Feat/fastapi modular server contribution

contributing/samples/fastapi_modular_server/.env.example

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+# Application Configuration
+DEBUG=true
+PORT=8881
+HOST=localhost
+LOG_LEVEL=INFO
+LOGGING=true
+
+# ADK Configuration
+SERVE_WEB_INTERFACE=true
+RELOAD_AGENTS=true
+
+# Google Gemini Configuration
+GOOGLE_API_KEY=YOUR_GOOGLE_API_KEY_HERE
+
+# Model Configuration
+MODEL_PROVIDER=google

contributing/samples/fastapi_modular_server/.gitignore

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+# Logs
+*.log
+
+# Environment variables
+.env
+
+# Python bytecode
+*.pyc
+__pycache__/
+
+# Test artifacts
+.pytest_cache/
+
+# Virtual environments
+.venv/
+venv/
+
+# IDE configuration
+.vscode/
+.idea/

contributing/samples/fastapi_modular_server/README.md

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+# 🚀 Google ADK FastAPI Modular Server
+
+A **production-ready template** for extending Google's Agent Development Kit (ADK) with custom FastAPI endpoints, optimized SSE streaming, and modular architecture patterns.
+
+## 🎯 Purpose & Value
+
+The **FastAPI Modular Server** serves as a **template and reference implementation** for teams who want to:
+
+- **Extend ADK's built-in server** without modifying core behavior
+- **Accelerate production deployment** with battle-tested patterns
+- **Add custom business logic** through modular router systems
+- **Enable hot-reload capabilities** for faster development cycles
+
+## ✨ Key Features
+
+### 🔧 **Modular Router Architecture**
+- Clean separation of concerns with dedicated router classes
+- Easy to add new endpoints without touching core server code
+
+### ⚡ **Optimized SSE Streaming**
+- **3 optimization levels** for different use cases:
+  - `MINIMAL`: Essential content only (author + text)
+  - `BALANCED`: Core data with invocation tracking
+  - `FULL_COMPAT`: Complete ADK event compatibility
+- Reduced payload sizes for improved performance
+- Custom event filtering and mapping
+
+### 🔄 **Hot-Reload Development**
+- Automatic agent reloading on file changes
+- File system monitoring with `watchdog`
+- Development-friendly with production stability
+
+
+## 📁 Project Structure
+
+```
+fastapi_modular_server/
+├── .env.example                 # Environment variables template
+├── README.md                    # Project documentation
+├── __init__.py                  # Package initialization
+├── app/                         # Main application directory
+│   ├── __init__.py              # App package initialization
+│   ├── agents/                  # Agent definitions
+│   │   └── greetings_agent/     # Greetings agent module
+│   │       ├── __init__.py      # Agent package init
+│   │       └── greetings_agent.py         # Greetings agent implementation
+│   ├── api/                     # API layer
+│   │   ├── __init__.py          # API package init
+│   │   ├── routers/             # API route definitions
+│   │   │   ├── __init__.py      # Routers package init
+│   │   │   └── agent_router.py         # Agent-related API routes
+│   │   └── custom_adk_server.py            # FastAPI server configuration
+│   ├── config/                  # Configuration management
+│   │   └── settings.py          # Application settings
+│   ├── core/                    # Core application components
+│   │   ├── __init__.py          # Core package init
+│   │   ├── dependencies.py      # Dependency injection
+│   │   ├── logging.py           # Logging configuration
+│   │   └── mapping/             # Data mapping utilities
+│   │       ├── __init__.py      # Mapping package init
+│   │       └── sse_event_mapper.py    # Server-Sent Events mapper
+│   └── models/                  # Data models
+│       ├── __init__.py          # Models package init
+│       └── streaming_request.py         # Streaming data models
+└── main.py                      # Application entry point
+```
+
+## 🚀 Quick Start
+
+### 1. **Configuration**
+```bash
+# Copy environment template
+cp .env.example .env
+
+# Edit .env with your settings
+vim .env
+
+# Set the API KEY
+
+```
+
+### 2. **Run the Server**
+```bash
+# Development mode with hot-reload
+python main.py
+
+# Production mode
+uvicorn main:app --host 0.0.0.0 --port 8881
+```
+
+
+## 🔧 Customization Guide
+
+### **Adding New Routers**
+
+Create a new router following the established pattern:
+
+```python
+# app/api/routers/my_custom_router.py
+from fastapi import APIRouter, Depends
+from app.core.dependencies import ADKServices, get_adk_services
+
+class MyCustomRouter:
+    def __init__(self, web_server_instance):
+        self.web_server = web_server_instance
+        self.router = APIRouter(prefix="/custom", tags=["Custom"])
+        self._setup_routes()
+
+    def _setup_routes(self):
+        @self.router.get("/endpoint")
+        async def my_endpoint(
+        ):
+            # Access any ADK service
+            sessions = await self.web_server.session_service.list_sessions()
+            return {"data": "custom response", "session_count": len(sessions)}
+
+    def get_router(self) -> APIRouter:
+        return self.router
+```
+
+Register it in the custom server:
+
+```python
+# In app/api/custom_adk_server.py - CustomAdkWebServer class
+def _initialize_routers(self):
+    try:
+        self.agent_router = AgentRouter(self)
+        self.my_custom_router = MyCustomRouter(self)  # Add this
+        logger.info("All routers initialized successfully.")
+    except Exception as e:
+        logger.error(f"Failed to initialize routers: {e}", exc_info=True)
+
+def _register_modular_routers(self, app: FastAPI):
+    # ... existing code ...
+
+    if self.my_custom_router:
+        app.include_router(self.my_custom_router.get_router())
+        logger.info("Registered MyCustomRouter.")
+```
+
+### **Overriding ADK Endpoints**
+
+#### **Method 1: Route Removal (Current Approach)**
+
+```python
+def _register_modular_routers(self, app: FastAPI):
+    # Remove specific ADK routes
+    routes_to_remove = []
+    for route in app.routes:
+        if route.path in [
+            "/run_sse", 
+            # You could add additional ADK routes here if you want to override them,
+            # e.g., "/apps/{app_name}/users/{user_id}/sessions"
+        ] and hasattr(route, 'methods') and 'POST' in route.methods:
+            routes_to_remove.append(route)
+
+    # Remove the routes
+    for route in routes_to_remove:
+        app.routes.remove(route)
+```
+
+#### **Method 2: Middleware Interception**
+
+For more complex overrides, use middleware:
+
+```python
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware
+
+class RouteOverrideMiddleware(BaseHTTPMiddleware):
+    async def dispatch(self, request: Request, call_next):
+        # Intercept specific routes
+        if request.url.path == "/run_sse" and request.method == "POST":
+            # Handle with custom logic
+            return await self.handle_custom_sse(request)
+
+        return await call_next(request)
+```
+
+### **Accessing ADK Services and Runners**
+
+#### **From Router Classes**
+```python
+class AgentRouter:
+    def __init__(self, web_server_instance):
+        self.web_server = web_server_instance
+
+    async def my_endpoint(self, adk_services: ADKServices = Depends(get_adk_services)):
+        # Access services
+        agents = self.web_server.agent_loader.list_agents()
+        session = await self.web_server.session_service.list_sessions()
+
+        # Access runners through web server
+         runner = await self.web_server.get_runner_async("your_app_name")
+
+        # Access other web server properties
+        runners_cache = self.web_server.runners_to_clean
+```
+
+### **Optimizing SSE Streaming**
+
+#### **Custom Event Filtering**
+
+Extend the SSE mapper for more sophisticated filtering:
+
+```python
+# app/models/streaming_request.py
+class OptimizationLevel(str, Enum):
+  """Enumeration for the available SSE optimization levels."""
+
+  MINIMAL = "minimal"
+  BALANCED = "balanced"
+  FULL_COMPAT = "full_compat"
+  ULTRA_MINIMAL = "ultra_minimal"
+
+# app/core/mapping/sse_mapper.py
+class AdvancedSSEEventMapper(SSEEventMapper):
+    def map_event_to_sse_message(self, event: Event, optimization_level: OptimizationLevel) -> Optional[str]:
+        # Custom filtering logic
+        if self._should_skip_event(event):
+            return None
+
+        # Custom payload creation
+        payload = self._create_custom_payload(event, optimization_level)
+
+        # Custom serialization
+        return self._serialize_payload(payload)
+
+    def _should_skip_event(self, event: Event) -> bool:
+        # Skip system events, debug events, empty events, etc.
+        if event.author in ["system", "debug"]:
+            return True
+        if not event.content or not event.content.parts:
+            return True
+        return False
+
+    def _create_custom_payload(self, event: Event, level: OptimizationLevel) -> Dict:
+        if level == OptimizationLevel.ULTRA_MINIMAL:
+            # Even more minimal than minimal
+            return {"t": self._extract_text_only(event)}
+
+        return super()._create_minimal_payload(event)
+```
+
+## 🤝 Contributing
+
+This template is designed to be extended and customized for your specific needs. Key extension points:
+
+1. **Router Classes**: Add domain-specific endpoints
+2. **SSE Mappers**: Custom event processing and optimization
+3. **Middleware**: Cross-cutting concerns
+4. **Services**: Additional business logic services
+5. **Configuration**: Environment-specific settings
+
+## 📚 Further Resources
+
+- **Google ADK Documentation**: https://google.github.io/adk-docs/
+- **FastAPI Documentation**: https://fastapi.tiangolo.com/
+- **Pydantic Settings**: https://docs.pydantic.dev/latest/concepts/pydantic_settings/
+- **Server-Sent Events**: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events

contributing/samples/fastapi_modular_server/app/agents/greetings_agent/__init__.py

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+from app.agents.greetings_agent.greetings_agent import root_agent

contributing/samples/fastapi_modular_server/app/agents/greetings_agent/greetings_agent.py

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+from google.adk.agents import LlmAgent
+
+root_agent = LlmAgent(
+    model="gemini-2.5-flash",
+    name="greetings_agent",
+    description="A friendly Google Gemini-powered agent",
+    instruction="You are a helpful AI assistant powered by Google Gemini.",
+    tools=[],
+)