Parallelizing document retrieval

This is an article I wrote a while ago, but apparently hadn't posted.

MongoDB 2.6 has a new feature that allows you to read all the documents from one collection with multiple cursors in parallel. This is done through a database command called parallelCollectionScan. The idea behind it is that it is faster then reading all the documents in a collection sequentially.

Just like the Aggregation Cursor, calling this command returns cursor information. However, it returns an array of these structures. Let run this example to see what it returns:

<?php
$m = new MongoClient;
$d = $m->demo;

$r = $d->command( [
    'parallelCollectionScan' => 'cities',
    'numCursors' => 3
] );

var_dump( $r );
?>

And this outputs (after some formatting):

array(2) {
  ["cursors"]=>
  array(3) {
    [0]=>
    array(2) {
      ["cursor"]=>
      array(3) {
        ["firstBatch"]=> array(0) { }
        ["ns"]=> string(14) "demo.cities"
        ["id"]=>
        object(MongoInt64)#5 (1) {
          ["value"]=> string(12) "339843550291"
        }
      }
      ["ok"]=> bool(true)
    }
    [1]=>
    array(2) {
      ["cursor"]=>
      array(3) {
        ["firstBatch"]=> array(0) { }
        ["ns"]=> string(14) "demo.cities"
        ["id"]=>
        object(MongoInt64)#6 (1) {
          ["value"]=> string(12) "340949759620"
        }
      }
      ["ok"]=> bool(true)
    }
    [2]=>
    array(2) {
      ...
    }
  }
  ["ok"]=> float(1)
}

With the MongoCommandCursor::createFromDocument from an earlier article you can create a MongoCommandCursor for each of the array elements:

<?php
$m = new MongoClient;
$d = $m->demo;

$r = $d->command( [
        'parallelCollectionScan' => 'cities',
        'numCursors' => 3
], null, $hash );

$cursors = [];
foreach( $r['cursors'] as $cursorInfo )
{
        $cursors[] = MongoCommandCursor::createFromDocument( $m, $hash, $cursorInfo );
}
?>

Instead of creating an array of cursors yourself, the driver implements the MongoCollection::parallelCollectionScan method. Making the above a little bit easier:

<?php
$m = new MongoClient;
$c = $m->demo->cities;

$cursors = $c->parallelCollectionScan( 3 );
?>

The idea is that with multiple cursors you can iterate over each of the segments in parallel, for example indifferent threads. Of course, PHP does not have threads so that you can't really run things in parallel. However, PHP does have a MultipleIterator class that allows you to iterate over multiple cursors at the same time:

<?php
$m = new MongoClient;
$c = $m->demo->cities;

$cursors = $c->parallelCollectionScan( 3 );


$multiple_it = new MultipleIterator( MultipleIterator::MIT_NEED_ANY );
foreach ( $cursors as $cursor )
{
        $multiple_it->attachIterator( $cursor );
}


foreach ( $multiple_it as $items )
{
        foreach ( $items as $item )
        {
                if ( $item !== NULL )
                {
                        echo $item['name'], "\n";
                }
        }
}
?>

There are three sections here. First we create the cursors with MongoCollection::parallelCollectionScan, then we collect the created cursors into a MultipleIterator and lastly we iterate over the $multiple_it iterator to get our results. Each iteration gives us an array of elements back. One element for each of the containing cursors (3 in our example). We need a second loop (foreach) to pick out the real document.

Not every contained cursor will provide the same amount of items, it is up to the MongoDB server to divide this. When a contained iterator is exhausted, the MultipleIterator sets the value to NULL. It is probably better to then remove that specific contained iterator from the MultipleIterator, but that is left as an excercise for the reader.

When running some benchmarks, I didn't actually see any performance benefit with multiple cursors over just one cursor, but that is likely because the cursors are still iterated over sequentially, and not in parallel. Perhaps using the pthreads PECL extension allows for a better benchmark, but right now, the PHP driver for MongoDB doesn't support threaded execution yet.

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