Write Concerns

MongoDB provides several different ways of selecting how durable a write to the database should be. These ways are called Write Concerns and span everything from completely ignoring all errors, to specifically targetting which servers are required to confirm the write before returning the operation.

When a write (such as with MongoCollection::insert, MongoCollection::update, and MongoCollection::remove) is given a Write Concern option ("w") the driver will send the query to MongoDB and piggy back a getLastError command (GLE) with the Write Concern option at the same time. The server only returns when the Write Concern condition is verified to be fullfilled, or the query times out (controlled with the "wtimeout" option, 10000 milliseconds is the default).

Warning

Even though a getLastError command times out the data will most likely have been written to the primary server and will be replicated to all the secondaries once they have cought up.

The typical reasons for a timeout to happen is if you specify a Write Concern which requires confirmation from more servers then you currently have available.

When using acknowledged writes and the replica set has failed over, the driver will automatically disconnect from the primary, throw an exception, and attempt to find a new primary on the next operation (your application must decide whether or not to retry the operation on the new primary).

When using unacknowledged writes (w=0) and the replica set has failed over, there will be no way for the driver to know about the change so it will continue and silently fail to write.

The default Write Concern for the MongoClient is 1: acknowledge write operations.

**Available Write Concerns**
Write Concern	Meaning	Description
w=0	Unacknowledged	A write will not be followed up with a GLE call, and therefore not checked ("fire and forget")
w=1	Acknowledged	The write will be acknowledged by the server (the primary on replica set configuration)
w=N	Replica Set Acknowledged	The write will be acknowledged by the primary server, and replicated to N-1 secondaries.
w=majority	Majority Acknowledged	The write will be acknowledged by the majority of the replica set (including the primary). This is a special reserved string.
w=<tag set>	Replica Set Tag Set Acknowledged	The write will be acknowledged by members of the entire tag set
j=true	Journaled	The write will be acknowledged by primary and the journal flushed to disk

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:

Example #1 Passing a WriteConcern to a write operation


<?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:

Example #2 Connection string WriteConcerns


<?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:

Example #3 MongoDB::setWriteConcern and MongoCollection::setWriteConcern


<?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.

Example #4 Unacknowledged WriteConcern, followed with Acknowledged Write


<?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. */
}
?>

If the last write throws an exception, you know that there's a problem with your database.

Acknowledged Writes

These type of write operations will make sure that the database has accepted the write operation before returning success. If the write failed, it will throw a MongoCursorException with an explanation of the failure. The MongoClient default behaviour is to acknowledge the write (w=1).

It is possible to specify how many members of an replica set have to acknowledge the write (i.e. have it replicated) before the write is deemed acknowledged and the operation returns.

Example #5 Acknowledged Writes


<?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));
?>

Keep in mind to select your Write Concern carefully. If you have a replica set with 5 members, and you select Write Concern of 4 you will risk the write blocking forever when one member of the replica set goes down for maintenance or a temporary network outage happens.

Warning

Passing in a string value for Write Concern has a specific meaning (Replica Set Tag Set Acknowledged). Please be careful of NOT using string values for numbers (i.e. array("w" => "1")) as it will be treated as a tag set name.

Majority Acknowledged Writes

Using the special majority Write Concern option is the recommended way for writes that are required to survive the apocalypse, as it will ensure the majority of your replica set will have the write and will therefore be guaranteed to survive all usual suspect outage scenarios.

Example #6 Majority Acknowledged Write


<?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:

Example #7 Acknowledged and Journaled Write

Forcing journal flush


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

Changelog

Version	Description
1.3.0	MongoClient was introduced and defaults to acknowledged writes. The deprecated Mongo defaults to unacknowledged writes.
1.3.0	The "safe" write option has been deprecated and is not available with the new MongoClient class. Use the "w" option instead.