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Posted to user@zookeeper.apache.org by Satish Bhatti <ct...@gmail.com> on 2009/06/10 21:05:40 UTC
zookeeper.getChildren asynchronous callback
I am using the asynchronous (callback) version of zookeeper.getChildren().
That call returns immediately, I then wait for a certain time interval for
nodes to appear, and if not I exit the method that made the
zookeeper.getChildren()
call. Later on, a node gets added under that node and I see in my logfile
that the Watcher.process() callback that I set above gets called. Now if I
make 10 failed attempts to get a node using the above technique, and at some
later time a node does get added, I see in the logfile that the
Watcher.process() ends up being called 10 times! Of course by this time I
have totally lost interest in those callbacks. Question: Is there a way to
remove that asynchronous callback? i.e. If I make a asynchronous
zookeeper.getChildren()
call, wait time t, give up, at that point can I remove the async callback?
Satish
Re: zookeeper.getChildren asynchronous callback
Posted by Satish <ct...@gmail.com>.
I guess I was talking about removing the trigger. Personally I don't
think there's a need to bend over backwards to remove "in-flight"
notifications! :)
Satish
On Jun 12, 2009, at 9:59 AM, Patrick Hunt <ph...@apache.org> wrote:
> We probably want to allow the caller to specify which type of watch
> they want to remove - a watch on the znode itself, on children of
> the znode, or both.
>
> zk.removeWatch(path, watcher, wtype)
> where:
> path is path to the znode
> watcher may be a specific watcher or null matching all watchers
> wtype is enum of TYPE_CHILD, TYPE_DATA, or TYPE_ALL (something like
> that)
>
> We also need to be careful of the semantics since "watch" wraps up a
> couple of concepts: the trigger and the notification. What are we
> removing? The trigger or both? If just the trigger then you may
> still be notified after removeWatch is called (if notification is
> "in flight"). Perhaps removal of the watch should itself trigger the
> notification to all watchers, with a new notification type of "watch
> removed"?
>
> Patrick
>
> Satish Bhatti wrote:
>> Actually I guess that would be zookeeper.removeWatch( rootPath,
>> tempWatcher
>> ) !!
>> On Thu, Jun 11, 2009 at 10:14 AM, Satish Bhatti
>> <ct...@gmail.com> wrote:
>>> That's right Ben. Basically, I would like to use it something
>>> like this:
>>> public boolean waitForChildrenChanged( String rootPath,
>>> long
>>> timeout )
>>> {
>>> BooleanLock blChildrenChanged = new BooleanLock();
>>>
>>> Watcher tempWatcher =
>>> new Watcher()
>>> {
>>> public void process( WatchedEvent event )
>>> {
>>> logger.debug( "waitForAnyEntry(): Got state event:
>>> " +
>>> ZooKeeperUtils.watchedEventToString( event ) );
>>> blChildrenChanged.setValue( true );
>>> }
>>> };
>>>
>>> zookeeper.getChildren( rootPath, watcher,
>>>
>>> new AsyncCallback.ChildrenCallback()
>>> {
>>> public void processResult( int rc, String path, Object
>>> ctx,
>>> List<String> children )
>>> {
>>> logger.debug( "waitForChildrenChanged():
>>> AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " +
>>> ctx + ", " +
>>> children );
>>> }
>>> }, null );
>>>
>>> blChildrenChanged.waitUntilTrue( timeout );
>>>
>>> zookeeper.removeWatch( tempWatcher );
>>>
>>> return blChildrenChanged.isTrue();
>>> }
>>>
>>> The only piece missing from the API is the zookeeper.removeWatch(
>>> tempWatcher );
>>>
>>> Satish
>>>
>>>
>>> On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <breed@yahoo-
>>> inc.com>wrote:
>>>
>>>> just to clarify i believe you are talking about callbacks on the
>>>> watch
>>>> object you are passing in the asynchronous call rather than the
>>>> asynchronous
>>>> completion callback. (Henry is making the same assumption.) when
>>>> you say you
>>>> are getting the callback 10 times, i believe your are talking
>>>> about 10
>>>> different watch objects getting called back once each. right?
>>>>
>>>> it turns out that the zookeeper client does know what you are
>>>> watching,
>>>> and the zookeeper server will only register one watch. the thing
>>>> that is
>>>> missing is the clearWatches call that Henry refers to. the thing
>>>> that
>>>> complicates things a bit, perhaps not for you, is the scenario
>>>> where we have
>>>> different modules sharing the same zookeeper handle. if different
>>>> modules
>>>> are interested in watching the same object, you don't want one
>>>> module to
>>>> simply clear a the watches for a path because one module may mess
>>>> up the
>>>> other.
>>>>
>>>> we have talked about adding this ability to clear watches for a
>>>> while. i
>>>> think the auto-watch reregistration patch made the issue slightly
>>>> more
>>>> pressing since it means that watches can survive for the entire
>>>> lifetime of
>>>> a session not just for the duration of a connection to a specific
>>>> server.
>>>> i've created ZOOKEEPER-442 to track this issue.
>>>>
>>>> ben
>>>>
>>>>
>>>> Henry Robinson wrote:
>>>>
>>>>> Hi Satish -
>>>>>
>>>>> As you've found out, you can set multiple identical watches per
>>>>> znode -
>>>>> the
>>>>> zookeeper client will not detect identical watches in case you
>>>>> really
>>>>> meant
>>>>> to call them several times. There's no way currently, as far as
>>>>> I know,
>>>>> to
>>>>> clear the watches once they've been set. So your options are
>>>>> either to
>>>>> avoid
>>>>> repeatedly setting them by detecting whether getChildren is a
>>>>> repeat
>>>>> call,
>>>>> or by dealing with multiple invocations on the callback path and
>>>>> not
>>>>> doing
>>>>> anything once you've established you're no longer interested.
>>>>>
>>>>> It might well make sense to add a clearWatches(path) call to the
>>>>> API,
>>>>> which
>>>>> would be useful particularly for clients where callbacks are
>>>>> expensive
>>>>> and
>>>>> require a context switch (which I think is true for all clients
>>>>> right
>>>>> now!).
>>>>>
>>>>> Henry
>>>>>
>>>>> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti
>>>>> <ct...@gmail.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>>> I am using the asynchronous (callback) version of
>>>>>> zookeeper.getChildren().
>>>>>> That call returns immediately, I then wait for a certain time
>>>>>> interval
>>>>>> for
>>>>>> nodes to appear, and if not I exit the method that made the
>>>>>> zookeeper.getChildren()
>>>>>> call. Later on, a node gets added under that node and I see in
>>>>>> my
>>>>>> logfile
>>>>>> that the Watcher.process() callback that I set above gets
>>>>>> called. Now
>>>>>> if I
>>>>>> make 10 failed attempts to get a node using the above
>>>>>> technique, and at
>>>>>> some
>>>>>> later time a node does get added, I see in the logfile that the
>>>>>> Watcher.process() ends up being called 10 times! Of course by
>>>>>> this time
>>>>>> I
>>>>>> have totally lost interest in those callbacks. Question: Is
>>>>>> there a
>>>>>> way
>>>>>> to
>>>>>> remove that asynchronous callback? i.e. If I make a asynchronous
>>>>>> zookeeper.getChildren()
>>>>>> call, wait time t, give up, at that point can I remove the async
>>>>>> callback?
>>>>>> Satish
>>>>>>
>>>>>>
>>>>>>
Re: zookeeper.getChildren asynchronous callback
Posted by Patrick Hunt <ph...@apache.org>.
We probably want to allow the caller to specify which type of watch they
want to remove - a watch on the znode itself, on children of the znode,
or both.
zk.removeWatch(path, watcher, wtype)
where:
path is path to the znode
watcher may be a specific watcher or null matching all watchers
wtype is enum of TYPE_CHILD, TYPE_DATA, or TYPE_ALL (something like that)
We also need to be careful of the semantics since "watch" wraps up a
couple of concepts: the trigger and the notification. What are we
removing? The trigger or both? If just the trigger then you may still be
notified after removeWatch is called (if notification is "in flight").
Perhaps removal of the watch should itself trigger the notification to
all watchers, with a new notification type of "watch removed"?
Patrick
Satish Bhatti wrote:
> Actually I guess that would be zookeeper.removeWatch( rootPath, tempWatcher
> ) !!
>
> On Thu, Jun 11, 2009 at 10:14 AM, Satish Bhatti <ct...@gmail.com> wrote:
>
>> That's right Ben. Basically, I would like to use it something like this:
>> public boolean waitForChildrenChanged( String rootPath,
>> long timeout )
>> {
>> BooleanLock blChildrenChanged = new BooleanLock();
>>
>> Watcher tempWatcher =
>> new Watcher()
>> {
>> public void process( WatchedEvent event )
>> {
>> logger.debug( "waitForAnyEntry(): Got state event: " +
>> ZooKeeperUtils.watchedEventToString( event ) );
>> blChildrenChanged.setValue( true );
>> }
>> };
>>
>> zookeeper.getChildren( rootPath, watcher,
>>
>> new AsyncCallback.ChildrenCallback()
>> {
>> public void processResult( int rc, String path, Object ctx,
>> List<String> children )
>> {
>> logger.debug( "waitForChildrenChanged():
>> AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " + ctx + ", " +
>> children );
>> }
>> }, null );
>>
>> blChildrenChanged.waitUntilTrue( timeout );
>>
>> zookeeper.removeWatch( tempWatcher );
>>
>> return blChildrenChanged.isTrue();
>> }
>>
>> The only piece missing from the API is the zookeeper.removeWatch(
>> tempWatcher );
>>
>> Satish
>>
>>
>> On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <br...@yahoo-inc.com>wrote:
>>
>>> just to clarify i believe you are talking about callbacks on the watch
>>> object you are passing in the asynchronous call rather than the asynchronous
>>> completion callback. (Henry is making the same assumption.) when you say you
>>> are getting the callback 10 times, i believe your are talking about 10
>>> different watch objects getting called back once each. right?
>>>
>>> it turns out that the zookeeper client does know what you are watching,
>>> and the zookeeper server will only register one watch. the thing that is
>>> missing is the clearWatches call that Henry refers to. the thing that
>>> complicates things a bit, perhaps not for you, is the scenario where we have
>>> different modules sharing the same zookeeper handle. if different modules
>>> are interested in watching the same object, you don't want one module to
>>> simply clear a the watches for a path because one module may mess up the
>>> other.
>>>
>>> we have talked about adding this ability to clear watches for a while. i
>>> think the auto-watch reregistration patch made the issue slightly more
>>> pressing since it means that watches can survive for the entire lifetime of
>>> a session not just for the duration of a connection to a specific server.
>>> i've created ZOOKEEPER-442 to track this issue.
>>>
>>> ben
>>>
>>>
>>> Henry Robinson wrote:
>>>
>>>> Hi Satish -
>>>>
>>>> As you've found out, you can set multiple identical watches per znode -
>>>> the
>>>> zookeeper client will not detect identical watches in case you really
>>>> meant
>>>> to call them several times. There's no way currently, as far as I know,
>>>> to
>>>> clear the watches once they've been set. So your options are either to
>>>> avoid
>>>> repeatedly setting them by detecting whether getChildren is a repeat
>>>> call,
>>>> or by dealing with multiple invocations on the callback path and not
>>>> doing
>>>> anything once you've established you're no longer interested.
>>>>
>>>> It might well make sense to add a clearWatches(path) call to the API,
>>>> which
>>>> would be useful particularly for clients where callbacks are expensive
>>>> and
>>>> require a context switch (which I think is true for all clients right
>>>> now!).
>>>>
>>>> Henry
>>>>
>>>> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com>
>>>> wrote:
>>>>
>>>>
>>>>
>>>>> I am using the asynchronous (callback) version of
>>>>> zookeeper.getChildren().
>>>>> That call returns immediately, I then wait for a certain time interval
>>>>> for
>>>>> nodes to appear, and if not I exit the method that made the
>>>>> zookeeper.getChildren()
>>>>> call. Later on, a node gets added under that node and I see in my
>>>>> logfile
>>>>> that the Watcher.process() callback that I set above gets called. Now
>>>>> if I
>>>>> make 10 failed attempts to get a node using the above technique, and at
>>>>> some
>>>>> later time a node does get added, I see in the logfile that the
>>>>> Watcher.process() ends up being called 10 times! Of course by this time
>>>>> I
>>>>> have totally lost interest in those callbacks. Question: Is there a
>>>>> way
>>>>> to
>>>>> remove that asynchronous callback? i.e. If I make a asynchronous
>>>>> zookeeper.getChildren()
>>>>> call, wait time t, give up, at that point can I remove the async
>>>>> callback?
>>>>> Satish
>>>>>
>>>>>
>>>>>
>
Re: zookeeper.getChildren asynchronous callback
Posted by Satish Bhatti <ct...@gmail.com>.
Actually I guess that would be zookeeper.removeWatch( rootPath, tempWatcher
) !!
On Thu, Jun 11, 2009 at 10:14 AM, Satish Bhatti <ct...@gmail.com> wrote:
> That's right Ben. Basically, I would like to use it something like this:
> public boolean waitForChildrenChanged( String rootPath,
> long timeout )
> {
> BooleanLock blChildrenChanged = new BooleanLock();
>
> Watcher tempWatcher =
> new Watcher()
> {
> public void process( WatchedEvent event )
> {
> logger.debug( "waitForAnyEntry(): Got state event: " +
> ZooKeeperUtils.watchedEventToString( event ) );
> blChildrenChanged.setValue( true );
> }
> };
>
> zookeeper.getChildren( rootPath, watcher,
>
> new AsyncCallback.ChildrenCallback()
> {
> public void processResult( int rc, String path, Object ctx,
> List<String> children )
> {
> logger.debug( "waitForChildrenChanged():
> AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " + ctx + ", " +
> children );
> }
> }, null );
>
> blChildrenChanged.waitUntilTrue( timeout );
>
> zookeeper.removeWatch( tempWatcher );
>
> return blChildrenChanged.isTrue();
> }
>
> The only piece missing from the API is the zookeeper.removeWatch(
> tempWatcher );
>
> Satish
>
>
> On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <br...@yahoo-inc.com>wrote:
>
>> just to clarify i believe you are talking about callbacks on the watch
>> object you are passing in the asynchronous call rather than the asynchronous
>> completion callback. (Henry is making the same assumption.) when you say you
>> are getting the callback 10 times, i believe your are talking about 10
>> different watch objects getting called back once each. right?
>>
>> it turns out that the zookeeper client does know what you are watching,
>> and the zookeeper server will only register one watch. the thing that is
>> missing is the clearWatches call that Henry refers to. the thing that
>> complicates things a bit, perhaps not for you, is the scenario where we have
>> different modules sharing the same zookeeper handle. if different modules
>> are interested in watching the same object, you don't want one module to
>> simply clear a the watches for a path because one module may mess up the
>> other.
>>
>> we have talked about adding this ability to clear watches for a while. i
>> think the auto-watch reregistration patch made the issue slightly more
>> pressing since it means that watches can survive for the entire lifetime of
>> a session not just for the duration of a connection to a specific server.
>> i've created ZOOKEEPER-442 to track this issue.
>>
>> ben
>>
>>
>> Henry Robinson wrote:
>>
>>> Hi Satish -
>>>
>>> As you've found out, you can set multiple identical watches per znode -
>>> the
>>> zookeeper client will not detect identical watches in case you really
>>> meant
>>> to call them several times. There's no way currently, as far as I know,
>>> to
>>> clear the watches once they've been set. So your options are either to
>>> avoid
>>> repeatedly setting them by detecting whether getChildren is a repeat
>>> call,
>>> or by dealing with multiple invocations on the callback path and not
>>> doing
>>> anything once you've established you're no longer interested.
>>>
>>> It might well make sense to add a clearWatches(path) call to the API,
>>> which
>>> would be useful particularly for clients where callbacks are expensive
>>> and
>>> require a context switch (which I think is true for all clients right
>>> now!).
>>>
>>> Henry
>>>
>>> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com>
>>> wrote:
>>>
>>>
>>>
>>>> I am using the asynchronous (callback) version of
>>>> zookeeper.getChildren().
>>>> That call returns immediately, I then wait for a certain time interval
>>>> for
>>>> nodes to appear, and if not I exit the method that made the
>>>> zookeeper.getChildren()
>>>> call. Later on, a node gets added under that node and I see in my
>>>> logfile
>>>> that the Watcher.process() callback that I set above gets called. Now
>>>> if I
>>>> make 10 failed attempts to get a node using the above technique, and at
>>>> some
>>>> later time a node does get added, I see in the logfile that the
>>>> Watcher.process() ends up being called 10 times! Of course by this time
>>>> I
>>>> have totally lost interest in those callbacks. Question: Is there a
>>>> way
>>>> to
>>>> remove that asynchronous callback? i.e. If I make a asynchronous
>>>> zookeeper.getChildren()
>>>> call, wait time t, give up, at that point can I remove the async
>>>> callback?
>>>> Satish
>>>>
>>>>
>>>>
>>>
>>
>
Re: zookeeper.getChildren asynchronous callback
Posted by Ted Dunning <te...@gmail.com>.
One way to deal with this is to wrap the getchildren call with an object
that maintains a list of watchers. Then you can have just one watch set
which notifies all of the listener. This single object can keep resetting
the watch each time there is a change unless you tell it otherwise. Keeping
a boolean in this object would allow you to remember whether to keep setting
the watch. Adding a close() method to clear the listener list and stop
resets would also help.
By concentrating the watch handling into a single place, you should be able
to get control over tihs process. You can't turn it off quite as quickly as
if you could cancel the watch, but the effect is nearly the same.
Another trick is that you can isolate the watch in question to a special
session. Closing that session will eliminate the watch (and all other
watches in that same session).
On Thu, Jun 11, 2009 at 10:14 AM, Satish Bhatti <ct...@gmail.com> wrote:
> That's right Ben. Basically, I would like to use it something like this:
> public boolean waitForChildrenChanged( String rootPath,
> long timeout )
> {
> BooleanLock blChildrenChanged = new BooleanLock();
>
> Watcher tempWatcher =
> new Watcher()
> {
> public void process( WatchedEvent event )
> {
> logger.debug( "waitForAnyEntry(): Got state event: " +
> ZooKeeperUtils.watchedEventToString( event ) );
> blChildrenChanged.setValue( true );
> }
> };
>
> zookeeper.getChildren( rootPath, watcher,
>
> new AsyncCallback.ChildrenCallback()
> {
> public void processResult( int rc, String path, Object ctx,
> List<String> children )
> {
> logger.debug( "waitForChildrenChanged():
> AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " + ctx + ", "
> +
> children );
> }
> }, null );
>
> blChildrenChanged.waitUntilTrue( timeout );
>
> zookeeper.removeWatch( tempWatcher );
>
> return blChildrenChanged.isTrue();
> }
>
> The only piece missing from the API is the zookeeper.removeWatch(
> tempWatcher );
>
> Satish
>
>
> On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <br...@yahoo-inc.com>
> wrote:
>
> > just to clarify i believe you are talking about callbacks on the watch
> > object you are passing in the asynchronous call rather than the
> asynchronous
> > completion callback. (Henry is making the same assumption.) when you say
> you
> > are getting the callback 10 times, i believe your are talking about 10
> > different watch objects getting called back once each. right?
> >
> > it turns out that the zookeeper client does know what you are watching,
> and
> > the zookeeper server will only register one watch. the thing that is
> missing
> > is the clearWatches call that Henry refers to. the thing that complicates
> > things a bit, perhaps not for you, is the scenario where we have
> different
> > modules sharing the same zookeeper handle. if different modules are
> > interested in watching the same object, you don't want one module to
> simply
> > clear a the watches for a path because one module may mess up the other.
> >
> > we have talked about adding this ability to clear watches for a while. i
> > think the auto-watch reregistration patch made the issue slightly more
> > pressing since it means that watches can survive for the entire lifetime
> of
> > a session not just for the duration of a connection to a specific server.
> > i've created ZOOKEEPER-442 to track this issue.
> >
> > ben
> >
> >
> > Henry Robinson wrote:
> >
> >> Hi Satish -
> >>
> >> As you've found out, you can set multiple identical watches per znode -
> >> the
> >> zookeeper client will not detect identical watches in case you really
> >> meant
> >> to call them several times. There's no way currently, as far as I know,
> to
> >> clear the watches once they've been set. So your options are either to
> >> avoid
> >> repeatedly setting them by detecting whether getChildren is a repeat
> call,
> >> or by dealing with multiple invocations on the callback path and not
> doing
> >> anything once you've established you're no longer interested.
> >>
> >> It might well make sense to add a clearWatches(path) call to the API,
> >> which
> >> would be useful particularly for clients where callbacks are expensive
> and
> >> require a context switch (which I think is true for all clients right
> >> now!).
> >>
> >> Henry
> >>
> >> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com>
> >> wrote:
> >>
> >>
> >>
> >>> I am using the asynchronous (callback) version of
> >>> zookeeper.getChildren().
> >>> That call returns immediately, I then wait for a certain time interval
> >>> for
> >>> nodes to appear, and if not I exit the method that made the
> >>> zookeeper.getChildren()
> >>> call. Later on, a node gets added under that node and I see in my
> >>> logfile
> >>> that the Watcher.process() callback that I set above gets called. Now
> if
> >>> I
> >>> make 10 failed attempts to get a node using the above technique, and at
> >>> some
> >>> later time a node does get added, I see in the logfile that the
> >>> Watcher.process() ends up being called 10 times! Of course by this
> time
> >>> I
> >>> have totally lost interest in those callbacks. Question: Is there a
> way
> >>> to
> >>> remove that asynchronous callback? i.e. If I make a asynchronous
> >>> zookeeper.getChildren()
> >>> call, wait time t, give up, at that point can I remove the async
> >>> callback?
> >>> Satish
> >>>
> >>>
> >>>
> >>
> >
>
--
Ted Dunning, CTO
DeepDyve
111 West Evelyn Ave. Ste. 202
Sunnyvale, CA 94086
http://www.deepdyve.com
858-414-0013 (m)
408-773-0220 (fax)
Re: zookeeper.getChildren asynchronous callback
Posted by Mahadev Konar <ma...@yahoo-inc.com>.
Agreed... It is a nice api to have and also reduces our memory footprint for
unwanted watches (as Ben suggested earlier).
mahadev
On 6/11/09 10:14 AM, "Satish Bhatti" <ct...@gmail.com> wrote:
> That's right Ben. Basically, I would like to use it something like this:
> public boolean waitForChildrenChanged( String rootPath,
> long timeout )
> {
> BooleanLock blChildrenChanged = new BooleanLock();
>
> Watcher tempWatcher =
> new Watcher()
> {
> public void process( WatchedEvent event )
> {
> logger.debug( "waitForAnyEntry(): Got state event: " +
> ZooKeeperUtils.watchedEventToString( event ) );
> blChildrenChanged.setValue( true );
> }
> };
>
> zookeeper.getChildren( rootPath, watcher,
>
> new AsyncCallback.ChildrenCallback()
> {
> public void processResult( int rc, String path, Object ctx,
> List<String> children )
> {
> logger.debug( "waitForChildrenChanged():
> AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " + ctx + ", " +
> children );
> }
> }, null );
>
> blChildrenChanged.waitUntilTrue( timeout );
>
> zookeeper.removeWatch( tempWatcher );
>
> return blChildrenChanged.isTrue();
> }
>
> The only piece missing from the API is the zookeeper.removeWatch(
> tempWatcher );
>
> Satish
>
>
> On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <br...@yahoo-inc.com> wrote:
>
>> just to clarify i believe you are talking about callbacks on the watch
>> object you are passing in the asynchronous call rather than the asynchronous
>> completion callback. (Henry is making the same assumption.) when you say you
>> are getting the callback 10 times, i believe your are talking about 10
>> different watch objects getting called back once each. right?
>>
>> it turns out that the zookeeper client does know what you are watching, and
>> the zookeeper server will only register one watch. the thing that is missing
>> is the clearWatches call that Henry refers to. the thing that complicates
>> things a bit, perhaps not for you, is the scenario where we have different
>> modules sharing the same zookeeper handle. if different modules are
>> interested in watching the same object, you don't want one module to simply
>> clear a the watches for a path because one module may mess up the other.
>>
>> we have talked about adding this ability to clear watches for a while. i
>> think the auto-watch reregistration patch made the issue slightly more
>> pressing since it means that watches can survive for the entire lifetime of
>> a session not just for the duration of a connection to a specific server.
>> i've created ZOOKEEPER-442 to track this issue.
>>
>> ben
>>
>>
>> Henry Robinson wrote:
>>
>>> Hi Satish -
>>>
>>> As you've found out, you can set multiple identical watches per znode -
>>> the
>>> zookeeper client will not detect identical watches in case you really
>>> meant
>>> to call them several times. There's no way currently, as far as I know, to
>>> clear the watches once they've been set. So your options are either to
>>> avoid
>>> repeatedly setting them by detecting whether getChildren is a repeat call,
>>> or by dealing with multiple invocations on the callback path and not doing
>>> anything once you've established you're no longer interested.
>>>
>>> It might well make sense to add a clearWatches(path) call to the API,
>>> which
>>> would be useful particularly for clients where callbacks are expensive and
>>> require a context switch (which I think is true for all clients right
>>> now!).
>>>
>>> Henry
>>>
>>> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com>
>>> wrote:
>>>
>>>
>>>
>>>> I am using the asynchronous (callback) version of
>>>> zookeeper.getChildren().
>>>> That call returns immediately, I then wait for a certain time interval
>>>> for
>>>> nodes to appear, and if not I exit the method that made the
>>>> zookeeper.getChildren()
>>>> call. Later on, a node gets added under that node and I see in my
>>>> logfile
>>>> that the Watcher.process() callback that I set above gets called. Now if
>>>> I
>>>> make 10 failed attempts to get a node using the above technique, and at
>>>> some
>>>> later time a node does get added, I see in the logfile that the
>>>> Watcher.process() ends up being called 10 times! Of course by this time
>>>> I
>>>> have totally lost interest in those callbacks. Question: Is there a way
>>>> to
>>>> remove that asynchronous callback? i.e. If I make a asynchronous
>>>> zookeeper.getChildren()
>>>> call, wait time t, give up, at that point can I remove the async
>>>> callback?
>>>> Satish
>>>>
>>>>
>>>>
>>>
>>
Re: zookeeper.getChildren asynchronous callback
Posted by Satish Bhatti <ct...@gmail.com>.
That's right Ben. Basically, I would like to use it something like this:
public boolean waitForChildrenChanged( String rootPath,
long timeout )
{
BooleanLock blChildrenChanged = new BooleanLock();
Watcher tempWatcher =
new Watcher()
{
public void process( WatchedEvent event )
{
logger.debug( "waitForAnyEntry(): Got state event: " +
ZooKeeperUtils.watchedEventToString( event ) );
blChildrenChanged.setValue( true );
}
};
zookeeper.getChildren( rootPath, watcher,
new AsyncCallback.ChildrenCallback()
{
public void processResult( int rc, String path, Object ctx,
List<String> children )
{
logger.debug( "waitForChildrenChanged():
AsyncCallback.ChildrenCallback(): " + rc + ", " + path + ", " + ctx + ", " +
children );
}
}, null );
blChildrenChanged.waitUntilTrue( timeout );
zookeeper.removeWatch( tempWatcher );
return blChildrenChanged.isTrue();
}
The only piece missing from the API is the zookeeper.removeWatch(
tempWatcher );
Satish
On Thu, Jun 11, 2009 at 7:09 AM, Benjamin Reed <br...@yahoo-inc.com> wrote:
> just to clarify i believe you are talking about callbacks on the watch
> object you are passing in the asynchronous call rather than the asynchronous
> completion callback. (Henry is making the same assumption.) when you say you
> are getting the callback 10 times, i believe your are talking about 10
> different watch objects getting called back once each. right?
>
> it turns out that the zookeeper client does know what you are watching, and
> the zookeeper server will only register one watch. the thing that is missing
> is the clearWatches call that Henry refers to. the thing that complicates
> things a bit, perhaps not for you, is the scenario where we have different
> modules sharing the same zookeeper handle. if different modules are
> interested in watching the same object, you don't want one module to simply
> clear a the watches for a path because one module may mess up the other.
>
> we have talked about adding this ability to clear watches for a while. i
> think the auto-watch reregistration patch made the issue slightly more
> pressing since it means that watches can survive for the entire lifetime of
> a session not just for the duration of a connection to a specific server.
> i've created ZOOKEEPER-442 to track this issue.
>
> ben
>
>
> Henry Robinson wrote:
>
>> Hi Satish -
>>
>> As you've found out, you can set multiple identical watches per znode -
>> the
>> zookeeper client will not detect identical watches in case you really
>> meant
>> to call them several times. There's no way currently, as far as I know, to
>> clear the watches once they've been set. So your options are either to
>> avoid
>> repeatedly setting them by detecting whether getChildren is a repeat call,
>> or by dealing with multiple invocations on the callback path and not doing
>> anything once you've established you're no longer interested.
>>
>> It might well make sense to add a clearWatches(path) call to the API,
>> which
>> would be useful particularly for clients where callbacks are expensive and
>> require a context switch (which I think is true for all clients right
>> now!).
>>
>> Henry
>>
>> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com>
>> wrote:
>>
>>
>>
>>> I am using the asynchronous (callback) version of
>>> zookeeper.getChildren().
>>> That call returns immediately, I then wait for a certain time interval
>>> for
>>> nodes to appear, and if not I exit the method that made the
>>> zookeeper.getChildren()
>>> call. Later on, a node gets added under that node and I see in my
>>> logfile
>>> that the Watcher.process() callback that I set above gets called. Now if
>>> I
>>> make 10 failed attempts to get a node using the above technique, and at
>>> some
>>> later time a node does get added, I see in the logfile that the
>>> Watcher.process() ends up being called 10 times! Of course by this time
>>> I
>>> have totally lost interest in those callbacks. Question: Is there a way
>>> to
>>> remove that asynchronous callback? i.e. If I make a asynchronous
>>> zookeeper.getChildren()
>>> call, wait time t, give up, at that point can I remove the async
>>> callback?
>>> Satish
>>>
>>>
>>>
>>
>
Re: zookeeper.getChildren asynchronous callback
Posted by Benjamin Reed <br...@yahoo-inc.com>.
just to clarify i believe you are talking about callbacks on the watch
object you are passing in the asynchronous call rather than the
asynchronous completion callback. (Henry is making the same assumption.)
when you say you are getting the callback 10 times, i believe your are
talking about 10 different watch objects getting called back once each.
right?
it turns out that the zookeeper client does know what you are watching,
and the zookeeper server will only register one watch. the thing that is
missing is the clearWatches call that Henry refers to. the thing that
complicates things a bit, perhaps not for you, is the scenario where we
have different modules sharing the same zookeeper handle. if different
modules are interested in watching the same object, you don't want one
module to simply clear a the watches for a path because one module may
mess up the other.
we have talked about adding this ability to clear watches for a while. i
think the auto-watch reregistration patch made the issue slightly more
pressing since it means that watches can survive for the entire lifetime
of a session not just for the duration of a connection to a specific
server. i've created ZOOKEEPER-442 to track this issue.
ben
Henry Robinson wrote:
> Hi Satish -
>
> As you've found out, you can set multiple identical watches per znode - the
> zookeeper client will not detect identical watches in case you really meant
> to call them several times. There's no way currently, as far as I know, to
> clear the watches once they've been set. So your options are either to avoid
> repeatedly setting them by detecting whether getChildren is a repeat call,
> or by dealing with multiple invocations on the callback path and not doing
> anything once you've established you're no longer interested.
>
> It might well make sense to add a clearWatches(path) call to the API, which
> would be useful particularly for clients where callbacks are expensive and
> require a context switch (which I think is true for all clients right now!).
>
> Henry
>
> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com> wrote:
>
>
>> I am using the asynchronous (callback) version of zookeeper.getChildren().
>> That call returns immediately, I then wait for a certain time interval for
>> nodes to appear, and if not I exit the method that made the
>> zookeeper.getChildren()
>> call. Later on, a node gets added under that node and I see in my logfile
>> that the Watcher.process() callback that I set above gets called. Now if I
>> make 10 failed attempts to get a node using the above technique, and at
>> some
>> later time a node does get added, I see in the logfile that the
>> Watcher.process() ends up being called 10 times! Of course by this time I
>> have totally lost interest in those callbacks. Question: Is there a way
>> to
>> remove that asynchronous callback? i.e. If I make a asynchronous
>> zookeeper.getChildren()
>> call, wait time t, give up, at that point can I remove the async callback?
>> Satish
>>
>>
Re: zookeeper.getChildren asynchronous callback
Posted by Henry Robinson <he...@cloudera.com>.
Hi Satish -
As you've found out, you can set multiple identical watches per znode - the
zookeeper client will not detect identical watches in case you really meant
to call them several times. There's no way currently, as far as I know, to
clear the watches once they've been set. So your options are either to avoid
repeatedly setting them by detecting whether getChildren is a repeat call,
or by dealing with multiple invocations on the callback path and not doing
anything once you've established you're no longer interested.
It might well make sense to add a clearWatches(path) call to the API, which
would be useful particularly for clients where callbacks are expensive and
require a context switch (which I think is true for all clients right now!).
Henry
On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <ct...@gmail.com> wrote:
> I am using the asynchronous (callback) version of zookeeper.getChildren().
> That call returns immediately, I then wait for a certain time interval for
> nodes to appear, and if not I exit the method that made the
> zookeeper.getChildren()
> call. Later on, a node gets added under that node and I see in my logfile
> that the Watcher.process() callback that I set above gets called. Now if I
> make 10 failed attempts to get a node using the above technique, and at
> some
> later time a node does get added, I see in the logfile that the
> Watcher.process() ends up being called 10 times! Of course by this time I
> have totally lost interest in those callbacks. Question: Is there a way
> to
> remove that asynchronous callback? i.e. If I make a asynchronous
> zookeeper.getChildren()
> call, wait time t, give up, at that point can I remove the async callback?
> Satish
>