# Skybridge FaaS Implementation Guide

This document explains the implementation of the Function-as-a-Service (FaaS) component in Skybridge, specifically focusing on the Docker runtime implementation that replaced the original mock implementation.

## Overview

The Skybridge FaaS platform allows users to deploy and execute functions in isolated containers. The implementation consists of several key components:

1. **Function Management**: CRUD operations for function definitions
2. **Execution Engine**: Runtime backend for executing functions
3. **Repository Layer**: Data persistence for functions and executions
4. **Services Layer**: Business logic implementation
5. **API Layer**: RESTful interface for managing functions

## Docker Runtime Implementation

The original implementation contained a mock Docker runtime (`faas/internal/runtime/docker/simple.go`) that didn't actually interact with Docker. The new implementation provides real container execution capabilities.

### Key Features Implemented

1. **Real Docker Client Integration**: Uses the official Docker client library to communicate with the Docker daemon
2. **Container Lifecycle Management**: Creates, starts, waits for, and cleans up containers
3. **Image Management**: Pulls images when they don't exist locally
4. **Resource Limiting**: Applies memory limits to containers
5. **Input/Output Handling**: Passes input to functions and captures output
6. **Logging**: Retrieves container logs for debugging
7. **Health Checks**: Verifies Docker daemon connectivity

### Implementation Details

#### Container Creation

The `createContainer` method creates a Docker container with the following configuration:

- **Environment Variables**: Function environment variables plus input data
- **Resource Limits**: Memory limits based on function configuration
- **Attached Streams**: STDOUT and STDERR for log capture

#### Function Execution Flow

1. **Container Creation**: Creates a container from the function's Docker image
2. **Container Start**: Starts the container execution
3. **Wait for Completion**: Waits for the container to finish execution
4. **Result Collection**: Gathers output, logs, and execution metadata
5. **Cleanup**: Removes the container to free resources

#### Error Handling

The implementation includes comprehensive error handling:

- **Connection Errors**: Handles Docker daemon connectivity issues
- **Container Errors**: Manages container creation and execution failures
- **Resource Errors**: Handles resource constraint violations
- **Graceful Cleanup**: Ensures containers are cleaned up even on failures

## Testing

### Unit Tests

Unit tests are located in `faas/test/integration/` and cover:

- Docker runtime health checks
- Container creation and execution
- Error conditions

### Example Function

An example "Hello World" function is provided in `faas/examples/hello-world/` to demonstrate:

- Function structure and implementation
- Docker image creation
- Local testing
- Deployment to Skybridge FaaS

## Deployment

### Prerequisites

1. Docker daemon running and accessible
2. Docker socket mounted to the FaaS service container (as shown in `docker-compose.yml`)
3. Required permissions to access Docker

### Configuration

The FaaS service reads configuration from environment variables:

- `FAAS_DEFAULT_RUNTIME`: Should be set to "docker"
- Docker socket path: Typically `/var/run/docker.sock`

## Security Considerations

The current implementation has basic security features:

- **Container Isolation**: Functions run in isolated containers
- **Resource Limits**: Prevents resource exhaustion
- **Image Verification**: Only pulls trusted images

For production use, consider implementing:

- Container user restrictions
- Network isolation
- Enhanced logging and monitoring
- Authentication and authorization for Docker operations

## Performance Optimizations

Potential performance improvements include:

- **Image Caching**: Pre-pull commonly used images
- **Container Pooling**: Maintain a pool of ready containers
- **Parallel Execution**: Optimize concurrent function execution
- **Resource Monitoring**: Track and optimize resource usage

## Future Enhancements

Planned enhancements include:

1. **Multiple Runtime Support**: Add support for Podman and other container runtimes
2. **Advanced Resource Management**: CPU quotas, disk limits
3. **Enhanced Monitoring**: Detailed metrics and tracing
4. **Improved Error Handling**: More granular error reporting
5. **Security Hardening**: Additional security measures for container execution

## API Usage

The FaaS API provides endpoints for:

- **Function Management**: Create, read, update, delete functions
- **Deployment**: Deploy functions to prepare for execution
- **Execution**: Execute functions synchronously or asynchronously
- **Monitoring**: View execution status, logs, and metrics

Refer to the API documentation endpoint (`/api/docs`) for detailed information.
