how to understand read preferences in mongo

Question

I'm just getting started with mongoDB. I am trying to understand how to set up my secondary database servers so that when there is no primary, the secondaries can be used to read data. I believe the read preference I'm going for is preferredPrimary.

Now that I kinda understand which of the read preferences I want to test out, I'm trying to understand how to set up my replica set for preferredPrimary. I've been reading through the following documentation:

http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets/

Questions:

1. Is this the right doc to follow to set up read preferences?
2. Assuming that it is, I want to verify that the tags names / values are anything that I come up with? So specifically, the key used in the example "dc" is NOT a keyword in mongo. Is that correct?
3. once I set up these tags, in my client, when I'm connecting to the mongo database, do i have to specify any settings? I'm using a php front end, and I found this: http://php.net/manual/en/mongodb.setreadpreference.php
4. can you confirm that these tags replace the rs.slaveOK() method?

Environment:

• mongoDB version 2.6.5
• replica set with 3 members - one primary and 2 secondary servers

Answer 1

1. Yes
2. Yes

3. Yes, but the link that you provided is only for readPreference You also need to supply custom writeConcern (extract from link in a question):

   db.users.insert( { id: "xyz", status: "A" }, { writeConcern: { w: "MultipleDC" } } )

Look into php driver documentation how to do that.

4. Yes, you may skip call to slaveOK in this case (especially, that in 95% cases you will be reading from primary)