
        return 42
    
    def _get_metric2(self):
        """Helper method for metric collection."""
        # Implementation here
        return "hello"
    
    async def cleanup(self):
        """
        Cleanup resources.
        
        This is called when the plugin is unloaded or the client shuts down.
        Use this to close connections, release resources, etc.
        """
        logger.info("My awesome plugin cleanup")


# Plugin instance for automatic discovery
plugin = MyAwesomePlugin
```

### Step 3: Test Your Plugin

Create a test script to verify your plugin works:

```python
#!/usr/bin/env python3
import asyncio
import sys
from pathlib import Path

# Add parent directory to path
sys.path.insert(0, str(Path(__file__).parent))

from hbd.plugins.my_awesome_plugin import MyAwesomePlugin

async def test():
    # Create plugin instance
    plugin = MyAwesomePlugin({'option1': 'test_value'})
    
    # Initialize
    if not await plugin.initialize():
        print("Failed to initialize")
        return False
    
    # Collect data
    data = await plugin.collect()
    print(f"Collected data: {data}")
    
    # Cleanup
    await plugin.cleanup()
    
    return True

if __name__ == '__main__':
    success = asyncio.run(test())
    sys.exit(0 if success else 1)
```

## Plugin Lifecycle

Understanding the plugin lifecycle helps you implement plugins correctly:

```
1. Plugin Discovery
   └─> Loader scans hbd/plugins/ directory
   └─> Finds Python files (except those starting with _)
   └─> Imports modules

2. Plugin Instantiation
   └─> Creates instance with configuration
   └─> __init__() is called

3. Plugin Initialization
   └─> initialize() is called
   └─> Plugin verifies dependencies, establishes connections
   └─> Returns True/False for success/failure

4. Plugin Registration
   └─> If initialization succeeds, plugin is registered
   └─> Plugin becomes active

5. Data Collection
   └─> For InfoPlugin: collect() called once after initialization
   └─> For MonitorPlugin: collect() called periodically based on interval
   └─> Data is sent to server via PLG message

6. Plugin Shutdown
   └─> cleanup() is called
   └─> Plugin releases resources, closes connections
```

## Server-initiated InfoPlugin refresh

When a heartbeat packet arrives from a host the server has no plugin data for (e.g. after a server restart), the server sets `request_update = 1` in the ACK reply. The client detects this flag and immediately re-runs all InfoPlugins — clearing their cached results first — then resends the data as PLG messages.

This means InfoPlugin data will always reach the server as soon as possible without requiring a client restart. No action is needed from plugin authors: the framework handles cache invalidation and re-collection automatically.

The lifecycle for this case looks like:

```
Server restarts, host reconnects
   └─> hbd receives HTB with no existing plugin_data for host
   └─> hbd sets request_update=1 in ACK

Client receives ACK
   └─> Detects request_update flag
   └─> Clears _cache on every registered InfoPlugin
   └─> Calls collect() on each InfoPlugin
   └─> Sends fresh PLG messages to server
```

If you write an `InfoPlugin` with side effects in `_collect_info()` (opening connections, writing files, etc.), be aware it may be called more than once per client session when this mechanism triggers.

## Configuration

### Plugin-Specific Configuration

Plugins receive configuration through the `config` parameter in `__init__`:

```python
def __init__(self, config: Optional[Dict[str, Any]] = None):
    super().__init__(config)
    
    # Access configuration with defaults
    self.interval = self.config.get('interval', 60)
    self.threshold = self.config.get('threshold', 80)
    self.enabled_features = self.config.get('features', ['feature1', 'feature2'])
```

### Client Configuration File

Users configure plugins in the client configuration YAML:

```yaml
plugins:
  my_awesome_plugin:
    enabled: true
    interval: 120
    option1: custom_value
    option2: false
```

## Best Practices

### 1. Error Handling

Always handle errors gracefully:

```python
async def collect(self) -> Dict[str, Any]:
    try:
        return await self._collect_metrics()
    except Exception as e:
        logger.error(f"Error collecting metrics: {e}")
        return {"error": str(e)}
```

### 2. Logging

Use appropriate log levels:

```python
logger.debug("Detailed information for debugging")
logger.info("Normal operation messages")
logger.warning("Warning messages for unusual but handled situations")
logger.error("Error messages for failures")
```

### 3. Dependencies

Check for optional dependencies:

```python
try:
    import some_optional_library
except ImportError:
    some_optional_library = None

# Later in __init__:
if some_optional_library is None:
    raise ImportError("some_optional_library is required")
```

### 4. Performance

- Keep collection methods fast (< 1 second)
- Use async/await for I/O operations
- Cache expensive computations
- Don't block the event loop

### 5. Data Structure

Return clean, structured data:

```python
{
    'metric_name': value,
    'nested_data': {
        'sub_metric': value
    },
    'list_data': [item1, item2],
    'timestamp': time.time()  # Optional timestamp
}
```

### 6. Documentation

Document your plugin thoroughly:

- Class docstring with description and configuration
- Method docstrings explaining purpose and return values
- Inline comments for complex logic

## Examples

### Example 1: Simple InfoPlugin

```python
from hbd.plugin import InfoPlugin
import platform

class SimpleInfoPlugin(InfoPlugin):
    """Collect basic system information."""
    
    name = "simple_info"
    interval = 0  # InfoPlugin
    
    async def initialize(self):
        return True
    
    async def collect(self) -> Dict[str, Any]:
        return {
            'hostname': platform.node(),
            'system': platform.system(),
            'python_version': platform.python_version()
        }
    
    async def cleanup(self):
        pass

plugin = SimpleInfoPlugin
```

### Example 2: MonitorPlugin with State

```python
from hbd.plugin import MonitorPlugin
import time

class CounterPlugin(MonitorPlugin):
    """Track a counter over time."""
    
    name = "counter"
    interval = 30
    
    def __init__(self, config=None):
        super().__init__(config)
        self._counter = 0
        self._start_time = time.time()
    
    async def initialize(self):
        return True
    
    async def collect(self) -> Dict[str, Any]:
        self._counter += 1
        uptime = time.time() - self._start_time
        
        return {
            'count': self._counter,
            'uptime': uptime,
            'rate': self._counter / uptime
        }
    
    async def cleanup(self):
        pass

plugin = CounterPlugin
```

### Example 3: Plugin with External Command

```python
from hbd.plugin import MonitorPlugin
import asyncio

class CommandPlugin(MonitorPlugin):
    """Execute external command and capture output."""
    
    name = "command_executor"
    interval = 60
    
    def __init__(self, config=None):
        super().__init__(config)
        self.command = self.config.get('command', 'echo "no command"')
    
    async def initialize(self):
        return True
    
    async def collect(self) -> Dict[str, Any]:
        try:
            process = await asyncio.create_subprocess_shell(
                self.command,
                stdout=asyncio.subprocess.PIPE,
                stderr=asyncio.subprocess.PIPE
            )
            stdout, stderr = await asyncio.wait_for(
                process.communicate(),
                timeout=30
            )
            
            return {
                'exit_code': process.returncode,
                'stdout': stdout.decode('utf-8'),
                'stderr': stderr.decode('utf-8')
            }
        except Exception as e:
            return {'error': str(e)}
    
    async def cleanup(self):
        pass

plugin = CommandPlugin
```

## Testing

### Unit Testing

Create unit tests for your plugins:

```python
import unittest
import asyncio

class TestMyPlugin(unittest.TestCase):
    def setUp(self):
        self.plugin = MyAwesomePlugin({'option1': 'test'})
    
    def test_initialization(self):
        result = asyncio.run(self.plugin.initialize())
        self.assertTrue(result)
    
    def test_collection(self):
        asyncio.run(self.plugin.initialize())
        data = asyncio.run(self.plugin.collect())
        
        self.assertIsInstance(data, dict)
        self.assertIn('metric1', data)
        self.assertGreater(data['metric1'], 0)
    
    def tearDown(self):
        asyncio.run(self.plugin.cleanup())

if __name__ == '__main__':
    unittest.main()
```

### Integration Testing

Test your plugin with the actual client:

```bash
# Create test configuration
cat > test_config.yaml <<EOF
server: localhost
plugins:
  my_awesome_plugin:
    enabled: true
    interval: 10
    option1: test_value
EOF

# Run client in test mode
python -m hbd.hbc -c test_config.yaml --verbose
```

## Troubleshooting

### My plugin isn't loading

1. Check filename doesn't start with underscore
2. Verify plugin class inherits from InfoPlugin or MonitorPlugin
3. Check `initialize()` returns True
4. Look for import errors in logs

### Plugin loads but doesn't collect data

1. Check `interval` is set correctly (0 for InfoPlugin, > 0 for MonitorPlugin)
2. Verify `collect()` returns a dictionary
3. Check for exceptions in `collect()` method
4. Enable DEBUG logging to see detailed errors

### Data isn't appearing on server

1. Verify client is connected to server
2. Check server logs for PLG message handling
3. Verify returned data is JSON-serializable
4. Check for large data sizes (may exceed UDP packet size)

## Further Reading

- [Plugin Framework Source](../hbd/plugin.py) - Core plugin implementation
- [Built-in Plugins](../hbd/plugins/) - Examples of working plugins
- [Nagios Integration](NAGIOS_INTEGRATION.md) - Running external plugins
- [Configuration Guide](../hbd/config_example.yaml) - Full configuration reference