TPS00-J. Use thread pools to enable graceful degradation of service during traffic bursts

Many programs must address the problem of handling a series of incoming requests. One simple concurrency strategy is the thread-per-message design pattern, which uses a new thread for each request [Lea 2000a]. This pattern is generally preferred over sequential executions of time-consuming, I/O-bound, session-based, or isolated tasks.

However, the pattern also introduces overheads not seen in sequential execution, including the time and resources required for thread creation and scheduling, for task processing, for resource allocation and deallocation, and for frequent context switching [Lea 2000a]. Furthermore, an attacker can cause a denial of service (DoS) by overwhelming the system with too many requests all at once, causing the system to become unresponsive rather than degrading gracefully. From a safety perspective, one component can exhaust all resources because of an intermittent error, consequently starving all other components.

Thread pools allow a system to limit the maximum number of simultaneous requests that it processes to a number that it can comfortably serve rather than terminating all services when presented with a deluge of requests. Thread pools overcome these issues by controlling the maximum number of worker threads that can execute concurrently. Each object that supports thread pools accepts a Runnable or Callable<T> task and stores it in a temporary queue until resources become available. Additionally, thread life-cycle management overhead is minimized because the threads in a thread pool can be reused and can be efficiently added to or removed from the pool.

Programs that use multiple threads to service requests should — and programs that may be subjected to DoS attacks must — ensure graceful degradation of service during traffic bursts. Use of thread pools is one acceptable approach to meeting this requirement.

Noncompliant Code Example (Thread-Per-Message)

This noncompliant code example demonstrates the thread-per-message design pattern. The RequestHandler class provides a public static factory method so that callers can obtain a RequestHandler instance. The handleRequest() method is subsequently invoked to handle each request in its own thread.

class Helper {
  public void handle(Socket socket) {
    //...
  }
}

final class RequestHandler {
  private final Helper helper = new Helper();
  private final ServerSocket server;

  private RequestHandler(int port) throws IOException {
    server = new ServerSocket(port);
  }

  public static RequestHandler newInstance() throws IOException {
    return new RequestHandler(0); // Selects next available port
  }

  public void handleRequest() {
    new Thread(new Runnable() {
        public void run() {
          try {
            helper.handle(server.accept());
          } catch (IOException e) {
            // Forward to handler
          }
        }
    }).start();
  }

}

The thread-per-message strategy fails to provide graceful degradation of service. As threads are created, processing continues normally until some scarce resource is exhausted. For example, a system may allow only a limited number of open file descriptors even though additional threads can be created to serve requests. When the scarce resource is memory, the system may fail abruptly, resulting in a DoS.

Compliant Solution (Thread Pool)

This compliant solution uses a fixed thread pool that places a strict limit on the number of concurrently executing threads. Tasks submitted to the pool are stored in an internal queue. This prevents the system from being overwhelmed when attempting to respond to all incoming requests and allows it to degrade gracefully by serving a fixed maximum number of simultaneous clients [Tutorials 2008].

// class Helper remains unchanged

final class RequestHandler {
  private final Helper helper = new Helper();
  private final ServerSocket server;
  private final ExecutorService exec;

  private RequestHandler(int port, int poolSize) throws IOException {
    server = new ServerSocket(port);
    exec = Executors.newFixedThreadPool(poolSize);
  }

  public static RequestHandler newInstance(int poolSize) 
                                           throws IOException {
    return new RequestHandler(0, poolSize);
  }

  public void handleRequest() {
    Future<?> future = exec.submit(new Runnable() {
        @Override public void run() {
          try {
            helper.handle(server.accept());
          } catch (IOException e) {
            // Forward to handler
          }
        }
    });
  }
  // ... other methods such as shutting down the thread pool 
  // and task cancellation ...
}

According to the Java API documentation for the Executor interface [API 2006]

[The interface Executor is] an object that executes submitted Runnable tasks. This interface provides a way of decoupling task submission from the mechanics of how each task will be run, including details of thread use, scheduling, etc. An Executor is normally used instead of explicitly creating threads.

The ExecutorService interface used in this compliant solution derives from the java.util.concurrent.Executor interface. The ExecutorService.submit() method allows callers to obtain a Future<V> object. This object both encapsulates the as-yet-unknown result of an asynchronous computation and also enables callers to perform additional functions such as task cancellation.

The choice of newFixedThreadPool is not always appropriate. Refer to the Java API documentation for guidance on choosing among the following methods to meet specific design requirements [API 2006]:

• newFixedThreadPool()
• newCachedThreadPool()
• newSingleThreadExecutor()
• newScheduledThreadPool()

Risk Assessment

Using simplistic concurrency primitives to process an unbounded number of requests could result in severe performance degradation, deadlock, or system resource exhaustion and DoS.

Related Guidelines

Bibliography