Hello,
I have experienced big slow applications when using MongoDB with ruby in case of a failover. At the time of Moped I even opened an issue about this https://github.com/mongoid/moped/issues/373 without any feeback ...

My servers are AWS ec2 instances and I run a replica set accross different availability zones in the same region. Whenever a host is down (the instance is shutdown) I experience very slow query performances. It turns out this is because mongodb driver takes a lot of time to figure out that a host is down. Quite suprisingly, it gets instant feeback when trying to connect to a localhost port where there is now mongod listening.

I have isolated a simple case illustrating my problem in the following gist : https://gist.github.com/rpechayr/782df4b8b270bde21d7c

What can we do about this ? It looks like `connect_timeout` that has a 5 seconds default could be used, but I am really wondering why the default is 5 seconds.

Real life mongodb failover don't happen with down localhost ports, but with situations similar to the one I experienced.