Question

Using php driver 1.3.2 and mongodb 2.2, I am trying to use readPreference to direct an aggregate query to one of the secondaries in my replica set. Seems whatever I try, the aggregate query is executed on the primary server.

Basic example:

$db = new \MongoClient('rs1.example:27017,rs2.example:27017,rs3.example:27017', array('replicaSet' => 'myRs') );
$db->setReadPreference( \MongoClient::RP_SECONDARY );
$results = $db->tracking->sessions->aggregate( array( ... ) );

I enabled MongoLog and got the following results:

The aggregate method shows: REPLSET INFO: - connection: type: PRIMARY

If I use find instead, it shows: REPLSET INFO: - connection: type: SECONDARY

Is this a bug with the php driver? Anyone else run into this? Thought I would toss it on SO before adding it as a bug in their Jira.

Answer 1

All command queries through the PHP driver are currently directed to primary servers. We have several old tickets related to this, originating with requests to direct count commands to secondaries, but it was a non-trivial change that requiring checking the command against a whitelist to ensure it is read-only. The current ticket tracking this fix is PHP-535, which I linked to the issue you opened, PHP-662.

If you need an immediate work-around, you can call find() on the $cmd collection directly, passing the same $command array argument that you’d typically pass to MongoDB::command() as the argument to find(). Kristina documented this solution in this JIRA comment, and although she used slaveOkay() in that example, it should also work with read preferences.