fixing busy waiting in abstraction and eliminate busy-waiting

harsh543 · harsh543 · commit 240e8a9b0051 · 2025-07-09T08:59:32.000-07:00
diff --git a/microservices-log-aggregation/src/main/java/com/iluwatar/logaggregation/LogAggregator.java b/microservices-log-aggregation/src/main/java/com/iluwatar/logaggregation/LogAggregator.java
@@ -23,12 +23,16 @@
  * THE SOFTWARE.
  */
 package com.iluwatar.logaggregation;
-
+import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ConcurrentLinkedQueue;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.CountDownLatch;
+import java.util.ArrayList;
+import java.util.List;
 import lombok.extern.slf4j.Slf4j;
 
 /**
@@ -41,11 +45,17 @@
 public class LogAggregator {
 
   private static final int BUFFER_THRESHOLD = 3;
+  private static final int FLUSH_INTERVAL_SECONDS = 5;
+  private static final int SHUTDOWN_TIMEOUT_SECONDS = 10;
+
   private final CentralLogStore centralLogStore;
   private final ConcurrentLinkedQueue<LogEntry> buffer = new ConcurrentLinkedQueue<>();
   private final LogLevel minLogLevel;
   private final ExecutorService executorService = Executors.newSingleThreadExecutor();
   private final AtomicInteger logCount = new AtomicInteger(0);
+   private final ScheduledExecutorService scheduledExecutor = Executors.newScheduledThreadPool(1);
+  private final CountDownLatch shutdownLatch = new CountDownLatch(1);
+  private volatile boolean running = true;
 
   /**
    * constructor of LogAggregator.
@@ -57,6 +67,15 @@ public LogAggregator(CentralLogStore centralLogStore, LogLevel minLogLevel) {
     this.centralLogStore = centralLogStore;
     this.minLogLevel = minLogLevel;
     startBufferFlusher();
+    // Add shutdown hook for graceful termination
+    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
+      try {
+        stop();
+      } catch (InterruptedException e) {
+        LOGGER.warn("Shutdown interrupted", e);
+        Thread.currentThread().interrupt();
+      }
+    }));
   }
 
   /**
@@ -65,6 +84,11 @@ public LogAggregator(CentralLogStore centralLogStore, LogLevel minLogLevel) {
    * @param logEntry The log entry to collect.
    */
   public void collectLog(LogEntry logEntry) {
+    if (!running) {
+      LOGGER.warn("LogAggregator is shutting down. Skipping log entry.");
+      return;
+    }
+    
     if (logEntry.getLevel() == null || minLogLevel == null) {
       LOGGER.warn("Log level or threshold level is null. Skipping.");
       return;
@@ -75,10 +99,17 @@ public void collectLog(LogEntry logEntry) {
       return;
     }
 
-    buffer.offer(logEntry);
+    // BlockingQueue.offer() is non-blocking and thread-safe
+    boolean added = buffer.offer(logEntry);
+    if (!added) {
+      LOGGER.warn("Failed to add log entry to buffer - queue may be full");
+      return;
+    }
 
+    // Check if immediate flush is needed due to threshold
     if (logCount.incrementAndGet() >= BUFFER_THRESHOLD) {
-      flushBuffer();
+      // Schedule immediate flush instead of blocking current thread
+      scheduledExecutor.execute(this::flushBuffer);
     }
   }
 
@@ -88,32 +119,123 @@ public void collectLog(LogEntry logEntry) {
    * @throws InterruptedException If any thread has interrupted the current thread.
    */
   public void stop() throws InterruptedException {
-    executorService.shutdownNow();
-    if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
-      LOGGER.error("Log aggregator did not terminate.");
+    LOGGER.info("Stopping LogAggregator...");
+    running = false;
+    
+    // Shutdown the scheduler gracefully
+    scheduledExecutor.shutdown();
+    
+    try {
+      // Wait for scheduled tasks to complete
+      if (!scheduledExecutor.awaitTermination(SHUTDOWN_TIMEOUT_SECONDS, TimeUnit.SECONDS)) {
+        LOGGER.warn("Scheduler did not terminate gracefully, forcing shutdown");
+        scheduledExecutor.shutdownNow();
+        
+        // Wait a bit more for tasks to respond to interruption
+        if (!scheduledExecutor.awaitTermination(2, TimeUnit.SECONDS)) {
+          LOGGER.error("Scheduler did not terminate after forced shutdown");
+        }
+      }
+    } finally {
+      // Final flush of any remaining logs
+      flushBuffer();
+      shutdownLatch.countDown();
+      LOGGER.info("LogAggregator stopped successfully");
     }
-    flushBuffer();
   }
 
+
+  /**
+   * Waits for the LogAggregator to complete shutdown.
+   * Useful for testing or controlled shutdown scenarios.
+   *
+   * @throws InterruptedException If any thread has interrupted the current thread.
+   */
+  public void awaitShutdown() throws InterruptedException {
+    shutdownLatch.await();
+  }
+
+
   private void flushBuffer() {
-    LogEntry logEntry;
-    while ((logEntry = buffer.poll()) != null) {
-      centralLogStore.storeLog(logEntry);
-      logCount.decrementAndGet();
+   if (!running && buffer.isEmpty()) {
+      return;
+    }
+    
+    try {
+      List<LogEntry> batch = new ArrayList<>();
+      int drained = 0;
+      
+      // Drain up to a reasonable batch size for efficiency
+      LogEntry logEntry;
+      while ((logEntry = buffer.poll()) != null && drained < 100) {
+        batch.add(logEntry);
+        drained++;
+      }
+      
+      if (!batch.isEmpty()) {
+        LOGGER.debug("Flushing {} log entries to central store", batch.size());
+        
+        // Process the batch
+        for (LogEntry entry : batch) {
+          centralLogStore.storeLog(entry);
+          logCount.decrementAndGet();
+        }
+        
+        LOGGER.debug("Successfully flushed {} log entries", batch.size());
+      }
+    } catch (Exception e) {
+      LOGGER.error("Error occurred while flushing buffer", e);
     }
   }
 
-  private void startBufferFlusher() {
-    executorService.execute(
+  /**
+   * Starts the periodic buffer flusher using ScheduledExecutorService.
+   * This eliminates the busy-waiting loop with Thread.sleep().
+   */
+  private void startPeriodicFlusher() {
+    scheduledExecutor.scheduleAtFixedRate(
         () -> {
-          while (!Thread.currentThread().isInterrupted()) {
+          if (running) {
             try {
-              Thread.sleep(5000); // Flush every 5 seconds.
               flushBuffer();
-            } catch (InterruptedException e) {
-              Thread.currentThread().interrupt();
+            } catch (Exception e) {
+              LOGGER.error("Error in periodic flush", e);
             }
           }
-        });
+        },
+        FLUSH_INTERVAL_SECONDS,  // Initial delay
+        FLUSH_INTERVAL_SECONDS,  // Period
+        TimeUnit.SECONDS
+    );
+    
+    LOGGER.info("Periodic log flusher started with interval of {} seconds", FLUSH_INTERVAL_SECONDS);
+  }
+   /**
+   * Gets the current number of buffered log entries.
+   * Useful for monitoring and testing.
+   *
+   * @return Current buffer size
+   */
+  public int getBufferSize() {
+    return buffer.size();
+  }
+
+  /**
+   * Gets the current log count.
+   * Useful for monitoring and testing.
+   *
+   * @return Current log count
+   */
+  public int getLogCount() {
+    return logCount.get();
+  }
+
+  /**
+   * Checks if the LogAggregator is currently running.
+   *
+   * @return true if running, false if stopped or stopping
+   */
+  public boolean isRunning() {
+    return running;
   }
 }
diff --git a/server-session/src/main/java/com/iluwatar/sessionserver/App.java b/server-session/src/main/java/com/iluwatar/sessionserver/App.java
@@ -24,12 +24,18 @@
  */
 package com.iluwatar.sessionserver;
 
+
+import java.util.HashMap;
 import com.sun.net.httpserver.HttpServer;
 import java.io.IOException;
 import java.net.InetSocketAddress;
-import java.time.Instant;
-import java.util.HashMap;
 import java.util.Iterator;
+import java.time.Instant;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.CountDownLatch;
 import java.util.Map;
 import lombok.extern.slf4j.Slf4j;
 
@@ -57,6 +63,12 @@ public class App {
   private static Map<String, Instant> sessionCreationTimes = new HashMap<>();
   private static final long SESSION_EXPIRATION_TIME = 10000;
 
+  // Scheduler for session expiration task
+  private static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
+  private static volatile boolean running = true;
+  private static final CountDownLatch shutdownLatch = new CountDownLatch(1);
+
+
   /**
    * Main entry point.
    *
@@ -78,39 +90,61 @@ public static void main(String[] args) throws IOException {
     sessionExpirationTask();
 
     LOGGER.info("Server started. Listening on port 8080...");
+    // Wait for shutdown signal
+    try {
+      shutdownLatch.await();
+    } catch (InterruptedException e) {
+      LOGGER.error("Main thread interrupted", e);
+      Thread.currentThread().interrupt();
+    }
   }
 
   private static void sessionExpirationTask() {
-    new Thread(
-            () -> {
-              while (true) {
-                try {
-                  LOGGER.info("Session expiration checker started...");
-                  Thread.sleep(SESSION_EXPIRATION_TIME); // Sleep for expiration time
-                  Instant currentTime = Instant.now();
-                  synchronized (sessions) {
-                    synchronized (sessionCreationTimes) {
-                      Iterator<Map.Entry<String, Instant>> iterator =
-                          sessionCreationTimes.entrySet().iterator();
-                      while (iterator.hasNext()) {
-                        Map.Entry<String, Instant> entry = iterator.next();
-                        if (entry
-                            .getValue()
-                            .plusMillis(SESSION_EXPIRATION_TIME)
-                            .isBefore(currentTime)) {
-                          sessions.remove(entry.getKey());
-                          iterator.remove();
-                        }
-                      }
-                    }
-                  }
-                  LOGGER.info("Session expiration checker finished!");
-                } catch (InterruptedException e) {
-                  LOGGER.error("An error occurred: ", e);
-                  Thread.currentThread().interrupt();
-                }
-              }
-            })
-        .start();
+    if (!running) {
+      return;
+    }
+    try {
+      LOGGER.info("Session expiration checker started...");
+      Instant currentTime = Instant.now();
+      
+      // Use removeIf for efficient removal without explicit synchronization
+      // ConcurrentHashMap handles thread safety internally
+      sessionCreationTimes.entrySet().removeIf(entry -> {
+        if (entry.getValue().plusMillis(SESSION_EXPIRATION_TIME).isBefore(currentTime)) {
+          sessions.remove(entry.getKey());
+          LOGGER.debug("Expired session: {}", entry.getKey());
+          return true;
+        }
+        return false;
+      });
+      
+      LOGGER.info("Session expiration checker finished! Active sessions: {}", sessions.size());
+    } catch (Exception e) {
+      LOGGER.error("An error occurred during session expiration check: ", e);
+    }
+  }
+
+   /**
+   * Gracefully shuts down the session expiration scheduler.
+   * This method is called by the shutdown hook.
+   */
+  private static void shutdown() {
+    LOGGER.info("Shutting down session expiration scheduler...");
+    running = false;
+    scheduler.shutdown();
+    
+    try {
+      if (!scheduler.awaitTermination(5, TimeUnit.SECONDS)) {
+        LOGGER.warn("Scheduler did not terminate gracefully, forcing shutdown");
+        scheduler.shutdownNow();
+      }
+    } catch (InterruptedException e) {
+      LOGGER.warn("Shutdown interrupted, forcing immediate shutdown");
+      scheduler.shutdownNow();
+      Thread.currentThread().interrupt();
+    }
+    
+    shutdownLatch.countDown();
+    LOGGER.info("Session expiration scheduler shut down complete");
   }
 }
diff --git a/twin/src/main/java/com/iluwatar/twin/BallThread.java b/twin/src/main/java/com/iluwatar/twin/BallThread.java
