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From "Henry Robinson (JIRA)" <j...@apache.org>
Subject [jira] Created: (ZOOKEEPER-423) Add getFirstChild API
Date Mon, 01 Jun 2009 16:11:07 GMT
Add getFirstChild API
---------------------

                 Key: ZOOKEEPER-423
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-423
             Project: Zookeeper
          Issue Type: New Feature
          Components: contrib-bindings, documentation, java client, server
            Reporter: Henry Robinson


When building the distributed queue for my tutorial blog post, it was pointed out to me that
there's a serious inefficiency here. 

Informally, the items in the queue are created as sequential nodes. For a 'dequeue' call,
all items are retrieved and sorted by name by the client in order to find the name of the
next item to try and take. This costs O(n) bandwidth and O(n.log n) sorting time - per dequeue
call! Clearly this doesn't scale very well. 

If the servers were able to maintain a data structure that allowed them to efficiently retrieve
the children of a node in order of the zxid that created them this would make successful dequeue
operations O(1) at the cost of O(n) memory on the server (to maintain, e.g. a singly-linked
list as a queue). This is a win if it is generally true that clients only want the first child
in creation order, rather than the whole set. 

We could expose this to the client via this API: getFirstChild(handle, path, name_buffer,
watcher) which would have much the same semantics as getChildren, but only return one znode
name. 

Sequential nodes would still allow the ordering of znodes to be made explicitly available
to the client in one RPC should it need it. Although: since this ordering would now be available
cheaply for every set of children, it's not completely clear that there would be that many
use cases left for sequential nodes if this API was augmented with a getChildrenInCreationOrder
call. However, that's for a different discussion. 

A halfway-house alternative with more flexibility is to add an 'order' parameter to getFirstChild
and have the server compute the first child according to the requested order (creation time,
update time, lexicographical order). This saves bandwidth at the expense of increased server
load, although servers can be implemented to spend memory on pre-computing commonly requested
orders. I am only in favour of this approach if servers maintain a data-structure for every
possible order, and then the memory implications need careful consideration.

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