Async Protocol Interceptors

In this lesson, you will learn how to design "Async Protocol Interceptors" to inspect and mutate network traffic in real-time without coupling your business logic to the underlying transport layer.

The Interceptor Pattern

An interceptor acts as a transparent layer—a "man-in-the-middle"—sitting between your application logic and the network stream. In asynchronous Rust, this is typically achieved by wrapping an AsyncRead or AsyncWrite type in a struct that processes bytes before passing them up or down the stack.

Think of this like a sandwich: the top slice of bread is the user request, the bottom slice is the actual socket, and your interceptor is the filling that inspects headers, enforces security policies (like stripping unauthorized tokens), or logs metadata.

Transparent Inspection

Since we are working with AsyncRead and AsyncWrite, your interceptor can leverage the buffer-driven nature of these traits to modify data as it flows. Here is a simplified mental model of an intercepting wrapper:

struct HeaderInterceptor<T> {
    inner: T,
    policy: SecurityPolicy,
}

impl<T: AsyncRead + Unpin> AsyncRead for HeaderInterceptor<T> {
    fn poll_read(mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut ReadBuf<'_>) -> Poll<io::Result<()>> {
        // 1. Let the inner transport do the work
        let result = Pin::new(&mut self.inner).poll_read(cx, buf);
        
        // 2. Perform "inspection" on the filled buffer
        if let Poll::Ready(Ok(_)) = result {
            self.policy.enforce(buf.filled());
        }
        
        result
    }
}

Why this matters

By treating the interceptor as a standard AsyncRead/Write type, you keep your pipeline composable. You can stack multiple interceptors—one for authentication, one for compression, and one for rate limiting—without the upstream application code ever needing to know the inspection is occurring. Because you are using standard async traits, these layers are non-blocking and efficient, fitting perfectly into the high-performance async pipelines we have explored in previous lessons.

Key Takeaway: Async Protocol Interceptors decouple network policy from application logic by wrapping I/O traits, allowing you to transparently transform or validate traffic as it streams through your system.