MongoDB PHP: Reading from Slaves and setting persistent connections with a heavy read environment

Question

I'm attempting to set all incoming read queries to hit slaves on my mongo servers.

I see in the PHP docs a reference to:

MongoCursor::$slaveOkay = true;

However, this just seems to setup queries to be fired off to slaves; not really to do anything else. My connections to my servers look like this:

$mongo = new Mongo("mongodb://my.server:27017", 
                      array("replicaSet" => 'replicaSet', "persist" => "pool")
                  );

• Will I need to do anything different with my persist connection when wanting to only connect to the slave for reads?

• How can I target queries to hit just the Slave so that the writes I have on the primary won't block incoming read requests.

PHP docs shows me this example:

$db->setSlaveOkay(true);
$c = $db->myCollection;

$cursor = $c->find();

However I'm confused at the difference this has between the above, and if both are needed or not.

Answer 1

SlaveOkay and Read Preferences

The SlaveOkay preference is effectively "secondary preferred", but still allows reading from the primary.

MongoDB 2.2 and the Mongo PHP 1.3.0 driver introduce several new Read Preference Modes so there is now:

• primary - only read from the primary
• primaryPreferred - read from the primary unless it is unavailable
• secondary - only read from secondaries
• secondaryPreferred - prefer reads from a secondary (equivalent semantics to slaveOK)
• nearest - read from the nearest member of the replica set (by ping time)

Another new feature in MongoDB 2.2 is support for Tag Sets, which allow you to specify custom read preferences by tagging replica set members. This allows you to target specific secondaries. For example, a replica set member could be tagged with: { 'group' : 'reporting' }.

For more information on read preferences in the PHP driver see the Mongo manual on PHP.net: Read Preferences.

Persistent Connections

Connection pooling has been rewritten for the PHP 1.3.0 driver release, and all connections are now persistent.

Per the changelog:

Removed the "persist" option, as all connections are now persistent. It can still be used, but it doesn't affect anything.