Is GridFS via PHP useable if primary is unavailable? - php

Question

Konfiguration
Ubuntu 14.04
PHP 5.5.9
MongoDB 3.0.2
MongoDB PHP driver 1.6.6
Setup
I have a globally distributed MongoDB replica set (for testing purpose) with 3 servers. One has priority = 1, others have priority = 0 so they will never become primary.
This setup is used to distribute files to the replicated servers by adding them directly on the primary server using GridFS. Distribution works fine.
I have created a simple php watcher script which is executed on the secondary servers using read preference \MongoClient::RP_NEAREST. I wanted to determine the timestamp when the replicated received all files from the primary.
I wanted to make sure that the php script on the secondary servers are using the mongodb instance on their server (and not the primary), and therefore I stopped the primary mongodb server. After doing this, the two servers are keeping their secondary role.
Issue
If the primary server is unavailable, I was still able to execute queries like count() and find() on regular collections (also fs.files collection).
But calls that use GridFS will throw an MongoConnectionException: No candidate servers found exception.
Script
I have created a little script with which you should be able to reproduce the error.
$serverList = '...';
$conn = new \MongoClient(
'mongodb://'.$serverList,
array(
'replicaSet' => 'r0',
'readPreference' => \MongoClient::RP_NEAREST,
'username' => 'bat',
'password' => '',
'db' => 'bat'
)
);
$db = $conn->selectDB('bat');
echo 'works fine...';
$files = $db->selectCollection('fs.files');
$documentsCount = $files->count();
$documents = $files->find();
foreach($documents as $document) {
echo $document['filename'] . ', ';
}
echo 'throws exception...';
$gridfs = $db->getGridFS();
$documentsCount = $gridfs->find()->count();
$documents = $gridfs->find();
foreach($documents as $document) {
echo $document->getFilename();
}
If the primary server is unavailable, the lines after echo 'works fine...'; will work fine, while the line after echo 'throws exception...'; will throw an exception.
Maybe this is related to an issue in the java driver JAVA-401, there was a similar problem with usage of secondary servers and gridfs. Maybe the gridfs ist trying to ensuring indices if the fs.files collection contains less than 1000 files which is not possible on secondary.

Answer 1

I figured out the problem, it was simply the incorrect line $gridfs->find()->count(). If you execute $gridfs->count() or $gridfs->find() it works fine.
So you'll have to use $gridfs->count() instead of $gridfs->find()->count().
I dont't know why $gridfs->find()->count() works correctly (and only correctly) if a primary server is available.