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https://issues.apache.org/jira/browse/ZOOKEEPER-423?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13027015#comment-13027015
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Henry Robinson commented on ZOOKEEPER-423:
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Lukas -
Good suggestion. Could you describe the use case you're thinking of, so we can
properly weigh up the idea?
Thanks -
Henry
> 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
> Attachments: ZOOKEEPER-423.patch
>
>
> 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.
> [edit - JIRA interprets ( n ) without the spaces as a thumbs-down. cute.]
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