[
https://issues.apache.org/jira/browse/ZOOKEEPER-423?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13027055#comment-13027055
]
Lukas commented on ZOOKEEPER-423:
---------------------------------
Hi Henry,
I hope i can make it more clear :)
Given a scenario of a job queue with multiple workers. Each worker shall
process one job at the time and the jobs should be processed in an ordered
fashion (lets say LIFO in this case, could also be FIFO). Each job is
represented as a child node of some zookeeper node.
Each worker tries to process the newest job which is currently not processed by
another worker. For this, he retrieves the newest not locked child and creates
his lock (=ephemeral+sequential node) under this child node (grand child so to
say :).
If the worker has finished the job, he deletes the lock (grand child) and the
child (yes, here could be a race, but this can be circumvented).
In this scenario it is beneficial, if the worker can get the first N children
and try to lock one of them as some of them are already locked by other workers.
Also, getAndDelete is not appropriate, as in case of a worker failure (e.g.
hardware failure) the job is not in the queue anymore, but also not finished
which gives a zombie job.
By locking, I mean the zookeeper locking pattern using ephemeral+sequential
nodes.
Best,
Lukas
> 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.]
--
This message is automatically generated by JIRA.
For more information on JIRA, see: http://www.atlassian.com/software/jira
