Yup. This would be really nice to have. More examples and documentaiton for them, would be really helpful for our users.
Thanks mahadev On 11/15/10 12:54 PM, "Patrick Hunt" <ph...@apache.org> wrote: > It would be great to have more examples as part of the release > artifact. Would you mind creating a JIRA/patch for this? > http://wiki.apache.org/hadoop/ZooKeeper/HowToContribute > > I'm thinking that we could have a src/contrib/examples or src/examples > ... what do you guys think? (mahadev?) > > Patrick > > > On Thu, Nov 11, 2010 at 12:46 PM, Robert Crocombe <rcroc...@gmail.com> wrote: >> On Tue, Nov 9, 2010 at 12:34 PM, Jeremy Hanna >> <jeremy.hanna1...@gmail.com>wrote: >> >>> Anyone know of a good blog post or docs anywhere that gives a simple >>> example of Watchers in action? I saw the one on: >>> >>> http://hadoop.apache.org/zookeeper/docs/current/javaExample.html#ch_Introduc >>> tion >>> >>> but it seems kind of overly complicated for an intro to Watchers. I >>> appreciate the example but wondered if there were other examples out there. >>> >> >> Appended is a Java example of using a Watcher simply to wait for the client >> to actually be connected to a server. I used it when I was confirming to my >> satisfaction that there was a bug in the ZooKeeper recipe for WriteLock >> awhile ago. I think this use is slightly unusual in that it is more >> interested in KeeperState than the event type. A more conventional Watcher >> might be like the following sketch (uhm, this is Groovy), though really >> you'd have to look at both: >> >> @Override >> public void process(WatchedEvent event) { >> switch (event?.getType()) { >> case EventType.NodeDeleted: >> // TODO: what should we do if the node being watched is itself >> // deleted? >> LOG.error("The node being watched '" + event.getPath + "' has been deleted: >> that's not good") >> break >> case EventType.NodeChildrenChanged: >> childrenChanged(event) >> break >> default: >> LOG.debug("Ignoring event type '" + event?.getType() + "'") >> break >> } >> } >> >> -- >> Robert Crocombe >> >> package derp; >> >> import java.io.IOException; >> import java.util.concurrent.atomic.AtomicBoolean; >> import java.util.concurrent.locks.Condition; >> import java.util.concurrent.locks.Lock; >> import java.util.concurrent.locks.ReentrantLock; >> >> import org.apache.commons.logging.Log; >> import org.apache.commons.logging.LogFactory; >> import org.apache.zookeeper.KeeperException; >> import org.apache.zookeeper.WatchedEvent; >> import org.apache.zookeeper.Watcher; >> import org.apache.zookeeper.ZooKeeper; >> import org.apache.zookeeper.Watcher.Event.KeeperState; >> import org.apache.zookeeper.recipes.lock.WriteLock; >> >> public class Test { >> private static final Log LOG = LogFactory.getLog(Test.class); >> >> private static final String ZOO_CONFIG = "10.2.1.54:2181/test"; >> private static final String LOCK_DIR = "/locking-test"; >> private static final int TIMEOUT_MILLIS = 10000; >> >> private static class ConnectWatcher implements Watcher { >> >> private final Lock connectedLock = new ReentrantLock(); >> private final Condition connectedCondition = connectedLock.newCondition(); >> >> private final AtomicBoolean connected = new AtomicBoolean(false); >> >> @Override >> public void process(WatchedEvent event) { >> LOG.debug("Event: " + event); >> >> KeeperState keeperState = event.getState(); >> switch (keeperState) { >> case SyncConnected: >> if (!connected.get()) { >> connected.set(true); >> signal(); >> } >> break; >> case Expired: >> case Disconnected: >> if (connected.get()) { >> connected.set(false); >> signal(); >> } >> } >> } >> >> public void waitForConnection() throws InterruptedException { >> connectedLock.lock(); >> try { >> while (!connected.get()) { >> LOG.debug("Waiting for condition to be signalled"); >> connectedCondition.await(); >> LOG.debug("Woken up on condition signalled"); >> } >> } finally { >> connectedLock.unlock(); >> } >> LOG.debug("After signalling, we are connected"); >> } >> >> @Override >> public String toString() { >> StringBuilder b = new StringBuilder("["); >> b.append("connectedLock:").append(connectedLock); >> b.append(",connectedCondition:").append(connectedCondition); >> b.append(",connected:").append(connected); >> b.append("]"); >> return b.toString(); >> } >> >> private void signal() { >> LOG.debug("Signaling after event"); >> connectedLock.lock(); >> try { >> connectedCondition.signal(); >> } finally { >> connectedLock.unlock(); >> } >> } >> } >> >> private static final void fine(ZooKeeper lowerId, ZooKeeper higherId) throws >> KeeperException, >> InterruptedException { >> WriteLock lower = new WriteLock(lowerId, LOCK_DIR, null); >> WriteLock higher = new WriteLock(higherId, LOCK_DIR, null); >> >> boolean lowerAcquired = lower.lock(); >> assert lowerAcquired; >> >> LOG.debug("Lower acquired lock successfully, so higher should fail"); >> >> boolean higherAcquired = higher.lock(); >> assert !higherAcquired; >> >> LOG.debug("Correct: higher session fails to acquire lock"); >> >> lower.unlock(); >> // Now that lower has unlocked, higher will acquire. Really should use >> // the version of WriteLock with the LockListener, but a short sleep >> // should do. >> Thread.sleep(2000); >> higher.unlock(); >> // make sure we let go. >> assert !higher.isOwner(); >> } >> >> /* >> * Using recipes from ZooKeeper 3.2.1. >> * >> * This bug occurs because the sort in ZooKeeperLockOperation.execute >> * (beginning @ line 221) orders the paths, but the paths contain the >> * session ID (lines 206-207), so that sorting the paths places all paths >> * with a lower session ID before those with a higher, i.e. the sorting is >> * not just by sequence number. I think this is a bug. >> * >> * The result is that a lock acquisition by a WriteLock coming from a >> * ZooKeeper with a high session ID will not be seen by another attempt to >> * lock coming from ZooKeeper with a lower session ID, because the sorting >> * will make the assumption: >> * >> * ownerId = sortedNames.first().getName(); >> * >> * line 226 false, and also the test >> * >> * if (!lessThanMe.isEmpty()) { >> * >> * line 228 will not reveal the presence of the other node that is already >> * locked. >> */ >> private static final void bug(ZooKeeper lowerId, ZooKeeper higherId) throws >> KeeperException, >> InterruptedException { >> WriteLock lower = new WriteLock(lowerId, LOCK_DIR, null); >> WriteLock higher = new WriteLock(higherId, LOCK_DIR, null); >> >> boolean higherAcquired = higher.lock(); >> assert higherAcquired; >> boolean lowerAcquired = false; >> try { >> LOG.debug("Higher acquired lock successfully, so lower should fail"); >> LOG.debug("Is higher owner (should be): " + higher.isOwner()); >> >> lowerAcquired = lower.lock(); >> >> LOG.debug("Is lower lock owner? " + lower.isOwner()); >> >> if (!lowerAcquired) { >> LOG.info("Okay: bug not triggered: lower lock did not acquire"); >> } else { >> LOG >> .error("BUG! Even though the higher had the lock, the lower managed to >> acquire it as well!"); >> } >> } finally { >> if (lowerAcquired) >> lower.unlock(); >> if (higherAcquired) >> higher.unlock(); >> } >> assert !lower.isOwner(); >> assert !higher.isOwner(); >> } >> >> public static void main(String[] args) throws IOException, KeeperException, >> InterruptedException { >> // Two ZooKeepers so we get two sessions: the bug only shows up for >> // two different session IDs. >> ConnectWatcher connectA = new ConnectWatcher(); >> ConnectWatcher connectB = new ConnectWatcher(); >> >> ZooKeeper zooA = new ZooKeeper(ZOO_CONFIG, TIMEOUT_MILLIS, connectA); >> ZooKeeper zooB = new ZooKeeper(ZOO_CONFIG, TIMEOUT_MILLIS, connectB); >> >> connectA.waitForConnection(); >> connectB.waitForConnection(); >> LOG.debug("Both connections connected"); >> >> final long idA = zooA.getSessionId(); >> final long idB = zooB.getSessionId(); >> LOG.debug("For A -> " + zooA + " session ID is " + idA + " -> " + >> Long.toHexString(idA)); >> LOG.debug("For B -> " + zooB + " session ID is " + idB + " -> " + >> Long.toHexString(idB)); >> >> // verify different sessions >> assert idA != idB; >> >> try { >> if (idA < idB) { >> fine(zooA, zooB); >> bug(zooA, zooB); >> } else { >> fine(zooB, zooA); >> bug(zooB, zooA); >> } >> } catch (Exception e) { >> e.printStackTrace(); >> } finally { >> try { >> LOG.debug("Closing session A -> " + Long.toHexString(idA)); >> zooA.close(); >> } catch (Exception e) { >> } >> try { >> LOG.debug("Closing session A -> " + Long.toHexString(idB)); >> zooB.close(); >> } catch (Exception e) { >> } >> } >> } >> } >> >
