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    <blockquote> <blockquote> <p>Taking a simple example of 2 mongoDB instances A (primary) and B (secondary).</p> </blockquote> </blockquote> <p>You should not use two instance as part of your replica set. You should either have an <a href="http://docs.mongodb.org/manual/administration/replica-sets/#replica-set-arbiters" rel="nofollow">arbiter</a> or another secondary so that elections can take place.</p> <p>Please see <a href="http://www.mongodb.org/display/DOCS/Gotchas" rel="nofollow">here</a> for gotchas, <a href="http://docs.mongodb.org/manual/core/replication/" rel="nofollow">here</a> for replication fundamentals and <a href="http://docs.mongodb.org/manual/core/replication/#replica-set-elections" rel="nofollow">here</a> for information on elections.</p> <blockquote> <blockquote> <p>If my client is happily querying A I understand that writes get replicated to B but what happens if server A becomes inaccessible?</p> </blockquote> </blockquote> <p>The service should <a href="http://docs.mongodb.org/manual/faq/replica-sets/#how-long-does-replica-set-failover-take" rel="nofollow">failover</a> with server B becoming primary. However, with two instances, server B will not be able to elect itself as primary because there is no majority.</p> <blockquote> <blockquote> <p>Whilst in terms of mongo replication I can see that B gets elected as the new primary, how does the client know to now channel its queries to B and not A? Is this all done internally to mongo?</p> </blockquote> </blockquote> <p>This is typically performed through the driver as the driver is aware of the replica set configuration. Please see <a href="http://www.mongodb.org/display/DOCS/Drivers" rel="nofollow">here</a> for the relevant driver documentation (depending on what driver you are using).</p>
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    1. COThanks for the reply. Good point about the even number of mongo instances - I'd missed that but it is mentioned in the gotchas link provided. I'm using the c# driver http://www.mongodb.org/display/DOCS/CSharp+Driver+Tutorial. I see it mentions specifying multiple servers in the connection string. It doesn't sound like this is necessary if all servers are part of the same set (and you specify the name of the replicaSet in the connection string) - so why would you provide multiple servers?
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    2. COI'm not an expert at all in the C# driver but I'd highly recommend following the documentation there. The guys on the team there are thorough.
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    3. COI'll try it out. Thanks for your help.
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