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From "Keith Turner (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-2027) ZooKeeperInstance.close() not freeing resources in multithreaded env
Date Mon, 16 Dec 2013 21:56:07 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-2027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13849772#comment-13849772
] 

Keith Turner commented on ACCUMULO-2027:
----------------------------------------

Multiple threads should be able to use the same connector w/ no problem.  I suspect this bug
could be triggered by background threads created by Accumulo client code (like batch scanner,
batch writer, conditional writer, and scanner created threads).  So it may not be limited
to the case where a user creates threads.  

> ZooKeeperInstance.close() not freeing resources in multithreaded env
> --------------------------------------------------------------------
>
>                 Key: ACCUMULO-2027
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2027
>             Project: Accumulo
>          Issue Type: Bug
>            Reporter: Keith Turner
>            Priority: Critical
>             Fix For: 1.4.5, 1.5.1, 1.6.0
>
>
> While looking at the changes related to ZooKeeperInstance.close() in the 1.4.5-SNAPSHOT
branch I noticed there were race conditions where resources were not properly released.  
One type of race condition is where a thread is between a closed check in ZooKeeperInstance
and calling a ZooCache method when ZooKeeperInstance.close() is called.  The following is
an example situation
>  # Thread 1 uses ZooKeeperInstance1 to get a zoocache.
>  # Thread 2 calls close() on ZooKeeperInstnce1 which calls close() on zoocache
>  # Thread 1 uses the zoocache it has reference to, causing a new zookeeper connection
to be created.
> Below is an example program that will trigger this behavior.   For me this little example
program reliably shows a connected zookeeper after all of the threads die.  If I use 0 threads
it will show a closed zookeeper connection at the end.
> {code:java}
>  static class WriteTask implements Runnable {
>     private BatchWriter writer;
>     private Random rand;
>     WriteTask(Connector conn) throws TableNotFoundException {
>       rand = new Random();
>       writer = conn.createBatchWriter("foo5", 10000000, 30000, 1);
>     }
>     @Override
>     public void run() {
>       try {
>         while (true) {
>           Mutation m1 = new Mutation(String.format("%06d", rand.nextInt(1000000)));
>           m1.put(String.format("%06d", rand.nextInt(100)), String.format("%06d", rand.nextInt(100)),
String.format("%06d", rand.nextInt(1000000)));
>           writer.addMutation(m1);
>           writer.flush();
>         }
>       } catch (Exception e) {
>         System.out.println(e.getMessage());
>       }
>     }
>   }
>   static class ReadTask implements Runnable {
>     private Scanner scanner;
>     ReadTask(Connector conn) throws TableNotFoundException {
>       scanner = conn.createScanner("foo5", new Authorizations());
>     }
>     @Override
>     public void run() {
>       try {
>         while (true) {
>           for (Entry<Key,Value> entry : scanner) {
>           }
>         }
>       } catch (Exception e) {
>         System.out.println(e.getMessage());
>       }
>     }
>   }
>   @Test(timeout = 30000)
>   public void test2() throws Exception {
>     ZooKeeperInstance zki = new ZooKeeperInstance(accumulo.getInstanceName(), accumulo.getZooKeepers());
>     Connector conn = zki.getConnector("root", "superSecret");
>     conn.tableOperations().create("foo5");
>     ArrayList<Thread> threads = new ArrayList<Thread>();
>     int numThreads = 10;
>     for (int i = 0; i < numThreads; i++) {
>       Thread t = new Thread(new WriteTask(conn));
>       t.start();
>       threads.add(t);
>     }
>     for (int i = 0; i < numThreads; i++) {
>       Thread t = new Thread(new ReadTask(conn));
>       t.start();
>       threads.add(t);
>     }
>     // let threads get spun up
>     Thread.sleep(1000);
>     ZooSession.printSessions();
>     zki.close();
>     // wait for the threads to die
>     for (Thread thread : threads) {
>       thread.join();
>     }
>     ZooSession.printSessions();
>   }
> {code}
> Below are some changes I made to ZooSession for debugging purposes.
> {noformat}
> diff --git a/src/core/src/main/java/org/apache/accumulo/core/zookeeper/ZooSession.java
b/src/core/src/main/java/org/apache/accumulo/core/zookeeper/ZooSession.java
> index b3db26f..475a21d 100644
> --- a/src/core/src/main/java/org/apache/accumulo/core/zookeeper/ZooSession.java
> +++ b/src/core/src/main/java/org/apache/accumulo/core/zookeeper/ZooSession.java
> @@ -20,6 +20,8 @@
>  import java.net.UnknownHostException;
>  import java.util.HashMap;
>  import java.util.Map;
> +import java.util.Map.Entry;
> +import java.util.Set;
>  
>  import org.apache.accumulo.core.util.UtilWaitThread;
>  import org.apache.log4j.Logger;
> @@ -29,7 +31,7 @@
>  import org.apache.zookeeper.ZooKeeper;
>  import org.apache.zookeeper.ZooKeeper.States;
>  
> -class ZooSession {
> +public class ZooSession {
>    
>    private static final Logger log = Logger.getLogger(ZooSession.class);
>    
> @@ -121,6 +123,8 @@
>      
>      ZooSessionInfo zsi = sessions.get(sessionKey);
>      if (zsi != null && zsi.zooKeeper.getState() == States.CLOSED) {
> +      System.out.println("Removing closed session ");
> +      new Exception().printStackTrace();
>        if (auth != null && sessions.get(readOnlySessionKey) == zsi)
>          sessions.remove(readOnlySessionKey);
>        zsi = null;
> @@ -137,4 +141,13 @@
>      }
>      return zsi.zooKeeper;
>    }
> +
> +  public static synchronized void printSessions() {
> +    Set<Entry<String,ZooSessionInfo>> es = sessions.entrySet();
> +
> +    for (Entry<String,ZooSessionInfo> entry : es) {
> +      System.out.println(entry.getKey() + " " + entry.getValue().zooKeeper.getState());
> +    }
> +  }
> +
>  }
> {noformat}
> With the above changes I will see an exception like the following when one of the race
conditions occurs.
> {noformat}
> Removing closed session 
> java.lang.Exception
> 	at org.apache.accumulo.core.zookeeper.ZooSession.getSession(ZooSession.java:127)
> 	at org.apache.accumulo.core.zookeeper.ZooReader.getSession(ZooReader.java:37)
> 	at org.apache.accumulo.core.zookeeper.ZooReader.getZooKeeper(ZooReader.java:41)
> 	at org.apache.accumulo.core.zookeeper.ZooCache.getZooKeeper(ZooCache.java:56)
> 	at org.apache.accumulo.core.zookeeper.ZooCache.retry(ZooCache.java:127)
> 	at org.apache.accumulo.core.zookeeper.ZooCache.get(ZooCache.java:233)
> 	at org.apache.accumulo.core.zookeeper.ZooCache.get(ZooCache.java:188)
> 	at org.apache.accumulo.core.client.ZooKeeperInstance.getInstanceID(ZooKeeperInstance.java:156)
> 	at org.apache.accumulo.core.client.impl.TabletLocator.getInstance(TabletLocator.java:96)
> 	at org.apache.accumulo.core.client.impl.ThriftScanner.scan(ThriftScanner.java:245)
> 	at org.apache.accumulo.core.client.impl.ScannerIterator$Reader.run(ScannerIterator.java:94)
> 	at org.apache.accumulo.core.client.impl.ScannerIterator.hasNext(ScannerIterator.java:176)
> 	at org.apache.accumulo.minicluster.MiniAccumuloClusterTest$ReadTask.run(MiniAccumuloClusterTest.java:109)
> 	at java.lang.Thread.run(Thread.java:662)
> {noformat}



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