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From Alexander Shraer <shra...@gmail.com>
Subject Re: Dynamic reconfiguration
Date Sat, 28 Jul 2012 23:36:55 GMT
On second thought, your scenario would not be possible even if you initiate a server with the
current config plus only itself.
I agree that you should not do union of old and new configs.
Alex

Sent from mobile

On Jul 28, 2012, at 3:26 PM, Jared Cantwell <jared.cantwell@gmail.com> wrote:

> Having to "make up" a configuration for new servers that are non voting followers is
something I keep getting stuck on, because if a couple of these servers start with just the
wrong configuration they can all have an unspecified version and actually form a standalone
quorum and not know a newer one exists. I would feel much better if new servers that are to
become non voting followers could simply copy the configuration file from an existing voting
participant (including  the version) and still start up. This is particularly an issue for
us because servers with arbitrarily old configurations can start up at any time, and if the
wrong things happen then they could form a quorum if they have the right "bootstrap" configurations
with unspecified versions. 
> 
> Does this make sense as a concern? We may patch the server to allow starting up without
its myid in the configuration. 
> 
> Jared 
> 
> On Jul 28, 2012, at 12:15 PM, Alexander Shraer <shralex@gmail.com> wrote:
> 
>> yes, if the server reboots (this is when it would read the config file). 
>> Otherwise, it has the last config in memory (this is held in a QuorumVerifier object
in QuorumPeer) and it doesn't look in the config file. 
>> 
>> BTW the config file (when overwritten by the system) has an auto-generated version
using which we know which config is later than which.  Users are not supposed to specify this
version at all - its supposed to be managed by the system. If you replace the file and set
the version to something low or not specify it at all, chances are that the config file will
be overwritten during synchronization with the leader or during communication with other servers
in FastLeaderElection.
>> If you set it to something high, its possible that your server will be able to convince
others that this is the latest config :)
>> 
>> Alex
>> 
>> 
>> On Sat, Jul 28, 2012 at 10:57 AM, Jared Cantwell <jared.cantwell@gmail.com>
wrote:
>> No that you would want to do this, but simply overwriting a config file would "uncommit"
a configuration and make that server think the last committed configuration was whatever is
in the file?
>> 

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