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I have a MongoDB Cluster deployed in Azure. Normally a MongoDB cluster has at least a couple of mongo router instances (mongos) which is where an application should connect (the application is also hosted in azure).

I want to 'failover' balance the traffic from the application to the mongodb instances. What I mean is that I want to connect my application to one instance an if it fails (and only if it fails) to the other. I don't want my traffic to be balanced with round-robin:

  • mongorouter1.myinternaldomain.dom:27017: This should be always the the endpoint
  • mongorouter2.myinternaldomain.dom:27017: This should be the endpoint only if the firstone fails.
  • mongorouter3.myinternaldomain.dom:27017: This should be the endpoint only if the previous both fail.

I see that this kind of things are managed with the Traffic Manager or with ILB (Internal Load balancer) in Azure. But I have a problem because as I understand the failover balancing is only available with the Traffic manager and that the ILB only allows round-robin.

Because my application is also in Azure I don't want to expose the mongodb ports to the public, so I understand that I can't use the Traffic Manager which is the service that allow failover load balancing.

What should the correct way to implement this?

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The only way to have software load balancer with failover in Azure is to use Traffic Manager. You can set ACLs, so only Your own app can connect with DB on specified endpoint. Or You can configure failover in Your application - without load balancer, because You do not need load balancing (just failover).

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