Updated README.md. All test case passed. Updated with Google Java Guidelines

Author: skamble2

pom.xml

@@ -199,6 +199,7 @@
     <module>proxy</module>
     <module>publish-subscribe</module>
     <module>queue-based-load-leveling</module>
+      <module>rate-limiting-pattern</module>
     <module>reactor</module>
     <module>registry</module>
     <module>repository</module>

rate-limiting-pattern/README.md

@@ -0,0 +1,328 @@
+---
+title: "Rate Limiting Pattern in Java: Controlling System Overload Gracefully"
+shortTitle: Rate Limiting
+description: "Explore multiple rate limiting strategies in Java—Token Bucket, Fixed Window, and Adaptive Limiting. Learn with diagrams, programmatic examples, and real-world simulation."
+category: Behavioral
+language: en
+tag:
+  - Resilience
+  - System Overload Protection
+  - API Throttling
+  - Concurrency
+  - Cloud Patterns
+---
+
+## Also known as
+
+- Throttling
+- Request Limiting
+- API Rate Limiting
+
+---
+
+## Intent of Rate Limiting Design Pattern
+
+To regulate the number of requests sent to a service in a specific time window, avoiding resource exhaustion and ensuring system stability. This is especially useful in distributed and cloud-native architectures.
+
+---
+
+## Detailed Explanation of Rate Limiting with Real-World Examples
+
+### Real-world example
+
+Imagine you're entering a concert hall that only allows 50 people per minute. If too many fans arrive at once, the gate staff slows down entry, allowing only a few at a time. This prevents overcrowding and ensures safety. Similarly, the rate limiter controls how many requests are processed to avoid overloading a server.
+
+### In plain words
+
+Regulate the number of requests a system handles within a time frame to protect availability and performance.
+
+
+### AWS says
+
+> "API Gateway limits the steady-state rate and burst rate of requests that it allows for each method in your REST APIs. When request rates exceed these limits, API Gateway begins to throttle requests."
+
+— [API Gateway quotas and important notes - AWS Documentation](https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html)
+
+---
+
+## Architecture Diagram
+
+![UML Class Diagram](etc/UMLClassDiagram.png)
+
+This UML shows the key components:
+- `RateLimiter` interface
+- `TokenBucketRateLimiter`, `FixedWindowRateLimiter`, `AdaptiveRateLimiter`
+- Supporting exception classes
+- `FindCustomerRequest` as a rate-limited operation
+
+---
+
+## Flowcharts
+
+### 1. Token Bucket Strategy
+
+![Token Bucket Rate Limiter](etc/TokenBucketRateLimiter.png)
+
+### 2. Fixed Window Strategy
+
+![Fixed Window Rate Limiter](etc/FixedWindowRateLimiter.png)
+
+### 3. Adaptive Rate Limiting Strategy
+
+![Adaptive Rate Limiter](etc/AdaptiveRateLimiter.png)
+
+---
+
+### Programmatic Example of Rate Limiter Pattern in Java
+
+The **Rate Limiter** design pattern helps protect systems from overload by restricting the number of operations that can be performed in a given time window. It is especially useful when accessing shared resources, APIs, or services that are sensitive to spikes in traffic.
+
+This implementation demonstrates three strategies for rate limiting:
+
+- **Token Bucket Rate Limiter**
+- **Fixed Window Rate Limiter**
+- **Adaptive Rate Limiter**
+
+Let’s walk through the key components.
+
+---
+
+#### 1. Token Bucket Rate Limiter
+
+The token bucket allows short bursts followed by a steady rate. Tokens are added periodically and requests are only allowed if a token is available.
+
+```java
+public class TokenBucketRateLimiter implements RateLimiter {
+  private final int capacity;
+  private final int refillRate;
+  private final ConcurrentHashMap<String, TokenBucket> buckets = new ConcurrentHashMap<>();
+  private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
+
+  public TokenBucketRateLimiter(int capacity, int refillRate) {
+    this.capacity = capacity;
+    this.refillRate = refillRate;
+    scheduler.scheduleAtFixedRate(this::refillBuckets, 1, 1, TimeUnit.SECONDS);
+  }
+
+  @Override
+  public void check(String serviceName, String operationName) throws RateLimitException {
+    String key = serviceName + ":" + operationName;
+    TokenBucket bucket = buckets.computeIfAbsent(key, k -> new TokenBucket(capacity));
+    if (!bucket.tryConsume()) {
+      throw new ThrottlingException(serviceName, operationName, 1000);
+    }
+  }
+
+  private void refillBuckets() {
+    buckets.forEach((k, b) -> b.refill(refillRate));
+  }
+
+  private static class TokenBucket {
+    private final int capacity;
+    private final AtomicInteger tokens;
+
+    TokenBucket(int capacity) {
+      this.capacity = capacity;
+      this.tokens = new AtomicInteger(capacity);
+    }
+
+    boolean tryConsume() {
+      while (true) {
+        int current = tokens.get();
+        if (current <= 0) return false;
+        if (tokens.compareAndSet(current, current - 1)) return true;
+      }
+    }
+
+    void refill(int amount) {
+      tokens.getAndUpdate(current -> Math.min(current + amount, capacity));
+    }
+  }
+}
+```
+
+---
+
+#### 2. Fixed Window Rate Limiter
+
+This strategy uses a simple counter within a fixed time window.
+
+```java
+public class FixedWindowRateLimiter implements RateLimiter {
+  private final int limit;
+  private final long windowMillis;
+  private final ConcurrentHashMap<String, WindowCounter> counters = new ConcurrentHashMap<>();
+
+  public FixedWindowRateLimiter(int limit, long windowSeconds) {
+    this.limit = limit;
+    this.windowMillis = TimeUnit.SECONDS.toMillis(windowSeconds);
+  }
+
+  @Override
+  public synchronized void check(String serviceName, String operationName) throws RateLimitException {
+    String key = serviceName + ":" + operationName;
+    WindowCounter counter = counters.computeIfAbsent(key, k -> new WindowCounter());
+
+    if (!counter.tryIncrement()) {
+      throw new RateLimitException("Rate limit exceeded for " + key, windowMillis);
+    }
+  }
+
+  private class WindowCounter {
+    private AtomicInteger count = new AtomicInteger(0);
+    private volatile long windowStart = System.currentTimeMillis();
+
+    synchronized boolean tryIncrement() {
+      long now = System.currentTimeMillis();
+      if (now - windowStart > windowMillis) {
+        count.set(0);
+        windowStart = now;
+      }
+      return count.incrementAndGet() <= limit;
+    }
+  }
+}
+```
+
+---
+
+#### 3. Adaptive Rate Limiter
+
+This version adjusts the rate based on system health, reducing the rate when throttling occurs and recovering periodically.
+
+```java
+public class AdaptiveRateLimiter implements RateLimiter {
+  private final int initialLimit;
+  private final int maxLimit;
+  private final AtomicInteger currentLimit;
+  private final ConcurrentHashMap<String, RateLimiter> limiters = new ConcurrentHashMap<>();
+  private final ScheduledExecutorService healthChecker = Executors.newScheduledThreadPool(1);
+
+  public AdaptiveRateLimiter(int initialLimit, int maxLimit) {
+    this.initialLimit = initialLimit;
+    this.maxLimit = maxLimit;
+    this.currentLimit = new AtomicInteger(initialLimit);
+    healthChecker.scheduleAtFixedRate(this::adjustLimits, 10, 10, TimeUnit.SECONDS);
+  }
+
+  @Override
+  public void check(String serviceName, String operationName) throws RateLimitException {
+    String key = serviceName + ":" + operationName;
+    int current = currentLimit.get();
+    RateLimiter limiter = limiters.computeIfAbsent(key, k -> new TokenBucketRateLimiter(current, current));
+
+    try {
+      limiter.check(serviceName, operationName);
+    } catch (RateLimitException e) {
+      currentLimit.updateAndGet(curr -> Math.max(initialLimit, curr / 2));
+      throw e;
+    }
+  }
+
+  private void adjustLimits() {
+    currentLimit.updateAndGet(curr -> Math.min(maxLimit, curr + (initialLimit / 2)));
+  }
+}
+```
+
+---
+
+#### 4. Simulated Demo Using All Limiters
+
+```java
+public final class App {
+  public static void main(String[] args) {
+    TokenBucketRateLimiter tb = new TokenBucketRateLimiter(2, 1);
+    FixedWindowRateLimiter fw = new FixedWindowRateLimiter(3, 1);
+    AdaptiveRateLimiter ar = new AdaptiveRateLimiter(2, 6);
+
+    ExecutorService executor = Executors.newFixedThreadPool(3);
+    for (int i = 1; i <= 3; i++) {
+      executor.submit(createClientTask(i, tb, fw, ar));
+    }
+  }
+
+  private static Runnable createClientTask(int clientId, RateLimiter tb, RateLimiter fw, RateLimiter ar) {
+    return () -> {
+      String[] services = {"s3", "dynamodb", "lambda"};
+      String[] operations = {"GetObject", "PutObject", "Query", "Scan", "PutItem", "Invoke", "ListFunctions"};
+      Random random = new Random();
+
+      while (true) {
+        String service = services[random.nextInt(services.length)];
+        String operation = operations[random.nextInt(operations.length)];
+        try {
+          switch (service) {
+            case "s3" -> tb.check(service, operation);
+            case "dynamodb" -> fw.check(service, operation);
+            case "lambda" -> ar.check(service, operation);
+          }
+          System.out.printf("Client %d: %s.%s - ALLOWED%n", clientId, service, operation);
+        } catch (RateLimitException e) {
+          System.out.printf("Client %d: %s.%s - THROTTLED%n", clientId, service, operation);
+        }
+
+        try {
+          Thread.sleep(30 + random.nextInt(50));
+        } catch (InterruptedException e) {
+          Thread.currentThread().interrupt();
+        }
+      }
+    };
+  }
+}
+```
+
+---
+
+This example highlights how the Rate Limiter pattern supports various throttling techniques and how they respond under simulated traffic pressure, making it invaluable for building scalable, resilient systems.
+
+## When to Use Rate Limiting
+
+- APIs receiving unpredictable traffic
+- Shared cloud resources (e.g., DB, compute)
+- Services requiring fair client usage
+- Preventing DoS or abuse
+
+---
+
+## Real-World Applications
+
+- **AWS API Gateway**
+- **Google Cloud Functions**
+- **Netflix Zuul API Gateway**
+- **Stripe API Throttling**
+
+---
+
+## Benefits and Trade-offs
+
+### Benefits
+
+- Protects backend from overload
+- Fair distribution of resources
+- Better user experience under load
+
+### Trade-offs
+
+- May delay valid requests
+- Requires tuning of limits
+- Could create bottlenecks if misused
+
+---
+
+## Related Java Design Patterns
+
+- [Circuit Breaker](https://java-design-patterns.com/patterns/circuit-breaker/)
+- [Retry](https://java-design-patterns.com/patterns/retry/)
+- [Throttling Queue](https://java-design-patterns.com/patterns/throttling/)
+
+---
+
+## References and Credits
+
+- [Microsoft Cloud Design Patterns](https://learn.microsoft.com/en-us/azure/architecture/patterns/throttling)
+- [AWS API Gateway Throttling](https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html)
+- *Designing Data-Intensive Applications* by Martin Kleppmann
+- [Resilience4j](https://resilience4j.readme.io/)
+- Java Design Patterns Project: [java-design-patterns](https://github.com/iluwatar/java-design-patterns)

rate-limiting-pattern/etc/Adaptive Rate Limiter.png renamed to rate-limiting-pattern/etc/AdaptiveRateLimiter.png

@@ -39,6 +39,24 @@
 
     <build>
         <plugins>
+            <plugin>
+                <groupId>com.diffplug.spotless</groupId>
+                <artifactId>spotless-maven-plugin</artifactId>
+                <version>2.44.2</version>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>check</goal> <!-- Fails the build if formatting is off -->
+                            <goal>apply</goal> <!-- Automatically formats code -->
+                        </goals>
+                    </execution>
+                </executions>
+                <configuration>
+                    <java>
+                        <googleJavaFormat />
+                    </java>
+                </configuration>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-surefire-plugin</artifactId>