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From Alexander Shraer <shra...@gmail.com>
Subject Re: Dynamic reconfiguration
Date Sat, 28 Jul 2012 00:20:56 GMT
Hi Jared,

Thanks for experimenting with this feature.

The idea is that new servers join as "non voting followers". Which means
that they act as normal followers but the leader ignores their votes since
they are not part of the current configuration. The leader only counts
their votes during the reconfiguration itself (to make sure a quorum of the
new config is ready before the new config can be committed/activated).
Defining them as observers is not a good idea, for example in your scenario
if they were observers they wouldn't be able to participate in the
reconfiguration protocol (which is similar to the protocol for committing
any other operation in which observers don't participate) and since we
don't have a quorum of followers in the new config that can ack,
reconfiguration would throw an exception (of
KeeperException.NEWCONFIGNOQUORUM type).
Of course if you intend them to be observers in the new config you can
define them as observers since their votes are not needed during reconfig

You're right, the new servers must be able to connect to the old quorum. At
minimum, their file should contain the current leader, but
you can also copy the current configuration file to the new members if you

In addition, you should add a line for the member itself, so that server F
appears in F's config file (Its not important that the other new servers
appear in F's file, but it won't hurt either, so you can do a union of old
and new if you wish). The constructor of QuorumPeer checks that the server
itself is in the configuration its started with, otherwise its not going to
run. This check has always been there, but I'm thinking of possibly
changing it in the future.

As soon as F connects to the leader, its config file will be overwritten
with the current config file as part of the synchronization process.


On Fri, Jul 27, 2012 at 10:06 AM, Jared Cantwell

> Hi,
> We are testing integration with 3.5.0 and dynamic membership and I have a
> question.  If I have a current set of servers in my ensemble {A,B,C,D,E}
> and I want to reconfigure the ensemble to {D,E,F,G,H}, how should the
> dynamic config file on servers F,G,H be configured on startup?  Should they
> have the old ensemble, the new ensemble, or the union of both ensembles?
>  It seems like these new servers need to  know about the old quorum, but
> since they aren't part of it yet its not clear to me how they should be
> configured.  Should there be an intermediate configuration with F,G, and H
> as simply Observers?
> I can't find much documentation on this so I want to make sure I understand
> things correctly.
> Thanks!
> ~Jared

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