mongodb, replicates and error: { "$err" : "not master and slaveOk=false", "code" : 13435 }

Question

You have to set "secondary okay" mode to let the mongo shell know that you're allowing reads from a secondary. This is to protect you and your applications from performing eventually consistent reads by accident. You can do this in the shell with:

rs.secondaryOk()

After that you can query normally from secondaries.

A note about "eventual consistency": under normal circumstances, replica set secondaries have all the same data as primaries within a second or less. Under very high load, data that you've written to the primary may take a while to replicate to the secondaries. This is known as "replica lag", and reading from a lagging secondary is known as an "eventually consistent" read, because, while the newly written data will show up at some point (barring network failures, etc), it may not be immediately available.

Edit: You only need to set secondaryOk when querying from secondaries, and only once per session.

To avoid typing rs.slaveOk() every time, do this:

Create a file named replStart.js, containing one line: rs.slaveOk()

Then include --shell replStart.js when you launch the Mongo shell. Of course, if you're connecting locally to a single instance, this doesn't save any typing.

in mongodb2.0

you should type

rs.slaveOk()

in secondary mongod node

THIS IS JUST A NOTE FOR ANYONE DEALING WITH THIS PROBLEM USING THE RUBY DRIVER

I had this same problem when using the Ruby Gem.

To set slaveOk in Ruby, you just pass it as an argument when you create the client like this:

mongo_client = MongoClient.new("localhost", 27017, { slave_ok: true })

https://github.com/mongodb/mongo-ruby-driver/wiki/Tutorial#making-a-connection

mongo_client = MongoClient.new # (optional host/port args)

Notice that 'args' is the third optional argument.

WARNING: slaveOk() is deprecated and may be removed in the next major release. Please use secondaryOk() instead. rs.secondaryOk()

slaveOk does not work anymore. One needs to use readPreference https://docs.mongodb.com/v3.0/reference/read-preference/#primaryPreferred

e.g.

const client = new MongoClient(mongoURL + "?readPreference=primaryPreferred", { useUnifiedTopology: true, useNewUrlParser: true });

I got here searching for the same error, but from Node.js native driver. The answer for me was combination of answers by campeterson and Prabhat.

The issue is that readPreference setting defaults to primary, which then somehow leads to the confusing slaveOk error. My problem is that I just wan to read from my replica set from any node. I don't even connect to it as to replicaset. I just connect to any node to read from it.

Setting readPreference to primaryPreferred (or better to the ReadPreference.PRIMARY_PREFERRED constant) solved it for me. Just pass it as an option to MongoClient.connect() or to client.db() or to any find(), aggregate() or other function.

• https://docs.mongodb.com/v3.0/reference/read-preference/#primaryPreferred
• http://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html (search readPreference)

const { MongoClient, ReadPreference } = require('mongodb');
const client = await MongoClient.connect(MONGODB_CONNECTIONSTRING, { readPreference: ReadPreference.PRIMARY_PREFERRED });