Serious counter-question: How do you run Postgres in a native and hands-off replicated failover setup? With MongoDB you create a three-node replica set and you're done. I've yet to see a simple guide for Postgres. But I might be wrong, so happy to hear how you would achieve the same with Postgres.
This isn't just a server property. It need to be implemented across the whole vertical, including client libraries and how queries are constructed in the application. With mongo it all is, and it's there out of the box.
Client libraries are made to do this failover and retry under the hood. Queries are more often made as upserts instead of assuming transactions, eg ObjectId rather than auto increment and so on. Even non upserting operations like $inc are safely retried if handled by queries and not application code.
It's magic to see it happen, with no extra code, the application just keeps running even when one server is down. Starting it up again and it reconciles with no hands-on.
I would love to see a postgres that does this, because relational data is often more useful than the document model.
