Curator intro

Curator
The Netflix ZooKeeper Client Library

Jordan Zimmerman
Senior Platform Engineer
Netflix, Inc.
jzimmerman@netflix.com
@rangalt

Agenda
• Background
• Overview of Curator
• The Recipes
• Some Low-Level Details
• Q&A

What’s wrong with this
code?

ZooKeeper client = new ZooKeeper(...);

client.create(“/foo”, data, ...);

ZooKeeper Surprise
• Almost no ZK client call is safe
• You cannot assume success
• You must handle exceptions

The Recipes Are Hard
Locks
Fully distributed locks that are globally synchronous, meaning at any snapshot in time no two clients think they hold the same lock. These can be
implemented using ZooKeeeper. As with priority queues, first define a lock node.

Note
There now exists a Lock implementation in ZooKeeper recipes directory. This is distributed with the release -- src/recipes/lock directory of the release artifact.

Clients wishing to obtain a lock do the following:

1. Call create( ) with a pathname of "_locknode_/guid-lock-" and the sequence and ephemeral flags set. The guid is needed in case the
create() result is missed. See the note below.
2. Call getChildren( ) on the lock node without setting the watch flag (this is important to avoid the herd effect).
3. If the pathname created in step 1 has the lowest sequence number suffix, the client has the lock and the client exits the protocol.
4. The client calls exists( ) with the watch flag set on the path in the lock directory with the next lowest sequence number.
5. if exists( ) returns false, go to step 2. Otherwise, wait for a notification for the pathname from the previous step before going to step 2.
The unlock protocol is very simple: clients wishing to release a lock simply delete the node they created in step 1.

Here are a few things to notice:

• The removal of a node will only cause one client to wake up since each node is watched by exactly one client. In this way, you avoid the
herd effect.
• There is no polling or timeouts.
• Because of the way you implement locking, it is easy to see the amount of lock contention, break locks, debug locking problems, etc.
Recoverable Errors and the GUID
• If a recoverable error occurs calling create() the client should call getChildren() and check for a node containing the guid used in the
path name. This handles the case (noted above) of the create() succeeding on the server but the server crashing before returning the
name of the new node.

Even the Distribution
Has Issues
from org.apache.zookeeper.recipes.lock.WriteLock
if (id == null) {
long sessionId = zookeeper.getSessionId();
String prefix = "x-" + sessionId + "-";
// lets try look up the current ID if we failed
// in the middle of creating the znode
findPrefixInChildren(prefix, zookeeper, dir);
idName = new ZNodeName(id);
}

Even the Distribution
Has Issues
from org.apache.zookeeper.recipes.lock.WriteLock
if (id == null) {
long sessionId = zookeeper.getSessionId();
String prefix = "x-" + sessionId + "-";
// lets try look up the current ID if we failed
// in the middle of creating the znode
findPrefixInChildren(prefix, zookeeper, dir);
idName = new ZNodeName(id);
}

Bad handling of Ephemeral-Sequential issue!

What About ZKClient?
• Unclear if it’s still being supported
Eleven open issues (back to 10/1/2009)
• README:
“+ TBD”
• No docs
• Little or no retries
• Design problems:
• All exceptions converted to RuntimeException
• Recipes/management code highly coupled
• Lots of foreground synchronization
• Small number of tests
• ... etc ...
• ...

Introducing Curator
Curator n ˈkyo͝orˌātər: a keeper or custodian of a
museum or other collection - A ZooKeeper
Keeper
Three components:
Client - A replacement/wrapper for the bundled ZooKeeper class

Framework - A high-level API that greatly simpliﬁes using
ZooKeeper

Recipes - Implementations of some of the common ZooKeeper
"recipes" built on top of the Curator Framework

The Curator Stack
• Client
• Framework
• Recipes
• Extensions

The Curator Stack
• Client
• Framework
• Recipes Curator Recipes

Curator Framework

• Extensions Curator Client

ZooKeeper

Curator is a platform
for writing ZooKeeper
Recipes

Curator Client
manages the
ZooKeeper
Connection

Curator Recipes

Curator Framework

Curator Client

ZooKeeper

Curator Client
manages the
ZooKeeper
Connection

Curator Framework
uses retry for all
operations and provides
a friendlier API

Curator Recipes

Curator Framework

Curator Client

ZooKeeper

Curator Framework
uses retry for all
operations and provides
a friendlier API

Curator Recipes:
implementations of all
recipes listed on the
ZK website (and more)

Curator Recipes

Curator Framework

Curator Client

ZooKeeper

Curator Recipes:
implementations of all
recipes listed on the
ZK website (and more)

The Recipes
• Leader Selector
• Distributed Locks
• Queues
• Barriers
• Counters
• Atomics
• ...

CuratorFramework
Instance
CuratorFrameworkFactory.newClient(...)
---------------------
CuratorFrameworkFactory.builder()
.connectString(“...”)
...
.build()

Usually injected as a singleton

Must Be Started

client.start();

// client is now ready for use

Leader Selector
By far the most common usage of
ZooKeeper

Distributed lock with a notiﬁcation
mechanism

public class CleanupLeader implements
LeaderSelectorListener
{
...
@Override
public void takeLeadership(CuratorFramework client)
throws Exception
{
while ( !Thread.currentThread().isInterrupted() )
{
sleepUntilNextPeriod();
doPeriodicCleanup();
}
}
}

...
LeaderSelector leaderSelector =
new LeaderSelector(client, path, new CleanupLeader());
leaderSelector.start();

Distributed Locks
• InterProcessMutex
• InterProcessReadWriteLock
• InterProcessMultiLock
• InterProcessSemaphore

Distributed Locks
• InterProcessMutex
• InterProcessReadWriteLock
• InterProcessMultiLock
• InterProcessSemaphore

Very similar to JDK locks

InterProcessMutex mutex =
new InterProcessMutex(client, lockPath);

mutex.acquire();
try
{
// do work in critical section
}
finally
{
mutex.release();
}

public void process(WatchedEvent event)
{
boolean wasConnected = isConnected.get();
boolean newIsConnected = wasConnected;
if ( event.getType() == Watcher.Event.EventType.None )
{
newIsConnected = (event.getState() == Event.KeeperState.SyncConnected);
if ( event.getState() == Event.KeeperState.Expired )
{
handleExpiredSession();
}
}

if ( newIsConnected != wasConnected )
{
isConnected.set(newIsConnected);
connectionStartMs = System.currentTimeMillis();
}

...
}

public static boolean shouldRetry(int rc)
{
return (rc == KeeperException.Code.CONNECTIONLOSS.intValue()) ||
(rc == KeeperException.Code.OPERATIONTIMEOUT.intValue()) ||
(rc == KeeperException.Code.SESSIONMOVED.intValue()) ||
(rc == KeeperException.Code.SESSIONEXPIRED.intValue());
}

public void takeException(Exception exception) throws Exception
{
boolean rethrow = true;
if ( isRetryException(exception) )
{
if ( retryPolicy.allowRetry(retryCount++, System.currentTimeMillis() - startTimeMs) )
{
rethrow = false;
}
}

if ( rethrow )
{
throw exception;
}
}

byte[] responseData = RetryLoop.callWithRetry
(
client.getZookeeperClient(),
new Callable<byte[]>()
{
@Override
public byte[] call() throws Exception
{
byte[] responseData;
responseData = client.getZooKeeper().getData(path,
...);
}
return responseData;
}
}
);
return responseData;

client.withProtectedEphemeralSequential()

final AtomicBoolean firstTime = new AtomicBoolean(true);
String returnPath = RetryLoop.callWithRetry
(
client.getZookeeperClient(),
new Callable<String>()
{
@Override
public String call() throws Exception
{
...

String createdPath = null;
if ( !firstTime.get() && doProtectedEphemeralSequential )
{
createdPath = findProtectedNodeInForeground(localPath);
}
...
}
}
);

public interface ConnectionStateListener
{
public void stateChanged(CuratorFramework
client, ConnectionState newState);
}

public enum ConnectionState
{
SUSPENDED,
RECONNECTED,
LOST
}

if ( e instanceof KeeperException.ConnectionLossException )
{
connectionStateManager.addStateChange(ConnectionState.LOST);
}

private void validateConnection(CuratorEvent curatorEvent)
{
if ( curatorEvent.getType() == CuratorEventType.WATCHED )
{
if ( curatorEvent.getWatchedEvent().getState() ==
Watcher.Event.KeeperState.Disconnected )
{
connectionStateManager.addStateChange(ConnectionState.SUSPENDED);
internalSync(this, "/", null);
}
else if ( curatorEvent.getWatchedEvent().getState() ==
Watcher.Event.KeeperState.Expired )
{
connectionStateManager.addStateChange(ConnectionState.LOST);
}
else if ( curatorEvent.getWatchedEvent().getState() ==
Watcher.Event.KeeperState.SyncConnected )
{
connectionStateManager.addStateChange(ConnectionState.RECONNECTED);
}
}
}

• TestingServer: manages an internally
running ZooKeeper server
// Create the server using a random port
public TestingServer()

• TestingCluster: manages an internally
running ensemble of ZooKeeper servers.
// Creates an ensemble comprised of n servers.
// Each server will use a temp directory and
// random ports
public TestingCluster(int instanceQty)

Extensions
• Discovery
• Discovery REST Server
• Exhibitor
• ???

Extensions
• Discovery
• Discovery REST Server
• Exhibitor Curator Recipes

• ???
Curator Framework
Extensions
Curator Client

ZooKeeper

Exhibitor
Sneak Peak

March or April 2012
Open Source on Github

Netﬂix Github

Netﬂix’s home for Open Source

Maven Central
Binaries pushed to Maven Central

<dependency>
<groupId>com.netflix.curator</groupId>
<artifactId>curator-recipes</artifactId>
<version>1.1.0</version>
</dependency>

Much%younger%–%much%thinner0

Jordan Zimmerman
@randgalt

Q&A

Much%younger%–%much%thinner0

Jordan Zimmerman
@randgalt

Curator intro

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Editor's Notes