Package org.apache.curator

Class SessionFailRetryLoop

java.lang.Object

org.apache.curator.SessionFailRetryLoop

All Implemented Interfaces:

Closeable, AutoCloseable

public class SessionFailRetryLoop
extends Object
implements Closeable

See RetryLoop for the main details on retry loops. All Curator/ZooKeeper operations should be done in a retry loop.

The standard retry loop treats session failure as a type of connection failure. i.e. the fact that it is a session failure isn't considered. This can be problematic if you are performing a series of operations that rely on ephemeral nodes. If the session fails after the ephemeral node has been created, future Curator/ZooKeeper operations may succeed even though the ephemeral node has been removed by ZooKeeper.

Here's an example:

• You create an ephemeral/sequential node as a kind of lock/marker
• You perform some other operations
• The session fails for some reason
• You attempt to create a node assuming that the lock/marker still exists
  • Curator will notice the session failure and try to reconnect
  • In most cases, the reconnect will succeed and, thus, the node creation will succeed even though the ephemeral node will have been deleted by ZooKeeper.

The SessionFailRetryLoop prevents this type of scenario. When a session failure is detected, the thread is marked as failed which will cause all future Curator operations to fail. The SessionFailRetryLoop will then either retry the entire set of operations or fail (depending on SessionFailRetryLoop.Mode)

Canonical usage:

 SessionFailRetryLoop    retryLoop = client.newSessionFailRetryLoop(mode);
 retryLoop.start();
 try
 {
     while ( retryLoop.shouldContinue() )
     {
         try
         {
             // do work
         }
         catch ( Exception e )
         {
             retryLoop.takeException(e);
         }
     }
 }
 finally
 {
     retryLoop.close();
 }
 

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static enum	SessionFailRetryLoop.Mode
static class	SessionFailRetryLoop.SessionFailedException

Method Summary

Modifier and Type	Method	Description
static <T> T	callWithRetry(CuratorZookeeperClient client, SessionFailRetryLoop.Mode mode, Callable<T> proc)	Convenience utility: creates a "session fail" retry loop calling the given proc
void	close()	Must be called in a finally handler when done with the loop
boolean	shouldContinue()	If true is returned, make an attempt at the set of operations
void	start()	SessionFailRetryLoop must be started
void	takeException(Exception exception)	Pass any caught exceptions here

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

callWithRetry

public static <T> T callWithRetry(CuratorZookeeperClient client,
 SessionFailRetryLoop.Mode mode,
 Callable<T> proc)
                           throws Exception

Convenience utility: creates a "session fail" retry loop calling the given proc

Type Parameters:

T - return type

Parameters:

client - Zookeeper

mode - how to handle session failures

proc - procedure to call with retry

Returns:

procedure result

Throws:

Exception - any non-retriable errors

start

public void start()

SessionFailRetryLoop must be started

shouldContinue

public boolean shouldContinue()

If true is returned, make an attempt at the set of operations

Returns:

true/false

close

public void close()

Must be called in a finally handler when done with the loop

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Closeable

takeException

public void takeException(Exception exception)
                   throws Exception

Pass any caught exceptions here

Parameters:

exception - the exception

Throws:

Exception - if not retry-able or the retry policy returned negative