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From Adam Rosien <a...@rosien.net>
Subject Re: ZKClient
Date Tue, 04 May 2010 21:58:43 GMT
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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