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From Patrick Hunt <ph...@apache.org>
Subject Re: Why are ephemeral nodes written to disk?
Date Wed, 17 Jan 2018 23:47:22 GMT
NP, sorry for the slow response but I've been out on vacation the past few
weeks. ;-)

Regards,

Patrick

On Wed, Jan 17, 2018 at 3:39 PM, Jeff Widman <jeff@jeffwidman.com> wrote:

> Thank you. I did not realize sessions could continue even if the ensemble
> was shutdown.
>
> On Jan 17, 2018 3:28 PM, "Patrick Hunt" <phunt@apache.org> wrote:
>
> > On Tue, Jan 9, 2018 at 12:38 PM, Jeff Widman <jeff@jeffwidman.com>
> wrote:
> >
> > > Ephemeral nodes only exist for the life of the client session.
> > >
> > > As far as I understand, by definition, a client session ends when the
> > > entire zookeeper ensemble goes down.
> > >
> > > So I would expect that ephemeral nodes are only written to memory, not
> > > disk. The ephemeral nodes would be sync'd across machines as a client
> > > session can span multiple connections if a single zk server fails, but
> > once
> > > the ensemble is down there is no need to recover the ephemeral nodes
> from
> > > disk.
> > >
> > > However, when I looked at a zookeeper ensemble that is 99% ephemeral
> > nodes,
> > > I see a bunch of disk I/O from the zookeeper processes. So it appears
> > that
> > > ephemeral nodes are still written to disk...
> > >
> > > Why is this?
> > >
> >
> > Ephemeral znodes are treated just like persistent znodes in the sense
> that
> > a quorum of nodes need to agree to any change. As such the znode is
> written
> > to the transaction log.
> >
> > "a client session ends when the entire zookeeper ensemble goes down"
> >
> > is not correct. A client session ends either when a client closes it's
> > session explicitly or the ZK quorum leader decides that the session has
> > expired (which is based on the negotiated session timeout). Only while a
> > leader is active can a session be expired (or closed for that matter).
> When
> > you shutdown an ensemble the sessions are maintained. If you were to, for
> > example, shut down an ensemble for an hour and then restart it the
> sessions
> > would still be active. The clock would "reset" when the new leader was
> > elected. If the client session is still active the session would
> continue,
> > any ephemeral znodes would still exist.
> >
> > Patrick
> >
> >
> > >
> > > --
> > >
> > > *Jeff Widman*
> > > jeffwidman.com <http://www.jeffwidman.com/> | 740-WIDMAN-J (943-6265)
> > > <><
> > >
> >
>

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