MongoDB replica set: Why the last remaining server becomes secondary

There are two questions here, one for MS clustering, and another one for Mongo.

MS Clustering

The decision of where to put the public, heart-beat, inter-node communication, and quorum drive is significant. Also cluster architecture makes a difference; you pick different quroum options if the two nodes are in adjacent racks than if they were in completely different datacenters.

Put the heartbeat on the same interface/subnet as the public interface

This theory holds that if you lose your public interface, you want the heartbeat to fail because this node is effectively down to users.

Put the heartbeat on it's own private interface/subnet

This theory holds that something outside of the cluster is arbiting who is doing what role, and unnecessary node-death is to be avoided.

Put the WFS on the heartbeat network

If the two nodes are in the same overall network (the same set of switches is supporting the non-public networks for both nodes) then putting the WFS on the heartbeat network doesn't introduce any new vulnerabilities.

If the two nodes are in different network fault domains (such as different datacenters), this is a bad idea. The heartbeat network provides the 'node majority' quorum option, and the WFS provides the 'File Share Majority' quorum option. You really want both options to be in separate fault domains.

Your revised diagram makes sense if both nodes are in the same data-center, though I myself would but the heartbeat on the public side.

MongoDB

MongoDB is a bit simpler. With even numbers of nodes, you absolutely want a third node to act as tie-breaker. They're pretty clear about that. However, your diagram states:

Up to 12 replica members (7 can vote).

7 is an odd number. You don't require an Arbiter.

Unlike Microsoft clusters, Mongo's cluster voting doesn't care about multiple avenues of network to break voting deadlocks. Because of this, separate arbitration and cluster-internal networks do not provide any meaningful increase in robustness. The only reason you'd want a separate arbitration network is if replication traffic was expected to be so heavy that election-packets (the heartbeat, actually) would get pushed so far down the stack that it would miss the 10 second timeout.

1. MongoDB replication doesn't work that way. When you set up the cluster the cluster will elect a Primary, and that will be the reference point for data (writes always, reads depending upon how you configure the client). If the server goes down, the remaining servers will elect a new Primary (assuming they have quorum), and sync from that.

2. Your client, if you use an official mongoDB one, can read from any Primary or Secondary (if configured to do so), and the cluster itself will tell your client that the Primary has changed, so that it can write to it.