mongodb not using local database in replica set

Question

I have 4 servers each hosting a website and a mongo replica set.

• MongoDB version: v3.4.13
• Driver: PHP

Server 1 is PRIMARY.

• Located on the west coast

Server 2 is SECONDARY with tag: { location: 'east' }

• Located on the east coast

• I'm connecting to the db with connection string: mongodb://localhost:27017/?replicaSet=rs&readPreference=nearest&readPreferenceTags=location:east

Server 3 and 4 are SECONDARY with no tags.

I want server 2 to read from it's local database, but instead it is reading from primary (or another secondary, I can't tell, but it's definitely not reading from it's local database)

• I suspect it's not reading from it's own SECONDARY db because there is about a 3 second lag for any query I run.

How can I tell server 2 to read from it's own local SECONDARY db?

Answer 1

SOLUTION: All servers within the cluster need to be able to connect to all other servers within the cluster.

Turns out, some of my secondary's weren't able to connect to the other secondary's because of some rules placed in my iptables.

Once all servers within the cluster were able to connect to each other the speed increased dramatically.

Answer 2

Include readPreferenceTags in your connection string. Refer to the online doc.

By the way, you shouldn't have a even number of nodes in a replica set unless one of them has no voting right.