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From Adam Rosien <a...@rosien.net>
Subject Re: ZKClient
Date Tue, 04 May 2010 22:24:26 GMT
I'll check it out, but it is repeated in this list and on the web site
that it's not as easy as it seems. I just want to enumerate the
failure points and create abstractions to avoid them.

.. Adam

On Tue, May 4, 2010 at 3:16 PM, Mahadev Konar <mahadev@yahoo-inc.com> wrote:
> Hi Adam,
>  I don't think zk is very very hard to get right. There are exmaples in
> src/recipes which implements locks/queues/others. There is ZOOKEEPER-22 to
> make it even more easier for application to use.
> Regarding re registration of watches, you can deifnitely write code and
> submit is as a part of well documented contrib module which lays out the
> assumptions/design of it. It could very well be useful for others. Its just
> that folks havent had much time to focus on these areas as yet.
> Thanks
> mahadev
> On 5/4/10 2:58 PM, "Adam Rosien" <adam@rosien.net> wrote:
>> I use zkclient in my work at kaChing and I have mixed feelings about
>> it. On one hand it makes "easy things easy" which is great, but on the
>> other hand I very few ideas what assumptions it makes "under the
>> hood". I also dislike some of the design choices such as unchecked
>> exceptions, but that's neither here nor there. It would take some
>> extensive documentation work by the authors to really enumerate the
>> model and assumptions, but the project doesn't seem to be active
>> (either from it being adequate for its current users or just
>> inactive). I'm not sure I could derive the assumptions myself.
>> I'm a bit frustrated that zk is "very, very hard to really get right".
>> At a project level, can't we create structures to avoid most of these
>> errors? Can there be a "standard model" with detailed assumptions and
>> implementations of all the recipes? How can we start this? Is there
>> something that makes this too hard?
>> I feel like a recipe page is a big fail; wouldn't an example app that
>> uses locks and barriers be that much more compelling?
>> For the common FAQ items like "you need to re-register the watch",
>> can't we just create code that implements this pattern? My goal is to
>> live up to the motto: a good API is impossible to use incorrectly.
>> .. Adam
>> On Tue, May 4, 2010 at 2:21 PM, Ted Dunning <ted.dunning@gmail.com> wrote:
>>> In general, writing this sort of layer on top of ZK is very, very hard to
>>> get really right for general use.  In a simple use-case, you can probably
>>> nail it but distributed systems are a Zoo, to coin a phrase.  The problem is
>>> that you are fundamentally changing the metaphors in use so assumptions can
>>> come unglued or be introduced pretty easily.
>>> One example of this is the fact that ZK watches *don't* fire for every
>>> change but when you write listener oriented code, you kind of expect that
>>> they will.  That makes it really, really easy to introduce that assumption
>>> in the heads of the programmer using the event listener library on top of
>>> ZK.  Another example is how the atomic get content/set watch call works in
>>> ZK is easy to violate in an event driven architecture because the thread
>>> that watches ZK probably resets the watch.  If you assume that the listener
>>> will read the data, then you have introduced a timing mismatch between the
>>> read of the data and the resetting of the watch.  That might be OK or it
>>> might not be.  The point is that these changes are subtle and tricky to get
>>> exactly right.
>>> On Tue, May 4, 2010 at 1:48 PM, Jonathan Holloway <
>>> jonathan.holloway@gmail.com> wrote:
>>>> Is there any reason why this isn't part of the Zookeeper trunk already?

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