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https://issues.apache.org/jira/browse/ACCUMULO-2027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13857606#comment-13857606
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William Slacum commented on ACCUMULO-2027:
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[~mdrob] there is a behavior change in that if there was some path that was
functioning, such as getting a connector, and another that was failing, such as
getting the master location, the ZKI would block until a timeout or exception
was thrown from the failing code path. Previously, each request could happen
concurrently because a given client/entity/thread/what-have-you didn't care
about what something else was doing. With the introduction of {{close()}},
clients to the ZKI are no longer shared-nothing so there isn't much we can do
beyond providing mutual exclusion.
With that said, it may be feasible to have a {{ReadWriteLock}} (or really, an
"open or closed") lock to alleviate performance woes. However, I'm apt to
choose the 4 line solution for code that, to me at least, doesn't appear to be
a set of performance critical code. [~elserj] and I talked about having
different implementations of connectors if the usage pattern of this class
doesn't meet all the needs of clients, but I'd would like to have data rather
than conjecture to go down that path.
> 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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