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From Jared Cantwell <jared.cantw...@gmail.com>
Subject Re: Dynamic reconfiguration
Date Sat, 28 Jul 2012 00:41:13 GMT
And a followup... does zookeeper only overwrite the dynamic configuration
file for nodes that are voting participants?  Such that if I started a
follower and then left it running through some reconfigurations, its file
would not get updated if it was never added as part of those


On Fri, Jul 27, 2012 at 6:35 PM, Jared Cantwell <jared.cantwell@gmail.com>wrote:

> So does just having the server started and pointing to the existing
> ensemble automatically make it a "non participating follower"?  In other
> words, there is no need to inform the existing nodes that this new node is
> joining as a follower?  And to extend that, there could be any number of
> followers that are simply listening in on the event stream?  I am assuming
> that's the case, and that it is a follower (and not participant) by virtue
> of not being in the official configuration stored in zookeeper itself.
> On Fri, Jul 27, 2012 at 6:29 PM, Alexander Shraer <shralex@gmail.com>wrote:
>> there are just two supported types - participant and observer.
>> (participant can act as either follower or leader).
>> So you can either write participant or leave it unspecified (which means
>> participant by default). Also, since the ip is the same for all your ports
>> you don't have to write it twice.  All of these should work in the same way:
>> server.5=;
>> server.5=;2181 <>
>> server.5=;
>> server.5=;2181 <>
>> On Fri, Jul 27, 2012 at 5:25 PM, Jared Cantwell <jared.cantwell@gmail.com
>> > wrote:
>>> Thanks Alex for the response.  Our current lines in the configuration
>>> look like this:
>>> server.5=;
>>> For the new servers is it ok for their entry to have "participant"?  Or
>>> should that be something different (e.g. "follower")?
>>> ~Jared
>>> On Fri, Jul 27, 2012 at 6:20 PM, Alexander Shraer <shralex@gmail.com>wrote:
>>>> 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
>>>> anyway.
>>>> 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 wish.
>>>> 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.
>>>> Alex
>>>> On Fri, Jul 27, 2012 at 10:06 AM, Jared Cantwell <
>>>> jared.cantwell@gmail.com> wrote:
>>>>> 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
>>>>> 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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