Matter AI | Code Reviewer Documentation

Synchronization Issues Overview

Excessive Synchronization

// Anti-pattern: Excessive synchronization
public class UserRepository {
    private final Map<Long, User> userCache = new HashMap<>();
    
    // Entire method is synchronized, blocking all threads
    public synchronized User getUser(long userId) {
        if (userCache.containsKey(userId)) {
            return userCache.get(userId);
        }
        
        // Slow I/O operation while holding lock
        User user = loadUserFromDatabase(userId);
        userCache.put(userId, user);
        return user;
    }
    
    private User loadUserFromDatabase(long userId) {
        // Simulating database access
        try {
            Thread.sleep(100); // Slow I/O operation
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        return new User(userId, "User " + userId);
    }
}

// Better approach: Minimizing synchronized blocks
public class OptimizedUserRepository {
    private final Map<Long, User> userCache = new ConcurrentHashMap<>();
    
    public User getUser(long userId) {
        // Check cache without synchronization
        User user = userCache.get(userId);
        if (user != null) {
            return user;
        }
        
        // Only synchronize when necessary
        synchronized (this) {
            // Double-check to avoid race condition
            user = userCache.get(userId);
            if (user != null) {
                return user;
            }
            
            // Load user outside synchronized block
            user = loadUserFromDatabase(userId);
            userCache.put(userId, user);
            return user;
        }
    }
    
    // Even better approach: Using computeIfAbsent
    public User getUserOptimized(long userId) {
        return userCache.computeIfAbsent(userId, this::loadUserFromDatabase);
    }
    
    private User loadUserFromDatabase(long userId) {
        // Same implementation as before
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        return new User(userId, "User " + userId);
    }
}

// Anti-pattern: Excessive locking in Node.js
class UserRepository {
  constructor() {
    this.userCache = new Map();
    this.mutex = new Mutex(); // Using a mutex library
  }
  
  async getUser(userId) {
    // Acquire lock for the entire operation
    const release = await this.mutex.acquire();
    try {
      if (this.userCache.has(userId)) {
        return this.userCache.get(userId);
      }
      
      // Slow I/O operation while holding lock
      const user = await this.loadUserFromDatabase(userId);
      this.userCache.set(userId, user);
      return user;
    } finally {
      // Release lock
      release();
    }
  }
  
  async loadUserFromDatabase(userId) {
    // Simulating database access
    await new Promise(resolve => setTimeout(resolve, 100));
    return { id: userId, name: `User ${userId}` };
  }
}

// Better approach: Minimizing critical sections
class OptimizedUserRepository {
  constructor() {
    this.userCache = new Map();
    this.mutex = new Mutex();
    this.pendingFetches = new Map();
  }
  
  async getUser(userId) {
    // Check cache without locking
    if (this.userCache.has(userId)) {
      return this.userCache.get(userId);
    }
    
    // Check if there's already a pending fetch for this user
    if (this.pendingFetches.has(userId)) {
      return this.pendingFetches.get(userId);
    }
    
    // Create a promise for this fetch
    const fetchPromise = (async () => {
      // Acquire lock only for the check and update
      const release = await this.mutex.acquire();
      try {
        // Double-check to avoid race condition
        if (this.userCache.has(userId)) {
          return this.userCache.get(userId);
        }
        
        // Release lock during I/O operation
        release();
        
        // Perform slow operation without holding lock
        const user = await this.loadUserFromDatabase(userId);
        
        // Acquire lock again to update cache
        const releaseAgain = await this.mutex.acquire();
        try {
          this.userCache.set(userId, user);
          return user;
        } finally {
          releaseAgain();
        }
      } catch (error) {
        // Ensure lock is released on error
        release();
        throw error;
      } finally {
        // Remove from pending fetches
        this.pendingFetches.delete(userId);
      }
    })();
    
    // Store the promise for other requests to use
    this.pendingFetches.set(userId, fetchPromise);
    return fetchPromise;
  }
  
  async loadUserFromDatabase(userId) {
    // Same implementation as before
    await new Promise(resolve => setTimeout(resolve, 100));
    return { id: userId, name: `User ${userId}` };
  }
}

Excessive synchronization, such as synchronizing entire methods or using coarse-grained locks, can lead to thread contention and reduced throughput, especially when the synchronized block contains slow operations like I/O.

To minimize synchronization overhead:

Use the smallest possible synchronized blocks
Avoid performing slow operations while holding locks
Consider using concurrent collections (ConcurrentHashMap, etc.)
Use non-blocking algorithms when possible
Consider using read-write locks for read-heavy workloads
Use atomic variables for simple counters and flags
Consider lock-free data structures for high-contention scenarios
Use higher-level concurrency utilities (e.g., CompletableFuture in Java)
Profile your application to identify synchronization bottlenecks
Consider using optimistic concurrency control when appropriate

Improper Lock Granularity

// Anti-pattern: Coarse-grained locking
public class InventoryManager {
    private final Map<String, Integer> inventory = new HashMap<>();
    private final Object lock = new Object();
    
    public void updateStock(String productId, int quantity) {
        // Single lock for all products
        synchronized (lock) {
            Integer currentStock = inventory.getOrDefault(productId, 0);
            inventory.put(productId, currentStock + quantity);
        }
    }
    
    public int getStock(String productId) {
        // Single lock for all products
        synchronized (lock) {
            return inventory.getOrDefault(productId, 0);
        }
    }
}

// Better approach: Fine-grained locking
public class OptimizedInventoryManager {
    private final Map<String, ProductStock> inventory = new ConcurrentHashMap<>();
    
    public void updateStock(String productId, int quantity) {
        // Get or create product stock with its own lock
        ProductStock stock = inventory.computeIfAbsent(productId, 
            id -> new ProductStock());
        
        // Lock only the specific product
        synchronized (stock) {
            stock.quantity += quantity;
        }
    }
    
    public int getStock(String productId) {
        ProductStock stock = inventory.get(productId);
        if (stock == null) {
            return 0;
        }
        
        // Lock only the specific product
        synchronized (stock) {
            return stock.quantity;
        }
    }
    
    private static class ProductStock {
        private int quantity;
    }
}

// Anti-pattern: Coarse-grained locking in Node.js
class InventoryManager {
  constructor() {
    this.inventory = new Map();
    this.mutex = new Mutex(); // Using a mutex library
  }
  
  async updateStock(productId, quantity) {
    // Single lock for all products
    const release = await this.mutex.acquire();
    try {
      const currentStock = this.inventory.get(productId) || 0;
      this.inventory.set(productId, currentStock + quantity);
    } finally {
      release();
    }
  }
  
  async getStock(productId) {
    // Single lock for all products
    const release = await this.mutex.acquire();
    try {
      return this.inventory.get(productId) || 0;
    } finally {
      release();
    }
  }
}

// Better approach: Fine-grained locking
class OptimizedInventoryManager {
  constructor() {
    this.inventory = new Map();
    this.mutexes = new Map(); // Map of mutexes per product
  }
  
  async updateStock(productId, quantity) {
    // Get or create mutex for this product
    let mutex = this.mutexes.get(productId);
    if (!mutex) {
      mutex = new Mutex();
      this.mutexes.set(productId, mutex);
    }
    
    // Lock only the specific product
    const release = await mutex.acquire();
    try {
      const currentStock = this.inventory.get(productId) || 0;
      this.inventory.set(productId, currentStock + quantity);
    } finally {
      release();
    }
  }
  
  async getStock(productId) {
    // Get mutex for this product
    const mutex = this.mutexes.get(productId);
    if (!mutex) {
      return 0; // No mutex means no product yet
    }
    
    // Lock only the specific product
    const release = await mutex.acquire();
    try {
      return this.inventory.get(productId) || 0;
    } finally {
      release();
    }
  }
}

Improper lock granularity, such as using a single lock for an entire collection instead of individual locks for each element, can lead to unnecessary contention and reduced parallelism.

To optimize lock granularity:

Use fine-grained locks for independent resources
Consider the trade-off between lock overhead and contention
Use concurrent collections with built-in fine-grained locking
Consider striped locks for large collections
Be aware of lock acquisition order to prevent deadlocks
Use read-write locks for read-heavy workloads
Consider lock-free algorithms for high-contention scenarios
Profile your application to identify lock contention hotspots
Consider using optimistic concurrency control when appropriate
Be mindful of the overhead of managing many locks

Unnecessary Thread Synchronization

// Anti-pattern: Unnecessary synchronization
public class ConfigManager {
    private Map<String, String> config;
    
    public ConfigManager() {
        // Load configuration at startup
        config = loadConfiguration();
    }
    
    // Synchronized unnecessarily for read-only data
    public synchronized String getConfig(String key) {
        return config.get(key);
    }
    
    private Map<String, String> loadConfiguration() {
        // Load configuration from file or database
        Map<String, String> result = new HashMap<>();
        result.put("app.name", "MyApp");
        result.put("app.version", "1.0");
        return result;
    }
}

// Better approach: Immutable configuration
public class OptimizedConfigManager {
    private final Map<String, String> config;
    
    public OptimizedConfigManager() {
        // Load configuration and make it immutable
        Map<String, String> loadedConfig = loadConfiguration();
        config = Collections.unmodifiableMap(loadedConfig);
    }
    
    // No synchronization needed for immutable data
    public String getConfig(String key) {
        return config.get(key);
    }
    
    private Map<String, String> loadConfiguration() {
        // Same implementation as before
        Map<String, String> result = new HashMap<>();
        result.put("app.name", "MyApp");
        result.put("app.version", "1.0");
        return result;
    }
}

// Anti-pattern: Unnecessary locking in Node.js
class ConfigManager {
  constructor() {
    this.config = this.loadConfiguration();
    this.mutex = new Mutex(); // Using a mutex library
  }
  
  // Locked unnecessarily for read-only data
  async getConfig(key) {
    const release = await this.mutex.acquire();
    try {
      return this.config[key];
    } finally {
      release();
    }
  }
  
  loadConfiguration() {
    // Load configuration from file or database
    return {
      'app.name': 'MyApp',
      'app.version': '1.0'
    };
  }
}

// Better approach: Immutable configuration
class OptimizedConfigManager {
  constructor() {
    // Load configuration and freeze it
    this.config = Object.freeze(this.loadConfiguration());
  }
  
  // No locking needed for immutable data
  getConfig(key) {
    return this.config[key];
  }
  
  loadConfiguration() {
    // Same implementation as before
    return {
      'app.name': 'MyApp',
      'app.version': '1.0'
    };
  }
}

Unnecessary thread synchronization, such as synchronizing read-only data or using synchronization when thread-safety isn’t required, adds overhead without providing any benefit.

To avoid unnecessary synchronization:

Use immutable objects for shared read-only data
Consider thread-local storage for thread-specific data
Use final fields for one-time initialization
Consider copy-on-write collections for rarely modified data
Use volatile variables for simple flags without compound operations
Consider using atomic variables for simple counters
Be aware of the thread-safety requirements of your application
Use synchronization only when necessary for thread safety
Consider using concurrent collections with built-in thread safety
Profile your application to identify unnecessary synchronization

Inefficient Reader-Writer Patterns

// Anti-pattern: Using synchronized for read-heavy workloads
public class ProductCatalog {
    private final Map<String, Product> products = new HashMap<>();
    private final Object lock = new Object();
    
    public Product getProduct(String productId) {
        synchronized (lock) {
            return products.get(productId);
        }
    }
    
    public void addProduct(String productId, Product product) {
        synchronized (lock) {
            products.put(productId, product);
        }
    }
    
    public List<Product> searchProducts(String keyword) {
        List<Product> results = new ArrayList<>();
        synchronized (lock) {
            for (Product product : products.values()) {
                if (product.getName().contains(keyword)) {
                    results.add(product);
                }
            }
        }
        return results;
    }
}

// Better approach: Using ReadWriteLock
public class OptimizedProductCatalog {
    private final Map<String, Product> products = new HashMap<>();
    private final ReadWriteLock rwLock = new ReentrantReadWriteLock();
    private final Lock readLock = rwLock.readLock();
    private final Lock writeLock = rwLock.writeLock();
    
    public Product getProduct(String productId) {
        readLock.lock();
        try {
            return products.get(productId);
        } finally {
            readLock.unlock();
        }
    }
    
    public void addProduct(String productId, Product product) {
        writeLock.lock();
        try {
            products.put(productId, product);
        } finally {
            writeLock.unlock();
        }
    }
    
    public List<Product> searchProducts(String keyword) {
        List<Product> results = new ArrayList<>();
        readLock.lock();
        try {
            for (Product product : products.values()) {
                if (product.getName().contains(keyword)) {
                    results.add(product);
                }
            }
        } finally {
            readLock.unlock();
        }
        return results;
    }
}

// Anti-pattern: Using single mutex for read-heavy workloads
class ProductCatalog {
  constructor() {
    this.products = new Map();
    this.mutex = new Mutex(); // Using a mutex library
  }
  
  async getProduct(productId) {
    const release = await this.mutex.acquire();
    try {
      return this.products.get(productId);
    } finally {
      release();
    }
  }
  
  async addProduct(productId, product) {
    const release = await this.mutex.acquire();
    try {
      this.products.set(productId, product);
    } finally {
      release();
    }
  }
  
  async searchProducts(keyword) {
    const results = [];
    const release = await this.mutex.acquire();
    try {
      for (const product of this.products.values()) {
        if (product.name.includes(keyword)) {
          results.push(product);
        }
      }
    } finally {
      release();
    }
    return results;
  }
}

// Better approach: Using read-write lock pattern
class OptimizedProductCatalog {
  constructor() {
    this.products = new Map();
    this.rwLock = new ReadWriteLock(); // Using a RW lock library
  }
  
  async getProduct(productId) {
    return this.rwLock.readLock(async () => {
      return this.products.get(productId);
    });
  }
  
  async addProduct(productId, product) {
    return this.rwLock.writeLock(async () => {
      this.products.set(productId, product);
    });
  }
  
  async searchProducts(keyword) {
    return this.rwLock.readLock(async () => {
      const results = [];
      for (const product of this.products.values()) {
        if (product.name.includes(keyword)) {
          results.push(product);
        }
      }
      return results;
    });
  }
}

Inefficient reader-writer patterns, such as using exclusive locks for read-heavy workloads, can lead to unnecessary contention and reduced throughput.

To optimize reader-writer patterns:

Use ReadWriteLock for read-heavy workloads
Consider concurrent collections with built-in reader-writer semantics
Use copy-on-write collections for rarely modified data
Consider snapshot isolation for read-heavy workloads
Be aware of the overhead of reader-writer locks
Consider optimistic concurrency control for low-contention scenarios
Use appropriate lock timeouts to prevent deadlocks
Profile your application to identify reader-writer contention
Consider using specialized concurrent data structures
Be mindful of the trade-offs between consistency and performance

Inefficient Thread Pool Configuration

// Anti-pattern: Inefficient thread pool configuration
public class TaskProcessor {
    // Fixed thread pool with too many threads
    private final ExecutorService executor = Executors.newFixedThreadPool(1000);
    
    public void processTask(Runnable task) {
        executor.submit(task);
    }
    
    // Method to shut down the executor
    public void shutdown() {
        executor.shutdown();
        try {
            if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
                executor.shutdownNow();
            }
        } catch (InterruptedException e) {
            executor.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}

// Better approach: Properly sized thread pool
public class OptimizedTaskProcessor {
    // Thread pool sized based on available processors
    private final ExecutorService executor;
    
    public OptimizedTaskProcessor() {
        int coreCount = Runtime.getRuntime().availableProcessors();
        // For CPU-bound tasks: use core count
        // For I/O-bound tasks: use more threads (e.g., core count * 2)
        executor = new ThreadPoolExecutor(
            coreCount,                  // Core pool size
            coreCount * 2,              // Max pool size
            60L, TimeUnit.SECONDS,      // Keep-alive time
            new LinkedBlockingQueue<>(1000), // Work queue
            new ThreadPoolExecutor.CallerRunsPolicy() // Rejection policy
        );
    }
    
    public void processTask(Runnable task) {
        executor.submit(task);
    }
    
    // Same shutdown method as before
    public void shutdown() {
        executor.shutdown();
        try {
            if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
                executor.shutdownNow();
            }
        } catch (InterruptedException e) {
            executor.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}

// Anti-pattern: Inefficient worker pool in Node.js
const { Worker, isMainThread, parentPort, workerData } = require('worker_threads');

class TaskProcessor {
  constructor() {
    // Creating too many workers
    this.workers = [];
    for (let i = 0; i < 1000; i++) {
      this.workers.push(new Worker('./worker.js'));
    }
    this.nextWorker = 0;
  }
  
  processTask(task) {
    // Round-robin assignment to workers
    const worker = this.workers[this.nextWorker];
    this.nextWorker = (this.nextWorker + 1) % this.workers.length;
    
    return new Promise((resolve, reject) => {
      worker.once('message', resolve);
      worker.once('error', reject);
      worker.postMessage(task);
    });
  }
  
  shutdown() {
    for (const worker of this.workers) {
      worker.terminate();
    }
  }
}

// Better approach: Properly sized worker pool
class OptimizedTaskProcessor {
  constructor() {
    // Create workers based on CPU cores
    const cpuCount = require('os').cpus().length;
    this.workers = [];
    for (let i = 0; i < cpuCount; i++) {
      this.workers.push(new Worker('./worker.js'));
    }
    this.nextWorker = 0;
  }
  
  processTask(task) {
    // Same implementation as before
    const worker = this.workers[this.nextWorker];
    this.nextWorker = (this.nextWorker + 1) % this.workers.length;
    
    return new Promise((resolve, reject) => {
      worker.once('message', resolve);
      worker.once('error', reject);
      worker.postMessage(task);
    });
  }
  
  shutdown() {
    for (const worker of this.workers) {
      worker.terminate();
    }
  }
}

Inefficient thread pool configuration, such as creating too many threads or using inappropriate queue sizes, can lead to resource exhaustion, increased context switching, and reduced performance.

To optimize thread pool configuration:

Size thread pools based on available processors and workload type
Use fewer threads for CPU-bound tasks (typically core count)
Use more threads for I/O-bound tasks (typically core count * N)
Consider using different thread pools for different types of tasks
Configure appropriate work queue sizes
Implement proper rejection policies
Monitor thread pool metrics (queue size, active threads, etc.)
Consider using a thread pool with dynamic sizing
Be mindful of thread pool starvation and deadlocks
Profile your application to identify optimal thread pool configuration

Deadlock-Prone Lock Ordering

// Anti-pattern: Inconsistent lock ordering leading to deadlocks
public class AccountManager {
    public void transfer(Account from, Account to, double amount) {
        // Lock accounts in arbitrary order (based on parameter order)
        synchronized (from) {
            synchronized (to) {
                if (from.getBalance() >= amount) {
                    from.withdraw(amount);
                    to.deposit(amount);
                }
            }
        }
    }
}

// Example of deadlock:
// Thread 1: transfer(accountA, accountB, 100)
// Thread 2: transfer(accountB, accountA, 50)
// Thread 1 locks accountA, Thread 2 locks accountB, both wait for the other's lock

// Better approach: Consistent lock ordering
public class SafeAccountManager {
    public void transfer(Account from, Account to, double amount) {
        // Determine a consistent locking order based on account ID
        Account firstLock = from.getId() < to.getId() ? from : to;
        Account secondLock = from.getId() < to.getId() ? to : from;
        
        // Always acquire locks in the same order
        synchronized (firstLock) {
            synchronized (secondLock) {
                // If we need to swap the order of operations based on which account is which
                if (from.getBalance() >= amount) {
                    from.withdraw(amount);
                    to.deposit(amount);
                }
            }
        }
    }
}

// Anti-pattern: Inconsistent lock ordering in Node.js
class AccountManager {
  async transfer(from, to, amount) {
    // Lock accounts in arbitrary order (based on parameter order)
    await from.lock.acquire();
    try {
      await to.lock.acquire();
      try {
        if (from.balance >= amount) {
          from.balance -= amount;
          to.balance += amount;
        }
      } finally {
        to.lock.release();
      }
    } finally {
      from.lock.release();
    }
  }
}

// Better approach: Consistent lock ordering
class SafeAccountManager {
  async transfer(from, to, amount) {
    // Determine a consistent locking order based on account ID
    const firstLock = from.id < to.id ? from : to;
    const secondLock = from.id < to.id ? to : from;
    
    // Always acquire locks in the same order
    await firstLock.lock.acquire();
    try {
      await secondLock.lock.acquire();
      try {
        // If we need to swap the order of operations based on which account is which
        if (from.balance >= amount) {
          from.balance -= amount;
          to.balance += amount;
        }
      } finally {
        secondLock.lock.release();
      }
    } finally {
      firstLock.lock.release();
    }
  }
}

Deadlock-prone lock ordering, such as acquiring locks in an inconsistent order across different threads, can lead to deadlocks where threads are permanently blocked waiting for locks held by each other.

To prevent deadlocks through proper lock ordering:

Always acquire locks in a consistent, predetermined order
Use a natural ordering (e.g., based on object IDs) for lock acquisition
Consider using tryLock with timeout to detect and recover from potential deadlocks
Minimize the number of locks held simultaneously
Keep critical sections as small as possible
Consider using higher-level concurrency utilities that handle lock ordering
Document the locking strategy and order for complex systems
Use deadlock detection tools during development and testing
Consider using lock hierarchies to enforce ordering
Implement proper error handling and recovery mechanisms

Busy Waiting

// Anti-pattern: Busy waiting
public class TaskCoordinator {
    private volatile boolean isTaskComplete = false;
    
    public void waitForTask() {
        // Continuously check flag, consuming CPU
        while (!isTaskComplete) {
            // Do nothing, just spin
        }
        
        // Process completed task
        processCompletedTask();
    }
    
    public void completeTask() {
        // Set flag when task is complete
        isTaskComplete = true;
    }
    
    private void processCompletedTask() {
        // Process the completed task
    }
}

// Better approach: Using proper synchronization primitives
public class OptimizedTaskCoordinator {
    private final Object lock = new Object();
    private boolean isTaskComplete = false;
    
    public void waitForTask() {
        synchronized (lock) {
            // Wait until notified, releasing CPU
            while (!isTaskComplete) {
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    return;
                }
            }
        }
        
        // Process completed task
        processCompletedTask();
    }
    
    public void completeTask() {
        synchronized (lock) {
            // Set flag and notify waiting threads
            isTaskComplete = true;
            lock.notifyAll();
        }
    }
    
    private void processCompletedTask() {
        // Process the completed task
    }
}

// Even better: Using higher-level concurrency utilities
public class ModernTaskCoordinator {
    private final CountDownLatch taskLatch = new CountDownLatch(1);
    
    public void waitForTask() {
        try {
            // Wait for latch to count down to zero
            taskLatch.await();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            return;
        }
        
        // Process completed task
        processCompletedTask();
    }
    
    public void completeTask() {
        // Count down the latch to release waiting threads
        taskLatch.countDown();
    }
    
    private void processCompletedTask() {
        // Process the completed task
    }
}

// Anti-pattern: Busy waiting in JavaScript
class TaskCoordinator {
  constructor() {
    this.isTaskComplete = false;
  }
  
  async waitForTask() {
    // Continuously check flag, consuming CPU
    while (!this.isTaskComplete) {
      // Small delay to prevent 100% CPU usage, but still inefficient
      await new Promise(resolve => setTimeout(resolve, 10));
    }
    
    // Process completed task
    this.processCompletedTask();
  }
  
  completeTask() {
    // Set flag when task is complete
    this.isTaskComplete = true;
  }
  
  processCompletedTask() {
    // Process the completed task
  }
}

// Better approach: Using proper async patterns
class OptimizedTaskCoordinator {
  constructor() {
    this.taskPromise = new Promise(resolve => {
      this.resolveTask = resolve;
    });
  }
  
  async waitForTask() {
    // Efficiently wait for promise resolution
    await this.taskPromise;
    
    // Process completed task
    this.processCompletedTask();
  }
  
  completeTask() {
    // Resolve the promise to notify waiting functions
    this.resolveTask();
  }
  
  processCompletedTask() {
    // Process the completed task
  }
}

Busy waiting, or spinning, is a synchronization anti-pattern where a thread continuously checks a condition without releasing the CPU, wasting computational resources and potentially causing performance issues.

To avoid busy waiting:

Use proper synchronization primitives (wait/notify, semaphores, etc.)
Consider using higher-level concurrency utilities (CountDownLatch, CyclicBarrier, etc.)
Use blocking queues for producer-consumer patterns
Implement proper backoff strategies when polling is necessary
Consider using event-driven architectures
Use proper asynchronous programming patterns
Be mindful of CPU usage in waiting threads
Consider using timeouts to prevent indefinite waiting
Use proper interrupt handling for cancellation
In JavaScript, use promises and async/await for asynchronous coordination

Synchronized Method Cascades

// Anti-pattern: Synchronized method cascades
public class UserService {
    private final UserRepository userRepository = new UserRepository();
    private final AuditService auditService = new AuditService();
    
    public synchronized User createUser(String username, String email) {
        // Check if user exists
        if (userRepository.findByUsername(username) != null) {
            throw new IllegalArgumentException("Username already exists");
        }
        
        // Create user
        User user = new User(username, email);
        userRepository.save(user);
        
        // Audit the action
        auditService.logUserCreation(user);
        
        return user;
    }
}

public class UserRepository {
    private final Map<String, User> users = new HashMap<>();
    
    public synchronized User findByUsername(String username) {
        return users.get(username);
    }
    
    public synchronized void save(User user) {
        users.put(user.getUsername(), user);
    }
}

public class AuditService {
    private final List<AuditLog> logs = new ArrayList<>();
    
    public synchronized void logUserCreation(User user) {
        logs.add(new AuditLog("USER_CREATED", user.getUsername()));
    }
}

// Better approach: Minimizing lock scope and avoiding cascades
public class OptimizedUserService {
    private final OptimizedUserRepository userRepository = new OptimizedUserRepository();
    private final OptimizedAuditService auditService = new OptimizedAuditService();
    
    public User createUser(String username, String email) {
        // Check if user exists
        User existingUser = userRepository.findByUsername(username);
        if (existingUser != null) {
            throw new IllegalArgumentException("Username already exists");
        }
        
        // Create user
        User user = new User(username, email);
        userRepository.save(user);
        
        // Audit the action
        auditService.logUserCreation(user);
        
        return user;
    }
}

public class OptimizedUserRepository {
    private final ConcurrentHashMap<String, User> users = new ConcurrentHashMap<>();
    
    public User findByUsername(String username) {
        return users.get(username);
    }
    
    public void save(User user) {
        users.put(user.getUsername(), user);
    }
}

public class OptimizedAuditService {
    private final Queue<AuditLog> logs = new ConcurrentLinkedQueue<>();
    
    public void logUserCreation(User user) {
        logs.add(new AuditLog("USER_CREATED", user.getUsername()));
    }
}

// Anti-pattern: Lock cascades in Node.js
class UserService {
  constructor() {
    this.userRepository = new UserRepository();
    this.auditService = new AuditService();
    this.mutex = new Mutex();
  }
  
  async createUser(username, email) {
    const release = await this.mutex.acquire();
    try {
      // Check if user exists
      const existingUser = await this.userRepository.findByUsername(username);
      if (existingUser) {
        throw new Error("Username already exists");
      }
      
      // Create user
      const user = { username, email };
      await this.userRepository.save(user);
      
      // Audit the action
      await this.auditService.logUserCreation(user);
      
      return user;
    } finally {
      release();
    }
  }
}

class UserRepository {
  constructor() {
    this.users = new Map();
    this.mutex = new Mutex();
  }
  
  async findByUsername(username) {
    const release = await this.mutex.acquire();
    try {
      return this.users.get(username);
    } finally {
      release();
    }
  }
  
  async save(user) {
    const release = await this.mutex.acquire();
    try {
      this.users.set(user.username, user);
    } finally {
      release();
    }
  }
}

class AuditService {
  constructor() {
    this.logs = [];
    this.mutex = new Mutex();
  }
  
  async logUserCreation(user) {
    const release = await this.mutex.acquire();
    try {
      this.logs.push({ action: "USER_CREATED", username: user.username });
    } finally {
      release();
    }
  }
}

// Better approach: Minimizing lock scope and avoiding cascades
class OptimizedUserService {
  constructor() {
    this.userRepository = new OptimizedUserRepository();
    this.auditService = new OptimizedAuditService();
  }
  
  async createUser(username, email) {
    // Check if user exists
    const existingUser = await this.userRepository.findByUsername(username);
    if (existingUser) {
      throw new Error("Username already exists");
    }
    
    // Create user
    const user = { username, email };
    await this.userRepository.save(user);
    
    // Audit the action
    await this.auditService.logUserCreation(user);
    
    return user;
  }
}

class OptimizedUserRepository {
  constructor() {
    // Using a concurrent map implementation or database would be better in practice
    this.users = new Map();
  }
  
  async findByUsername(username) {
    return this.users.get(username);
  }
  
  async save(user) {
    this.users.set(user.username, user);
  }
}

class OptimizedAuditService {
  constructor() {
    // Using a concurrent queue or database would be better in practice
    this.logs = [];
    this.mutex = new Mutex(); // Still need synchronization for array operations
  }
  
  async logUserCreation(user) {
    const release = await this.mutex.acquire();
    try {
      this.logs.push({ action: "USER_CREATED", username: user.username });
    } finally {
      release();
    }
  }
}

Synchronized method cascades occur when synchronized methods call other synchronized methods, potentially leading to nested locks, increased contention, and reduced concurrency.

To avoid synchronized method cascades:

Minimize the scope of synchronization
Use concurrent collections instead of synchronized methods
Consider using atomic operations for simple state changes
Avoid calling synchronized methods from within synchronized blocks
Break down large synchronized methods into smaller, non-synchronized ones
Use lock striping to reduce contention
Consider using optimistic concurrency control
Be aware of the locking hierarchy in your application
Document synchronization dependencies
Profile your application to identify synchronization bottlenecks

Contended Locks

// Anti-pattern: Highly contended locks
public class GlobalCounter {
    private int count = 0;
    private final Object lock = new Object();
    
    public void increment() {
        synchronized (lock) {
            count++;
        }
    }
    
    public int getCount() {
        synchronized (lock) {
            return count;
        }
    }
}

// Usage that leads to contention
public class ContentionExample {
    private final GlobalCounter counter = new GlobalCounter();
    
    public void runHighContentionWorkload() {
        // Create many threads that all increment the same counter
        ExecutorService executor = Executors.newFixedThreadPool(100);
        for (int i = 0; i < 1000000; i++) {
            executor.submit(() -> counter.increment());
        }
        executor.shutdown();
    }
}

// Better approach: Using atomic variables
public class AtomicCounter {
    private final AtomicInteger count = new AtomicInteger(0);
    
    public void increment() {
        count.incrementAndGet();
    }
    
    public int getCount() {
        return count.get();
    }
}

// Even better: Using striped locks for different counters
public class StripedCounter {
    private static final int STRIPE_COUNT = 16;
    private final AtomicInteger[] counters = new AtomicInteger[STRIPE_COUNT];
    
    public StripedCounter() {
        for (int i = 0; i < STRIPE_COUNT; i++) {
            counters[i] = new AtomicInteger(0);
        }
    }
    
    public void increment(Object key) {
        // Use the key's hash to determine which stripe to use
        int stripe = Math.abs(key.hashCode() % STRIPE_COUNT);
        counters[stripe].incrementAndGet();
    }
    
    public int getCount() {
        int sum = 0;
        for (AtomicInteger counter : counters) {
            sum += counter.get();
        }
        return sum;
    }
}

// Anti-pattern: Highly contended locks in Node.js
class GlobalCounter {
  constructor() {
    this.count = 0;
    this.mutex = new Mutex();
  }
  
  async increment() {
    const release = await this.mutex.acquire();
    try {
      this.count++;
    } finally {
      release();
    }
  }
  
  async getCount() {
    const release = await this.mutex.acquire();
    try {
      return this.count;
    } finally {
      release();
    }
  }
}

// Usage that leads to contention
async function runHighContentionWorkload() {
  const counter = new GlobalCounter();
  const promises = [];
  
  // Create many promises that all increment the same counter
  for (let i = 0; i < 10000; i++) {
    promises.push(counter.increment());
  }
  
  await Promise.all(promises);
}

// Better approach: Using atomic operations
class AtomicCounter {
  constructor() {
    this.count = 0;
    this.mutex = new Mutex();
  }
  
  // In JavaScript, we can use a more efficient approach with batching
  async increment(batchSize = 1) {
    const release = await this.mutex.acquire();
    try {
      this.count += batchSize;
    } finally {
      release();
    }
  }
  
  async getCount() {
    // No need for lock for a simple read in JS
    return this.count;
  }
}

// Even better: Using striped locks
class StripedCounter {
  constructor(stripeCount = 16) {
    this.stripeCount = stripeCount;
    this.counters = Array(stripeCount).fill(0);
    this.mutexes = Array(stripeCount).fill(null).map(() => new Mutex());
  }
  
  async increment(key) {
    // Use the key's hash to determine which stripe to use
    const stripe = Math.abs(this.hashCode(key) % this.stripeCount);
    const release = await this.mutexes[stripe].acquire();
    try {
      this.counters[stripe]++;
    } finally {
      release();
    }
  }
  
  async getCount() {
    // Sum all counters (might not be perfectly accurate due to race conditions)
    return this.counters.reduce((sum, count) => sum + count, 0);
  }
  
  // Simple hash function for JavaScript
  hashCode(obj) {
    const str = String(obj);
    let hash = 0;
    for (let i = 0; i < str.length; i++) {
      hash = ((hash << 5) - hash) + str.charCodeAt(i);
      hash |= 0; // Convert to 32-bit integer
    }
    return hash;
  }
}

Contended locks occur when many threads compete for the same lock, leading to significant thread blocking, context switching overhead, and reduced throughput.

To reduce lock contention:

Use atomic variables for simple counters and flags
Implement lock striping to distribute contention
Consider using concurrent collections with built-in concurrency control
Reduce the scope and duration of synchronized blocks
Use thread-local variables for thread-specific data
Consider using optimistic concurrency control
Batch operations to reduce lock acquisition frequency
Use non-blocking algorithms when possible
Consider using specialized concurrent data structures
Profile your application to identify contention hotspots

Synchronization Best Practices Checklist

Synchronization Best Practices Checklist:

1. Minimize Synchronization Scope
   - Keep synchronized blocks as small as possible
   - Avoid performing I/O or expensive operations while holding locks
   - Use fine-grained locking for independent resources
   - Consider using read-write locks for read-heavy workloads
   - Use immutable objects for shared read-only data

2. Choose Appropriate Synchronization Mechanisms
   - Use concurrent collections when possible (ConcurrentHashMap, etc.)
   - Use atomic variables for simple counters and flags
   - Consider lock-free algorithms for high-contention scenarios
   - Use higher-level concurrency utilities (CountDownLatch, etc.)
   - Choose the right thread pool configuration for your workload

3. Prevent Deadlocks
   - Acquire locks in a consistent, predetermined order
   - Use tryLock with timeout to detect and recover from potential deadlocks
   - Minimize the number of locks held simultaneously
   - Document locking strategies and dependencies
   - Use deadlock detection tools during development

4. Optimize for Contention
   - Implement lock striping for high-contention resources
   - Consider optimistic concurrency control for low-contention scenarios
   - Batch operations to reduce lock acquisition frequency
   - Use thread-local storage for thread-specific data
   - Monitor and profile lock contention in production

5. Follow Concurrency Best Practices
   - Prefer immutable objects for shared data
   - Document thread-safety guarantees for classes and methods
   - Use final fields for thread safety
   - Consider the memory model implications of your code
   - Test thoroughly for concurrency issues

Proper thread synchronization is a balancing act between ensuring data consistency and maintaining good performance. By following best practices, you can minimize synchronization overhead while still ensuring thread safety.

Key principles for efficient synchronization:

Synchronize only when necessary
Minimize the scope and duration of synchronization
Choose the right synchronization mechanism for each use case
Be aware of potential deadlocks and contention issues
Use higher-level concurrency utilities when possible
Profile and measure synchronization performance
Document thread-safety guarantees and requirements
Test thoroughly for concurrency issues
Consider the trade-offs between consistency and performance
Stay updated on modern concurrency patterns and libraries

Introduction

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Synchronization Issues