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From "ASF subversion and git services (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-2027) ZooKeeperInstance.close() not freeing resources in multithreaded env
Date Fri, 27 Dec 2013 17:11:56 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-2027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13857565#comment-13857565
] 

ASF subversion and git services commented on ACCUMULO-2027:
-----------------------------------------------------------

Commit 975e8c05e8d11f3848e6c800f4d2772026f6c3a3 in branch refs/heads/1.4.5-SNAPSHOT from Bill
Slacum
[ https://git-wip-us.apache.org/repos/asf?p=accumulo.git;h=975e8c0 ]

ACCUMULO-2027 Synchronized access to ZooKeeperInstance methods that mutated state

The usage pattern of a ZooKeeperInstance is to have many clients use the same
instance. Internall, the ZooKeeperInstances keeps a cache of information in a
ZooCache, which will manage connections to Zookeeper for getting and putting
data.

With the addition of close semantics on the ZKI, it is expected the ZKI will
free resources and halt operations for all clients. This requires ZKI method
calls to be linearizable, otherwise race conditions as described in
ACCUMULO-2027 will occur when a ZKI is simultaneously asked to close and
retrieve information.

This patch resolves that race condition by synchronizing all methods on a ZKI
that interact with an ZKI instance's ZooCache.


> 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
>            Assignee: William Slacum
>            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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