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From Jeff Widman <j...@jeffwidman.com>
Subject Re: Why are ephemeral nodes written to disk?
Date Wed, 17 Jan 2018 23:39:08 GMT
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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