Mongo::Error::NoServerAvailable - a few possible causes
Summer-time, perfect to make a major version upgrade – from MongoDB 3.2 to 3.4 – on the database servers.
Not without troubles. In my case: Mongo::Error::NoServerAvailable No server is available matching preference
A quick internet search will tell you that this is no uncommon error and the solutions
for it vary widely. Mind you, I’m not a server admin, my focus programming the backend of web apps.
In hope to help others a little, here’s what went wrong with our db servers. None of the mistakes caused an issue
on MongoDB 3.2, only after the upgrade they system got a bit picky.
DB hosts couldn’t connect to each other: For whatever reason I had the wrong IPs in the /etc/hosts file
and in the replica set I used those short names rather than IPs. A bad choice.
Make sure you can successfully connect to each db server from each other db server.
Mongoid didn’t know the replica-set name. This wasn’t an issue on 3.2 but suddenly was now, an easy fix in the mongoid.yml.
As always when upgrade a replica set, start with the secondaries: db.shutdownServer(), upgrade the software and start it before moving
to the next and doing the primary last.