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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: NamespaceWatcher hashCode and equals still bugging me
Date Wed, 10 Feb 2016 16:42:50 GMT
Yeah, a weak map would’ve made things easier but the map itself is unnecessary. When I wrote
it I wasn’t sure how ZK was implemented internally. Of course, I’m now taking advantage
of internal knowledge of ZK but there’s a lot of that in Curator and I feel pretty confident
it won’t change anytime soon.

NamespaceWatcher is a package protected internal class and is only ever used to wrap passed
in Watchers/CuratorWatchers and then passed into ZK. So, the missing comparisons don’t concern
me. 

> The only part that bugs me is having NamespaceWatcher.equals(raw Watcher).

This is required and is the “magic” that makes removing the Map possible. This way, I
can pass in new NamespaceWatcher instances each time but have them compare equal to the wrapped
Watcher. This is vital. What this is doing is creating a proxy that allows a passed in Watcher
to be wrapped but appear as equal inside of ZK.

-Jordan

> On Feb 10, 2016, at 11:30 AM, Scott Blum <dragonsinth@gmail.com> wrote:
> 
> Here's where I am right this second.  I looked back over commit ff8a795e61d0d44622bdbaf2144c25c70e31e864,
and I think I understand it about 90%.  I suspect the issue might have been solved by simply
having the original NamespaceWatcherMap have weak keys and weak values-- it only had weak
values, but again I don't have the 100% view on this.
> 
> That said, the new code seems much cleaner to me.  And in general, having NamespaceWatcher.equals(NamespaceWatcher)
seems 100% legit to me.  If we're only ever passing NamespaceWatcher instances to the ZK layer
to add and remove, that seems great.
> 
> The only part that bugs me is having NamespaceWatcher.equals(raw Watcher).  If we're
relying on this behavior anywhere, it's a recipe for problems.  If we're NOT relying on this
behavior, then we should rip some code out of NamespaceWatcher and have it so that a NamespaceWatcher
can only equals another NamespaceWatcher.
> 
> What do you think?
> 
> 
> On Wed, Feb 10, 2016 at 8:48 AM, Jordan Zimmerman <jordan@jordanzimmerman.com <mailto:jordan@jordanzimmerman.com>>
wrote:
> Scott - are you OK with a release or should I wait for more discussion on this issue?
> 
> -Jordan
> 
>> On Feb 9, 2016, at 1:50 PM, Scott Blum <dragonsinth@gmail.com <mailto:dragonsinth@gmail.com>>
wrote:
>> 
>> Sounds like a job for weak hash map. Will follow up later with more
>> 
>> On Feb 9, 2016 12:01 PM, "Jordan Zimmerman" <jordan@jordanzimmerman.com <mailto:jordan@jordanzimmerman.com>>
wrote:
>> > So.... taking a step back, what was underlying motivation for the hashCode /
equality changes?  IE, what's the bigger problem we were trying to solve?
>> 
>> Before this change, we were maintaining a map from Watcher to NamespaceWatcher so
that we could track/remove the wrapped watcher. This is necessary due to this guarantee of
ZooKeeper:
>> 
>> http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#sc_WatchGuarantees
<http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#sc_WatchGuarantees>
>> 
>> "if the same watch object is registered for an exists and a getData call for the
same file and that file is then deleted, the watch object would only be invoked once with
the deletion notification for the file.”
>> 
>> Given that NamespaceWatcher is an internal wrapper, Curator needs to generate the
same NamespaceWatcher for a given client’s Watcher/CuratorWatcher. The map handled this.
In the past, this was difficult to manage and had potential memory leaks if the map wasn’t
managed correctly. It occurred to me that the map isn’t needed if NamespaceWatcher could
have equality/hash values the same as the Watcher that it wraps. My testing proved this.
>> 
>> Thoughts?
>> 
>> -Jordan
>> 
>> 
>> > On Feb 9, 2016, at 11:49 AM, Scott Blum <dragonsinth@gmail.com <mailto:dragonsinth@gmail.com>>
wrote:
>> >
>> > Hi guys,
>> >
>> > I'm a practical guy, not a purist, but the 3.0 implementations of NamespaceWatcher.hashCode()
and equals() are bothering me.  The reason I care is that I want to avoid subtle bugs cropping
up.
>> >
>> > So here's the problem.
>> >
>> > 1) equals() is not reflexive between NamespaceWatcher and Watcher
>> >
>> > Assuming you have a NamespaceWatcher nw wrapping a Watcher w, the following
code might or might not work:
>> >
>> > container.add(nw)
>> > container.remove(w)
>> >
>> > It depends on whether the underlying container ultimately does "nw.equals(w)"
or "w.equals(nw)".  Set.contains() would have the same problem.
>> >
>> > 2) hashCode() and equals() inconsistent with each other
>> >
>> > Because nw.hashCode() != w.hashCode(), lookups in a hashSet or hashMap will
practically never work except by luck.
>> >
>> > hashSet.put(nw)
>> > hashSet.contains(w)
>> >
>> > Most of the time this will return false, except in the exact case where nw and
w happen to have hashCodes that map into the same bucket, and the equality check is done the
"right" order.
>> >
>> >
>> > So.... taking a step back, what was underlying motivation for the hashCode /
equality changes?  IE, what's the bigger problem we were trying to solve?
>> >
>> > Scott
>> >
>> 
> 
> 


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