“Secondary” vs “Slave” in Mongodb

Question

I am trying to clarify the terminology difference, if any, between "Secondary" and "Slave" in mongodb.

Confusion stems from connecting w/ the node-mongodb-native documentation of the Db constructor :

db_opts = {db: {
   readPreference: ReadPreference.SECONDARY
   slaveOk: true 
}}

MongoClient.connect('mongodb://example', db_opts, ...)

What is the difference between the readPreference option and the slaveOk option there?

Answer 1

Don't get caught up on Secondary vs. Slave here. What you're really asking is about slaveOk vs readPreference.

slaveOk is essentially deprecated in favor of using readPreference . The MongoClient docs do explicitly state for slaveOk: "legacy option allowing reads from secondary, use readPrefrence instead". They also state that setting a readPreference overrides any slaveOk value.