Chapter 16: Interceptors and Middleware

What Interceptors Do

Interceptors are middleware that runs before/after every RPC. Used for cross-cutting concerns without modifying each handler.

Client call → [Client Interceptor] → Network → [Server Interceptor] → Handler │ │ ├── Add auth token ├── Validate auth ├── Start trace span ├── Log request ├── Record metrics ├── Rate limit └── Retry on failure └── Record metrics

Server Interceptor (C++)

class LoggingInterceptor : public grpc::experimental::Interceptor {
  void Intercept(grpc::experimental::InterceptorBatchMethods* methods) override {
    if (methods->QueryInterceptionHookPoint(
        grpc::experimental::InterceptionHookPoints::PRE_SEND_INITIAL_METADATA)) {
      // Log before processing
      LOG_INFO("RPC started: %s", method_name);
    }
    methods->Proceed();  // continue to next interceptor or handler
  }
};

Common Interceptor Use Cases

Use Case	Client Side	Server Side
Authentication	Attach token to metadata	Validate token, reject if invalid
Logging	Log request/response	Log request/response + duration
Metrics	Record latency, errors	Record QPS, latency, error rate
Tracing	Start span, propagate context	Extract span, create child span
Retry	Retry on UNAVAILABLE	N/A
Rate limiting	N/A	Reject with RESOURCE_EXHAUSTED

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