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From: Ted Dunning <ted.dunning@gmail.com>
Date: Wed, 21 Jul 2010 10:39:56 -0700
Message-ID: <AANLkTin6KKwwaTsZQt2ocrw4CDx4AsfIndjroDwsP-2L@mail.gmail.com>
Subject: Re: Adding observers
To: zookeeper-user@hadoop.apache.org
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On Wed, Jul 21, 2010 at 10:30 AM, Avinash Lakshman <
avinash.lakshman@gmail.com> wrote:

>
> (1) Is it possible to increase the number of observers in the cluster
> dynamically?
>

Not quite, but practically speaking you can do as good as this.

In general, pretty much any ZK configuration change can be done without
service interruption by using a rolling restart.


> (2) How many observers can I add given that I will seldom write into the
> cluster but will have a lot of reads coming into the system? Can I run a
> cluster with say 100 observers?
>

Others will give more authoritative answers, but I am pretty sure that the
limitation on the number of observers is strictly related to write rate x
number of observers.  This is related to the fact that writes need to come
from the current master.  It isn't hard to imagine how to write a reflector
that watches for all changes and writes these to a secondary cluster.  That
would essentially eliminate the limit on number of observers.  Something
like that may already be possible within the current system (I couldn't say
since I haven't looked into observers that much).

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