Java server framework to listen to PostgreSQL NOTIFY statements

Question

I need to write a server which listens to PostgreSQL NOTIFY statements and considers each notification as a request to serve (actually, more like a task to process). My main requirements are:

1) A mechanism to poll on PGConnection (Ideally this would be a listener, but in the PgJDBC implementation, we are required to poll for pending notifications. Reference)

2) Execute a callback based on the "request" (using channel name in the NOTIFY notification), on a separate thread.

3) Has thread management stuff built in. (create/delete threads when a task is processed/finished, put on a queue when too many tasks being concurrently processed etc.)

Requirements 1 and 2 are something which are easy for me to implement myself. But I would prefer not to write thread management myself.

Is there an existing framework meeting this requirements? An added advantage would be if the framework automatically generates request statistics.

araqnid · Accepted Answer · 2011-12-27 16:19:11Z

To be honest, requirement 3 could probably be easily satistied just using standard ExecutorService implementations from Executors, which will allow you to, for example, get a fixed-size thread pool and submit work to them in the form of Runnable or Callable implementations. They will deal with the gory details of creating threads up to the limit etc.. You can then have your listener implement a thin layer of Runnable to collect statistics etc.

Something like:

private final ExecutorService threadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
private final NotificationCallback callback;
private int waiting, executing, succeeded, failed;

public void pollAndDispatch() {
   Notification notification;
   while ((notification = pollDatabase()) != null) {
      final Notification ourNotification = notification;
      incrementWaitingCount();
      threadPool.submit(new Runnable() {
         public void run() {
           waitingToExecuting();
           try {
             callback.processNotification(ourNotification);
             executionCompleted();
           } catch (Exception e) {
             executionFailed();
             LOG.error("Exeception thrown while processing notification: " + ourNotification, e);
           }
         }
      });
   }
}
// check PGconn for notification and return it, or null if none received
protected Notification pollDatabase() { ... }
// maintain statistics
private synchronized void incrementWaitingCount() { ++waiting; }
private synchronized void waitingToExecuting() { --waiting; ++executing; }
private synchronized void executionCompleted() { --executing; ++succeeded; }
private synchronized void executionFailed() { --executing; ++failed; }

If you want to be fancy, put the notifications onto a JMS queue and use its infrastructure to listen for new items and process them.

I looked around and writing it myself looked like a good idea. Thanks for the reply, and code snippet was really helpful.

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