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Message from discussion communication between mongo DB machines
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Jason Zucchetto  
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 More options Oct 15 2012, 5:52 pm
From: Jason Zucchetto <jason.zucche...@10gen.com>
Date: Mon, 15 Oct 2012 14:52:18 -0700 (PDT)
Local: Mon, Oct 15 2012 5:52 pm
Subject: Re: communication between mongo DB machines
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Hi Syed, every member of a replica set will send a ping/heartbeat to one
another every 2 seconds.  The two machines in data center B will not be
able to receive heartbeats from the machines in data center A, this will
cause them to believe only 2 of the 5 machines in the replica set are
healthy (since they can only communicate with one another).  

A new primary election between the two machines in data center B will then
occur (since both are currently secondary and a primary is needed).  When a
new primary election begins, the first member to receive a majority of
votes from the machines in the replica set will become the new primary.
 Since there are only 2/5 machines in data center B, no majority can be
reached and both machines will step down.

There is a great overview of replica set internals
here: http://docs.mongodb.org/manual/core/replication-internals/#replica-se...

I hope this answers your question, please let me know if you need more help.

Thanks--
Jason

On Wednesday, October 3, 2012 1:47:45 AM UTC-7, Syed Mehdi wrote:

> Hi,
> we have two data centers (Say "A" and "B") and one replica set with 5
> members.
> 3 of its machines are in "A" and 2 are in "B". Till now everything was
> working fine,
> but then network operator stopped imitation of communication from "B" to
> "A" (though "A" to "B" is still allowed).
> now our 2 machines (in "B") are shown as down in replica set status
> (checked on DB machine of "A").

> since primary is in "A", and "A" to "B" initiated communication is
> allowed, shouldn't the 2 machine show up as fine?

> also how can we deal with such a problem, if network operator will keep
> this rule and we still have to have machines in "A" and "B".

> thanks for any help
> syed


 
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