Dealing with session expired
I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Re: Dealing with session expired
Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Re: Dealing with session expired
So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Re: Dealing with session expired
Hi Tom, The session expired event means that the the server expired the client and that means the watches and ephemrals will go away for that node. How are you running your zookeeper quorum? Session expiry event should be really rare event . If you have a quorum of servers it should rarely happen. mahadev On 2/12/09 11:17 AM, Tom Nichols tmnich...@gmail.com wrote: So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Re: Dealing with session expired
Ephemerals and watches are maintained across disconnect/reconnect btw the client and server however session expiration (or closing the session explicitly) will trigger deletion of ephemeral nodes associated with the session. Right - once the session is expired the id is invalid. You need to create a new session (new id). Btw, the timeout value you provide to when constructing the zookeeper client session directly effects the session expiration - the server uses this timeout as the session expiration time. Patrick Tom Nichols wrote: So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Re: Dealing with session expired
Regardless of frequency Tom's code still has to handle this situation. I would suggest that the two classes Tom is referring to in his mail, the ones that use ZK client object, should either be able to reinitialize with a new zk session, or they themselves should be discarded and new instances created using the new session (not sure what makes more sense for his archi...) Regardless of whether we reuse the session object or create a new one I believe the code using the session needs to reinitialize in some way -- there's been a dramatic break from the cluster. As I mentioned, you can decrease the likelihood of expiration by increasing the timeout - but the downside is that you are less sensitive to clients dying (because their ephemeral nodes don't get deleted till close/expire and if you are doing something like leader election among your clients it will take longer for the followers to be notified). Patrick Mahadev Konar wrote: Hi Tom, The session expired event means that the the server expired the client and that means the watches and ephemrals will go away for that node. How are you running your zookeeper quorum? Session expiry event should be really rare event . If you have a quorum of servers it should rarely happen. mahadev On 2/12/09 11:17 AM, Tom Nichols tmnich...@gmail.com wrote: So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
RE: Dealing with session expired
idleness is not a problem. the client library sends heartbeats to keep the session alive. the client library will also handle reconnects automatically if a server dies. since session expiration really is a rare catastrophic event. (or at least it should be.) it is probably easiest to deal with it by starting with a fresh instance if your session expires. ben From: Tom Nichols [tmnich...@gmail.com] Sent: Thursday, February 12, 2009 11:53 AM To: zookeeper-user@hadoop.apache.org Subject: Re: Dealing with session expired I'm using a timeout of 5000ms. Now let me ask this: Suppose all of my clients are waiting on some external event -- not ZooKeeper -- so they are all idle and are not touching ZK nodes, nor are they calling exists, getChildren, etc etc. Can that idleness cause session expiry? I'm running a local quorum of 3 nodes. That is, I have an Ant script that kicks off 3 java tasks in parallel to run ConsumerPeerMain, each with its own config file. Regarding handling of the failure, I suspect I will just have to reinitialize by creating a new instance of my client(s) that themselves will have a new ZK instance. I'm using Spring to wire everything together, which is why it's particularly difficult to simply re-create a new ZK instance and pass it to the classes using it (those classes have no knowledge of each other). But I _can_ just pull a freshly-created (prototype) instance from the Spring application context, which is where a new ZK client will be wired in. The only ramification there is I have to throw the KeeperException as a fatal exception rather than letting that client try to re-elect. Or maybe add in some logic to say if I can't re-elect, _then_ throw an exception and consider it fatal. Thanks guys. -Tom On Thu, Feb 12, 2009 at 2:39 PM, Patrick Hunt ph...@apache.org wrote: Regardless of frequency Tom's code still has to handle this situation. I would suggest that the two classes Tom is referring to in his mail, the ones that use ZK client object, should either be able to reinitialize with a new zk session, or they themselves should be discarded and new instances created using the new session (not sure what makes more sense for his archi...) Regardless of whether we reuse the session object or create a new one I believe the code using the session needs to reinitialize in some way -- there's been a dramatic break from the cluster. As I mentioned, you can decrease the likelihood of expiration by increasing the timeout - but the downside is that you are less sensitive to clients dying (because their ephemeral nodes don't get deleted till close/expire and if you are doing something like leader election among your clients it will take longer for the followers to be notified). Patrick Mahadev Konar wrote: Hi Tom, The session expired event means that the the server expired the client and that means the watches and ephemrals will go away for that node. How are you running your zookeeper quorum? Session expiry event should be really rare event . If you have a quorum of servers it should rarely happen. mahadev On 2/12/09 11:17 AM, Tom Nichols tmnich...@gmail.com wrote: So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining a one to one relationship between a client handle and a session makes it much simpler for users to understand the existence and disappearance of ephemeral nodes and watches created by a zookeeper client. thanks mahadev On 2/12/09 10:58 AM, Tom Nichols tmnich...@gmail.com wrote: I've come across the situation where a ZK instance will have an expired connection and therefore all operations fail. Now AFAIK the only way to recover is to create a new ZK instance with the old session ID, correct? Now, my problem is, the ZK instance may be shared -- not between threads -- but maybe two classes in the same thread synchronize on different nodes by using different watchers. So it makes sense that one ZK client instance can handle this. Except that even if I detect the session expiration by catching the KeeperException, if I want to resume the session, I have to create a new ZK instance and pass it to any classes who were previously sharing the same instance. Does this make sense so far? Anyway, bottom line is, it would be nice if a ZK instance could itself recover a session rather than discarding that instance and creating a new one. Thoughts? Thanks in advance, -Tom
Hadoop User Group Meeting (Bay Area) 2/18
The next Bay Area Hadoop User Group meeting is scheduled for Wednesday, February 18th at Yahoo! 2811 Mission College Blvd, Santa Clara, Building 2, Training Rooms 5 6 from 6:00-7:30 pm. Agenda: Fair Scheduler for Hadoop - Matei Zaharia Interfacing with MySQL - Aaron Kimball Registration: http://upcoming.yahoo.com/event/1776616/ As always, suggestions for topics for future meetings are welcome. Please send them to me directly at aan...@yahoo-inc.com Look forward to seeing you there! Ajay
Re: Dealing with session expired
Tom, you might try changing the log4j default log level to DEBUG for the rootlogger and appender if you have not already done so (servers and clients both). You'll get more information to aid debugging if it does occur again. http://hadoop.apache.org/zookeeper/docs/r3.0.1/zookeeperAdmin.html#sc_logging Also, are you seeing timeouts on the client, or just session expiration on the server? The stat command, detailed here, may also be of use to you: http://hadoop.apache.org/zookeeper/docs/r3.0.1/zookeeperAdmin.html#sc_zkCommands Knowing more about your env, OS java version in particular, would also help us help you narrow things down. :-) Patrick Tom Nichols wrote: On Thu, Feb 12, 2009 at 4:11 PM, Benjamin Reed br...@yahoo-inc.com wrote: idleness is not a problem. the client library sends heartbeats to keep the session alive. the client library will also handle reconnects automatically if a server dies. That's odd then that I'm seeing this problem. I have a local, 3-node zookeeper quorum, and I have 3 instances of the client also running on the same box. The session expiry doesn't seem to be in response to any severe load on the machine or anything like that. I'll keep an eye on it and see if I can't reproduce the behavior in a distributed environment. I've realized a relatively easy way to deal with this problem -- I can let my thread throw a fatal unchecked exception and then use a ThreadGroup implementation that catches the exception. This in turn spawns a new client thread and adds it back to the same threadGroup. Thanks again guys. -Tom since session expiration really is a rare catastrophic event. (or at least it should be.) it is probably easiest to deal with it by starting with a fresh instance if your session expires. ben From: Tom Nichols [tmnich...@gmail.com] Sent: Thursday, February 12, 2009 11:53 AM To: zookeeper-user@hadoop.apache.org Subject: Re: Dealing with session expired I'm using a timeout of 5000ms. Now let me ask this: Suppose all of my clients are waiting on some external event -- not ZooKeeper -- so they are all idle and are not touching ZK nodes, nor are they calling exists, getChildren, etc etc. Can that idleness cause session expiry? I'm running a local quorum of 3 nodes. That is, I have an Ant script that kicks off 3 java tasks in parallel to run ConsumerPeerMain, each with its own config file. Regarding handling of the failure, I suspect I will just have to reinitialize by creating a new instance of my client(s) that themselves will have a new ZK instance. I'm using Spring to wire everything together, which is why it's particularly difficult to simply re-create a new ZK instance and pass it to the classes using it (those classes have no knowledge of each other). But I _can_ just pull a freshly-created (prototype) instance from the Spring application context, which is where a new ZK client will be wired in. The only ramification there is I have to throw the KeeperException as a fatal exception rather than letting that client try to re-elect. Or maybe add in some logic to say if I can't re-elect, _then_ throw an exception and consider it fatal. Thanks guys. -Tom On Thu, Feb 12, 2009 at 2:39 PM, Patrick Hunt ph...@apache.org wrote: Regardless of frequency Tom's code still has to handle this situation. I would suggest that the two classes Tom is referring to in his mail, the ones that use ZK client object, should either be able to reinitialize with a new zk session, or they themselves should be discarded and new instances created using the new session (not sure what makes more sense for his archi...) Regardless of whether we reuse the session object or create a new one I believe the code using the session needs to reinitialize in some way -- there's been a dramatic break from the cluster. As I mentioned, you can decrease the likelihood of expiration by increasing the timeout - but the downside is that you are less sensitive to clients dying (because their ephemeral nodes don't get deleted till close/expire and if you are doing something like leader election among your clients it will take longer for the followers to be notified). Patrick Mahadev Konar wrote: Hi Tom, The session expired event means that the the server expired the client and that means the watches and ephemrals will go away for that node. How are you running your zookeeper quorum? Session expiry event should be really rare event . If you have a quorum of servers it should rarely happen. mahadev On 2/12/09 11:17 AM, Tom Nichols tmnich...@gmail.com wrote: So if a session expires, my ephemeral nodes and watches have already disappeared? I suppose creating a new ZK instance with the old session ID would not do me any good in that case. Correct? Thanks. -Tom On Thu, Feb 12, 2009 at 2:12 PM, Mahadev Konar maha...@yahoo-inc.com wrote: Hi Tom, We prefer to discard the zookeeper instance if a session expires. Maintaining
