While I agree DS is hard, I don't think we should lose the useful feedback given by Jonathan/Adam - that getting started with ZK is challenging and can be frustrating. We need to learn from this feedback and create some action items to address. One of the main things I've heard so far that we can act on today is that we should add examples/docs to round things out. I agree with this. Also the recipes page should be updated to point to the recipe implementations we recently added to the release.

One suggestion, it's much easier for new contributors/users to contribute to the examples than it is to jump into ZK core development. New users feel the pain most directly (recently), I'd encourage you to contribute back by creating an example or two. I'm sure the existing contributors would be happy to work with you to get them committed and released.



On 05/04/2010 03:43 PM, Ted Dunning wrote:
Creating recipes is a great thing, but that doesn't change the fact that
distributed systems are inherently a bit tricky, especially if you start
with the assumption (as many people do) that Peter Deutsch was wrong.

One of the great contributions of MapReduce style parallelism or the java
concurrent package is that it provides safe trails in a pretty scary forest.
  Good Zookeeper recipes could provide similar guidance with similar positive

On Tue, May 4, 2010 at 3:24 PM, Adam Rosien<a...@rosien.net>  wrote:

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<maha...@yahoo-inc.com>
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
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
that folks havent had much time to focus on these areas as yet.


On 5/4/10 2:58 PM, "Adam Rosien"<a...@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.dunn...@gmail.com>
In general, writing this sort of layer on top of ZK is very, very hard
get really right for general use.  In a simple use-case, you can
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
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
they will.  That makes it really, really easy to introduce that
in the heads of the programmer using the event listener library on top
ZK.  Another example is how the atomic get content/set watch call works
ZK is easy to violate in an event driven architecture because the
that watches ZK probably resets the watch.  If you assume that the
will read the data, then you have introduced a timing mismatch between
read of the data and the resetting of the watch.  That might be OK or
might not be.  The point is that these changes are subtle and tricky to
exactly right.

On Tue, May 4, 2010 at 1:48 PM, Jonathan Holloway<
jonathan.hollo...@gmail.com>  wrote:

Is there any reason why this isn't part of the Zookeeper trunk

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