examples/rate-limiting: add example for session based rate limiting (#87)

Added an example for session based rate limiting after session.ID() was
exposed.

Fixes #22

examples/rate-limiting/go.mod

@@ -5,6 +5,6 @@ go 1.23.0
 toolchain go1.24.4
 
 require (
-	github.com/modelcontextprotocol/go-sdk v0.0.0-20250625185707-09181c2c2e89
+	github.com/modelcontextprotocol/go-sdk v0.1.0
 	golang.org/x/time v0.12.0
 )

examples/rate-limiting/go.sum

@@ -1,7 +1,7 @@
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
-github.com/modelcontextprotocol/go-sdk v0.0.0-20250625185707-09181c2c2e89 h1:kUGBYP25FTv3ZRBhLT4iQvtx4FDl7hPkWe3isYrMxyo=
-github.com/modelcontextprotocol/go-sdk v0.0.0-20250625185707-09181c2c2e89/go.mod h1:DcXfbr7yl7e35oMpzHfKw2nUYRjhIGS2uou/6tdsTB0=
+github.com/modelcontextprotocol/go-sdk v0.1.0 h1:ItzbFWYNt4EHcUrScX7P8JPASn1FVYb29G773Xkl+IU=
+github.com/modelcontextprotocol/go-sdk v0.1.0/go.mod h1:DcXfbr7yl7e35oMpzHfKw2nUYRjhIGS2uou/6tdsTB0=
 golang.org/x/time v0.12.0 h1:ScB/8o8olJvc+CQPWrK3fPZNfh7qgwCrY0zJmoEQLSE=
 golang.org/x/time v0.12.0/go.mod h1:CDIdPxbZBQxdj6cxyCIdrNogrJKMJ7pr37NYpMcMDSg=
 golang.org/x/tools v0.34.0 h1:qIpSLOxeCYGg9TrcJokLBG4KFA6d795g0xkBkiESGlo=

examples/rate-limiting/main.go

@@ -7,6 +7,8 @@ package main
 import (
 	"context"
 	"errors"
+	"log"
+	"sync"
 	"time"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
@@ -43,6 +45,43 @@ func PerMethodRateLimiterMiddleware[S mcp.Session](limiters map[string]*rate.Lim
 	}
 }
 
+// PerSessionRateLimiterMiddleware creates a middleware that applies rate limiting
+// on a per-session basis for receiving requests.
+func PerSessionRateLimiterMiddleware[S mcp.Session](limit rate.Limit, burst int) mcp.Middleware[S] {
+	// A map to store limiters, keyed by the session ID.
+	var (
+		sessionLimiters = make(map[string]*rate.Limiter)
+		mu              sync.Mutex
+	)
+
+	return func(next mcp.MethodHandler[S]) mcp.MethodHandler[S] {
+		return func(ctx context.Context, session S, method string, params mcp.Params) (mcp.Result, error) {
+			// It's possible that session.ID() may be empty at this point in time
+			// for some transports (e.g., stdio) or until the MCP initialize handshake
+			// has completed.
+			sessionID := session.ID()
+			if sessionID == "" {
+				// In this situation, you could apply a single global identifier
+				// if session ID is empty or bypass the rate limiter.
+				// In this example, we bypass the rate limiter.
+				log.Printf("Warning: Session ID is empty for method %q. Skipping per-session rate limiting.", method)
+				return next(ctx, session, method, params) // Skip limiting if ID is unavailable
+			}
+			mu.Lock()
+			limiter, ok := sessionLimiters[sessionID]
+			if !ok {
+				limiter = rate.NewLimiter(limit, burst)
+				sessionLimiters[sessionID] = limiter
+			}
+			mu.Unlock()
+			if !limiter.Allow() {
+				return nil, errors.New("JSON RPC overloaded")
+			}
+			return next(ctx, session, method, params)
+		}
+	}
+}
+
 func main() {
 	server := mcp.NewServer("greeter1", "v0.0.1", nil)
 	server.AddReceivingMiddleware(GlobalRateLimiterMiddleware[*mcp.ServerSession](rate.NewLimiter(rate.Every(time.Second/5), 10)))
@@ -50,5 +89,8 @@ func main() {
 		"callTool":  rate.NewLimiter(rate.Every(time.Second), 5),  // once a second with a burst up to 5
 		"listTools": rate.NewLimiter(rate.Every(time.Minute), 20), // once a minute with a burst up to 20
 	}))
+	server.AddReceivingMiddleware(PerSessionRateLimiterMiddleware[*mcp.ServerSession](rate.Every(time.Second/5), 10))
 	// Run Server logic.
+	log.Println("MCP Server instance created with Middleware (but not running).")
+	log.Println("This example demonstrates configuration, not live interaction.")
 }