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From Jonathan Holloway <jonathan.hollo...@gmail.com>
Subject Re: Sequence Number Generation With Zookeeper
Date Fri, 06 Aug 2010 02:59:15 GMT
Great, I just need to overcome the Integer.MAX_VALUE bit, maybe by having
SEQUENCE_LOW and SEQUENCE_HIGH nodes, being careful with the rollover.
 Thanks again.

On 5 August 2010 17:54, Ted Dunning <ted.dunning@gmail.com> wrote:

> Sounds right to me.  Much simpler as well.
>
> On Thu, Aug 5, 2010 at 5:38 PM, Jonathan Holloway <
> jonathan.holloway@gmail.com> wrote:
>
> > Hi Ted,
> >
> > Thanks for the comments.
> >
> > I might have overlooked something here, but is it also possible to do the
> > following:
> >
> > 1. Create a PERSISTENT node
> > 2. Have multiple clients set the data on the node, e.g.  Stat stat =
> > zookeeper.setData(SEQUENCE, ArrayUtils.EMPTY_BYTE_ARRAY, -1);
> > 3. Use the version number from stat.getVersion() as the sequence
> (obviously
> > I'm limited to Integer.MAX_VALUE)
> >
> > Are there any weird race conditions involved here which would mean that a
> > client would receive the wrong Stat object back?
> >
> > Many thanks again,
> > Jon.
> >
> > On 5 August 2010 16:09, Ted Dunning <ted.dunning@gmail.com> wrote:
> >
> > > (b)
> > >
> > > BUT:
> > >
> > > Sequential numbering is a special case of "now".  In large diameters,
> now
> > > gets very expensive.  This is a special case of that assertion.  If
> there
> > > is
> > > a way to get away from this presumption of the need for sequential
> > > numbering, you will be miles better off.
> > >
> > > HOWEVER:
> > >
> > > ZK can do better than you suggest.  Incrementing a counter does involve
> > > potential contention, but you will very likely be able to get to pretty
> > > high
> > > rates before the optimistic locking begins to fail.  If you code your
> > > update
> > > with a few tries at full speed followed by some form of retry back-off,
> > you
> > > should get pretty close to the best possible performance.
> > >
> > > You might also try building a lock with an ephemeral file before
> updating
> > > the counter.  I would expect that this will be slower than the back-off
> > > option if only because involves more transactions in ZK.  IF you wanted
> > to
> > > get too complicated for your own good, you could have a secondary
> > strategy
> > > flag that is only sampled by all clients every few seconds and is
> updated
> > > whenever a client needs to back-off more than say 5 steps.  If this
> flag
> > > has
> > > been updated recently, then clients should switch to the locking
> > protocol.
> > >  You might even have several locks so that you don't exclude all other
> > > updaters, merely thin them out a bit.  This flagged strategy would run
> as
> > > fast as optimistic locking as long as optimistic locking is fast and
> then
> > > would limit the total number of transactions needed under very high
> load.
> > >
> > > On Thu, Aug 5, 2010 at 3:31 PM, Jonathan Holloway <
> > > jonathan.holloway@gmail.com> wrote:
> > >
> > > > My so far involve:
> > > > a) Creating a node with PERSISTENT_SEQUENTIAL then deleting it - this
> > > gives
> > > > me the monotonically increasing number, but the sequence number isn't
> > > > contiguous
> > > > b) Storing the sequence number in the data portion of a persistent
> node
> > -
> > > > then updating this (using the version number - aka optimistic
> locking).
> > > >  The
> > > > problem with this is that under high load I'm assuming there'll be a
> > lot
> > > of
> > > > contention and hence failures with regards to updates.
> > > >
> > >
> >
>

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