Write Concerns

Using WriteConcerns

Each of the methods that causes writes ( MongoCollection::insert(), MongoCollection::update(), MongoCollection::remove(), and MongoCollection::batchInsert()) allow an optional argument to send a set of options to the MongoDB server. With this option array you can set the WriteConcern as the following example illustrates:

<?php
// Setting w=0 for insert:
$collection->insert($someDoc, array("w" => 0));

// Setting w=majority for update:
$collection->update($someDoc, $someUpdates, array("w" => "majority"));

// Setting w=5 and j=true for remove:
$collection->update($someDoc, array("w" => 5, "j" => true));

// Setting w="AllDCs" for batchInsert:
$collection->update(array($someDoc1, $someDoc2), array("w" => "AllDCs"));
?>

Besides setting WriteConcerns per operation as an option argument, it is also possible to set a default WriteConcern in different ways.

The first way is through the connection string. The connection string accepts the journal, w, and wTimeoutMS options:

<?php
$m = new MongoClient("mongodb://localhost/?journal=true&w=majority&wTimeoutMS=20000");
?>

Since driver version 1.5 it is also possible to call MongoDB::setWriteConcern() and MongoCollection::setWriteConcern() to set a default WriteConcern for all operations created from that specific MongoDB or MongoCollection object:

<?php
$m = new MongoClient("mongodb://localhost/");
$d = $m->demoDb;
$c = $d->demoCollection;

// Set w=3 on the database object with a timeout of 25000ms
$d->setWriteConcern(3, 25000);

// Set w=majority on the collection object without changing the timeout
$c->setWriteConcern("majority");
?>

Unacknowledged Writes

By not requiring the server to acknowledge writes the writes can be performed extremely quickly, but you don't know whether or not they actually succeeded. Writes can fail for a number of reasons: if there are network problems, if a database server goes down, or if the write was simply invalid (e.g., writing to a system collection; or duplicate key errors).

While developing, you should always use acknowledged writes (to protect against inadvertent mistakes, such as syntax errors, invalid operators, duplicate key errors and so on). In production, unacknowledged writes can be used for "unimportant" data. Unimportant data varies on application, but it's generally automatically (instead of user generated) data, such as click tracking or GPS locations, where you can get thousands of records per second.

It is strongly recommended that you do an acknowledged write at the end of series of unacknowledged writes. Doing so will not incur in a too large performance penalty, but still allow you to catch any errors that may have occurred.

<?php
$collection->insert($someDoc, array("w" => 0));
$collection->update($criteria, $newObj, array("w" => 0));
$collection->insert($somethingElse, array("w" => 0));
try {
	$collection->remove($something, array("w" => 1));
} catch(MongoCursorException $e) {
	/* Handle the exception.. */
	/* Here we should issue find() queries on the IDs generated for
	$somethingElse and $someDoc to verify they got written to the database and
	attempt to figureout where in the chain something happened. */
}
?>

<?php
// Force acknowledgement from the primary only
$collection->insert($doc, array("w" => 1));

// Force acknowledgement from the primary, and one other member of the
// replica set
$collection->insert($doc, array("w" => 2));

// Force acknowledgement from the primary, and six other members of the
// replica set (you probably never should do this):
$collection->insert($doc, array("w" => 7));
?>

<?php
$collection->insert($someDoc, array("w" => "majority"));
?>

Journaled Writes

When connecting to a replica set the default Write Concern is only to have the primary server acknowledge the write. There is however a 100ms window until the write gets journaled and flushed to disk. It is possible to force the write to be journaled before acknowledging the write by setting the j option:

<?php
$options = array(
	"w" => 1,
	"j" => true,
);
try {
	$collection->insert($document, $options);
} catch(MongoCursorException $e) {
	/* handle the exception */
}
?>

Voir aussi

• » MongoDB WriteConcern docs

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