Starlette is a lightweight ASGI framework and toolkit designed for building performant asynchronous web services in Python. It provides a robust and production-ready foundation that emphasizes high performance and minimal complexity. Created to be modular and flexible, it can function as a full-featured web framework or as a set of independent components for building custom ASGI applications. Because it is fully type-annotated and thoroughly tested, it is well-suited for reliable, high-performance production environments.

Functionality is centered around the Asynchronous Server Gateway Interface (ASGI) standard, enabling full compatibility with modern async features of Python. Starlette provides an integrated suite of tools for handling HTTP requests, managing WebSocket connections, routing, template rendering, and middleware integration. It is designed to be lightweight, maintaining few hard dependencies while offering high performance against industry-standard benchmarks.

Some of the key features are:

• Lightweight Framework: Offers a low-complexity, high-performance foundation for building async web applications.
• WebSocket Support: Provides native, class-based, and functional support for bidirectional WebSocket communication.
• Built-in Middleware: Includes robust middleware for CORS, GZip, session management, and host security.
• Test Client: Comes with a built-in test client powered by httpx to ensure high test coverage and reliability.
• Type Safety: Features a 100% type-annotated codebase for better developer tooling and code clarity.
• Modular Architecture: Components can be used independently as a toolkit, allowing developers to pick and choose parts for specific needs.
• Template Support: Offers seamless integration with template engines like Jinja2 for server-side rendering.
• Background Tasks: Allows for easy handling of in-process background tasks to keep endpoints responsive.

Operationally, developers instantiate a Starlette application and pass it a list of routes, middleware, and exception handlers. The framework relies on an ASGI server, such as Uvicorn, to execute the application. Its routing system supports path parameters, reverse URL generation, and submounting for large-scale applications. Developers can interact with requests and responses through user-friendly abstractions that hide the complexities of raw ASGI scope and communication channels while maintaining the performance benefits of an asynchronous architecture.

Some common use cases include:

• Microservices: Rapidly building high-performance, asynchronous microservices that require low overhead and fast execution.
• Real-time Applications: Implementing chat systems, live dashboards, or real-time notifications using WebSocket support.
• API Development: Creating clean, well-documented RESTful APIs with built-in validation and schema generation.
• Custom Web Infrastructure: Acting as the foundation for larger frameworks, such as FastAPI, by providing core request/response handling and routing capabilities.