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From Jordan Zimmerman <jzimmer...@netflix.com>
Subject Re: Use cases for ZooKeeper
Date Thu, 12 Jan 2012 17:52:58 GMT
Ted - are you interested in writing this on top of Curator? If not, I'll
give it a whack.


On 1/5/12 12:50 AM, "Ted Dunning" <ted.dunning@gmail.com> wrote:

>Jordan, I don't think that leader election does what Josh wants.
>I don't think that consistent hashing is particularly good for that either
>because the loss of one node causes the sequential state for lots of
>entities to move even among nodes that did not fail.
>What I would recommend is a variant of micro-sharding.  The key space is
>divided into many micro-shards.  Then nodes that are alive claim the
>micro-shards using ephemerals and proceed as Josh described.  On loss of a
>node, the shards that node was handling should be claimed by the remaining
>nodes.  When a new node appears or new work appears, it is helpful to
>direct nodes to effect a hand-off of traffic.
>In my experience, the best way to implement shard balancing is with and
>external master instance much in the style of hbase or katta.  This
>external master can be exceedingly simple and only needs to wake up on
>various events like loss of a node or change in the set of live shards.
>can also wake up at intervals if desired to backstop the normal
>notifications or to allow small changes for certain kinds of balancing.
> Typically, this only requires a few hundred lines of code.
>This external master can, of course, be run on multiple nodes and which
>master is in current control can be adjudicated with yet another leader
>You can view this as a package of many leader elections.  Or as
>consistent hashing.  The distinctions are a bit subtle but are very
>important.  These include,
>- there is a clean division of control between the master which determines
>who serves what and the nodes that do the serving
>- there is no herd effect because the master drives the assignments
>- node loss causes the minimum amount of change of assignments since no
>assignments to surviving nodes are disturbed.  This is a major win.
>- balancing is pretty good because there are many shards compared to the
>number of nodes.
>- the balancing strategy is highly pluggable.
>This pattern would make a nice addition to Curator, actually.  It comes up
>repeatedly in different contexts.
>On Thu, Jan 5, 2012 at 12:11 AM, Jordan Zimmerman
>> OK - so this is two options for doing the same thing. You use a Leader
>> Election algorithm to make sure that only one node in the cluster is
>> operating on a work unit. Curator has an implementation (it's really
>> a distributed lock with a slightly different API).
>> -JZ
>> On 1/5/12 12:04 AM, "Josh Stone" <pacesysjosh@gmail.com> wrote:
>> >Thanks for the response. Comments below:
>> >
>> >On Wed, Jan 4, 2012 at 10:46 PM, Jordan Zimmerman
>> ><jzimmerman@netflix.com>wrote:
>> >
>> >> Hi Josh,
>> >>
>> >> >Second use case: Distributed locking
>> >> This is one of the most common uses of ZooKeeper. There are many
>> >> implementations - one included with the ZK distro. Also, there is
>> >>Curator:
>> >> https://github.com/Netflix/curator
>> >>
>> >> >First use case: Distributing work to a cluster of nodes
>> >> This sounds feasible. If you give more details I and others on this
>> >> can help more.
>> >>
>> >
>> >Sure. I basically want to handle race conditions where two commands
>> >operate on the same data are received by my cluster of znodes,
>> >concurrently. One approach is to lock on the data that is effected by
>> >command (distributed lock). Another approach is make sure that all of
>> >commands that operate on any set of data are routed to the same node,
>> >where
>> >they can be processed serially using local synchronization. Consistent
>> >hashing is an algorithm that can be used to select a node to handle a
>> >message (where the inputs are the key to hash and the number of nodes
>> >the cluster).
>> >
>> >There are various implementations for this floating around. I'm just
>> >interesting to know how this is working for anyone else.
>> >
>> >Josh
>> >
>> >
>> >>
>> >> -JZ
>> >>
>> >> ________________________________________
>> >> From: Josh Stone [pacesysjosh@gmail.com]
>> >> Sent: Wednesday, January 04, 2012 8:09 PM
>> >> To: user@zookeeper.apache.org
>> >> Subject: Use cases for ZooKeeper
>> >>
>> >> I have a few use cases that I'm wondering if ZooKeeper would be
>> >> for and would appreciate some feedback.
>> >>
>> >> First use case: Distributing work to a cluster of nodes using
>> >> hashing to ensure that messages of some type are consistently
>>handled by
>> >> the same node. I haven't been able to find any info about ZooKeeper +
>> >> consistent hashing. Is anyone using it for this? A concern here
>>would be
>> >> how to redistribute work as nodes come and go from the cluster.
>> >>
>> >> Second use case: Distributed locking. I noticed that there's a recipe
>> >>for
>> >> this on the ZooKeeper wiki. Is anyone doing this? Any issues? One
>> >>concern
>> >> would be how to handle orphaned locks if a node that obtained a lock
>> >>goes
>> >> down.
>> >>
>> >> Third use case: Fault tolerance. If we utilized ZooKeeper to
>> >> messages to workers, can it be made to handle a node going down by
>> >> re-distributing the work to another node (perhaps messages that are
>> >> ack'ed within a timeout are resent)?
>> >>
>> >> Cheers,
>> >> Josh
>> >>

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