Mongodb – How to MongoDB elect a new primary when a datacenter has failed

I do not understand why the failover to DC2 has to be done manually (even if other parts have to be done manually: one thing less on your to do list in case of a major failure is always a good thing!).

In general, my feeling is that there are conceptual flaws in your setup.

Here is how I would do it and why.

1. I would not have manual failover. It is better to have slow access than none. What will happen in the current configuration is that if the primary fails, there will be a tie and therefor the whole set would enter secondary state, effectively turning the cluster into read-only mode. So even when everything else is fine in DC1 and there is no need for failing over to DC2, a failing primary will be a show stopper. With setup, you are artificially creating a single point of failure, effectively gainst the whole idea of a cluster, let alone a multi DC setup. Sounds like a Very Bad Idea™ to me. Automatic failover, even to DC2 sounds like a better idea. Slower reads and (depending on your write concern) slower writes still are better than read only mode.
2. I would have a third datacenter with only one instance: an arbiter. An arbiter can easily be run on a micro-machine as it will only be called in case of an election and an election is a cheap task in terms of RAM and computation power. The arbiter will help the set to always have a majority: If one DC gets disconnected for whatever reason, the other DC and the arbiter will form a majority. So if one DC goes down, you have only to worry about your other parts of your application. You don't have to wait
3. I am pretty sure that automatic failover for the other parts of your application can be achieved with some time and effort. Especially if you store all data in mongoDB and you have some sort of session replication available, it should be quite easy. Whether implementing automatic failover is worth the effort is pretty easy to calculate: Get your average downtime, find out how big the losses are created by this downtime in terms of money and customer satisfaction (if applicable). If the costs of implementing automatic failover is below or equal, go for automatic failover. I can help you with that if needed.

Yes, you can do this, and in fact with just 2 data centers you really have no other choice. Essentially, whenever you do not have a primary you can still do a reconfig but you must pass in {force:true} to make it happen on a non-primary node.

This is listed in the docs for the rs.reconfig() command and in detail (with examples) as part of a tutorial which describes exactly what needs to be done in the scenario you described:

http://docs.mongodb.org/manual/tutorial/reconfigure-replica-set-with-unavailable-members/

If you are interested, I cover this and other availability issues in the (free) M202: Advanced Deployment and Operations course on MongoDB University