On production we get a steady stream of seemingly random Mongo::Error::NoServerAvailable errors. Maybe 3 a day.
The replica set in question has not experienced any elections, and there is nothing unusual about its performance during these episodes.
Here's the full exception:
Mongo::Error::NoServerAvailable: No server is available matching preference: #<Mongo::ServerSelector::Primary:0x70286341840160 tag_sets=[] server_selection_timeout=30 local_threshold=0.015>
Here's the mongoid portion of the stack trace:
vendor/bundle/ruby/2.3.0/gems/mongoid-5.1.3/lib/mongoid/criteria/findable.rb:75 in multiple_from_db
vendor/bundle/ruby/2.3.0/gems/mongoid-5.1.3/lib/mongoid/criteria/findable.rb:19 in execute_or_raise
vendor/bundle/ruby/2.3.0/gems/mongoid-5.1.3/lib/mongoid/criteria/findable.rb:40 in find
vendor/bundle/ruby/2.3.0/gems/mongoid-5.1.3/lib/mongoid/findable.rb:94