code.grnet.gr Git - pithos/blob - src/gr/ebs/gss/server/rest/TransactionHelper.java

   1 /*
   2  * Copyright 2009 Electronic Business Systems Ltd.
   3  *
   4  * This file is part of GSS.
   5  *
   6  * GSS is free software: you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation, either version 3 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * GSS is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License
  17  * along with GSS.  If not, see <http://www.gnu.org/licenses/>.
  18  */
  19 package gr.ebs.gss.server.rest;
  20
  21 import java.util.concurrent.Callable;
  22 import java.util.concurrent.ExecutionException;
  23 import java.util.concurrent.Executors;
  24 import java.util.concurrent.ScheduledExecutorService;
  25 import java.util.concurrent.ScheduledFuture;
  26 import java.util.concurrent.TimeUnit;
  27
  28 import javax.ejb.EJBTransactionRolledbackException;
  29
  30 import org.apache.commons.logging.Log;
  31 import org.apache.commons.logging.LogFactory;
  32
  33 /**
  34  * A helper class that provides a method for repeatedly trying a transaction
  35  * that is rolled back due to optimistic locking exceptions.
  36  *
  37  * @author  past
  38  */
  39 public class TransactionHelper<T> {
  40         /**
  41          * The logger.
  42          */
  43         private static Log logger = LogFactory.getLog(TransactionHelper.class);
  44
  45         /**
  46          * Number of times to retry a transaction that was rolled back due to
  47          * optimistic locking.
  48          */
  49         private static final int TRANSACTION_RETRIES = 10;
  50
  51         /**
  52          * The minimum retry timeout in milliseconds.
  53          */
  54         private static final int MIN_TIMEOUT = 100;
  55
  56         /**
  57          * Execute the supplied command until it completes, ignoring transaction
  58          * rollbacks. Try at least TRANSACTION_RETRIES times before giving up,
  59          * each time waiting a random amount of time, using an exponential
  60          * backoff scheme. See http://en.wikipedia.org/wiki/Exponential_backoff
  61          * for the basic idea.
  62          *
  63          * @param command the command to execute
  64          * @return the value returned by the command
  65          * @throws Exception any other exception thrown by the command
  66          */
  67         public T tryExecute(final Callable<T> command) throws Exception {
  68                 T returnValue = null;
  69                 // Schedule a Future task to call the command after delay milliseconds.
  70                 int delay = 0;
  71                 ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
  72                 for (int i = 0; i < TRANSACTION_RETRIES; i++) {
  73                         final int retry = i;
  74                         ScheduledFuture<T> future = executor.schedule(new Callable<T>() {
  75
  76                                 @Override
  77                                 public T call() throws Exception {
  78                                         return command.call();
  79                                 }
  80                         }, delay, TimeUnit.MILLISECONDS);
  81
  82                         try {
  83                                 returnValue = future.get();
  84                                 break;
  85                         } catch (ExecutionException e) {
  86                                 Throwable cause = e.getCause();
  87                                 if (!(cause instanceof EJBTransactionRolledbackException) ||
  88                                                         retry == TRANSACTION_RETRIES - 1) {
  89                                         executor.shutdownNow();
  90                                         throw new Exception(cause);
  91                                 }
  92                                 delay = MIN_TIMEOUT + (int) (MIN_TIMEOUT * Math.random() * (i + 1));
  93                                 String origCause = cause.getCause() == null ?
  94                                                         cause.getClass().getName() :
  95                                                         cause.getCause().getClass().getName();
  96                                 logger.info("Transaction retry #" + (i+1) + " scheduled in " + delay +
  97                                                         " msec due to " + origCause);
  98                         }
  99
 100                 }
 101                 executor.shutdownNow();
 102                 return returnValue;
 103         }
 104 }