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Many programs must address the problem of handling a series of incoming requests. The Thread-Per-Message design pattern is the simplest concurrency strategy wherein a new thread is created for each request \[[Lea 2000|AA. JavaBibliography#Lea References#Lea 00]\]. This pattern is generally preferred to sequential executions of time-consuming, I/O-bound, session-based, or isolated tasks. |
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However, this pattern also has several pitfalls, including overheads of thread-creation and scheduling, task processing, resource allocation and deallocation, and frequent context switching \[[Lea 2000|AA. JavaBibliography#Lea References#Lea 00]\]. Furthermore, an attacker can cause a denial of service by overwhelming the system with too many requests all at once. Instead of degrading gracefully, the system becomes unresponsive, causing a denial of service. From a safety perspective, one component can exhaust all resources because of some intermittent error, starving all other components. |
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This compliant solution uses a fixed-thread pool that places an upper bound 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 trying to respond to all incoming requests and allows it to degrade gracefully by serving a fixed number of clients at a particular time \[[Tutorials 2008|AA. Java References#TutorialsBibliography#Tutorials 08]\]. |
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// 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 ...
}
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According to the Java API documentation for the {{Executor}} interface \[[API 2006|AA. Java References#APIBibliography#API 06]\]: |
Wiki Markup \[The Interface {{Executor}} is\] 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.
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The choice of the unbounded {{newFixedThreadPool}} is not always optimal. Refer to the Java API documentation for choosing between the following to meet specific design requirements \[[API 2006|AA. Java References#APIBibliography#API 06]\]: |
newFixedThreadPool()newCachedThreadPool()newSingleThreadExecutor()newScheduledThreadPool()
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\[[API 2006|AA. JavaBibliography#API References#API 06]\] [Interface Executor|http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/Executor.html] \[[Lea 2000|AA. JavaBibliography#Lea References#Lea 00]\] Section 4.1.3 Thread-Per-Message and 4.1.4 Worker Threads \[[Tutorials 2008|AA. Java References#TutorialsBibliography#Tutorials 08]\] [Thread Pools|http://java.sun.com/docs/books/tutorial/essential/concurrency/pools.html] \[[Goetz 2006|AA. Java References#GoetzBibliography#Goetz 06]\] Chapter 8, Applying Thread Pools \[[MITRE 2009|AA. Java References#MITREBibliography#MITRE 09]\] [CWE ID 405|http://cwe.mitre.org/data/definitions/405.html] "Asymmetric Resource Consumption (Amplification)", [CWE ID 410|http://cwe.mitre.org/data/definitions/410.html] "Insufficient Resource Pool" |
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