Querying
Distributing queries to secondaries
All queries (reads and writes) are only sent to the primary member of a
replica set by default. This is however easily configurable by using the
Read Preferences which allow
you to set some generic read preferences (such as allowing secondary reads
of the nearest server), and also provide ways to specifically target a
server in a specific country, datacenter, or even hardware, by the use of
replica set tag sets.
Read preferences can be configured at every level of the driver:
Each class inherits its read preference setting from the "parent" context.
Example #1 Inheriting read preferences from the database level down to the cursor
<?php
$db->setReadPreference(MongoClient::RP_SECONDARY_PREFERRED);
$c = $db->myCollection;
$cursor = $c->find();
?>
In this example, the query will be executed against a secondary. The
collection inherits MongoClient::RP_SECONDARY_PREFERRED
from the database and the cursor inherits it from the collection.
How secondaries are chosen
Each instance of MongoClient chooses its own
secondary using the available secondary with the lowest ping time. So, if we
had a PHP client in Europe and one in Australia and we had one secondary in
each of these data centers, we could do:
<?php
$options = array("replicaSet" => "setName", "readPreference" => MongoClient::RP_SECONDARY_PREFERRED);
// on the Australian client
$m = new MongoClient("mongodb://primary,australianhost.secondary,europeanhost.secondary", $options);
$cursor = $m->foo->bar->find();
$cursor->getNext();
echo "Reading from: ", $cursor->info()["server"], "\n";
// on the European client
$m = new MongoClient("mongodb://primary,australianhost.secondary,europeanhost.secondary", $options);
$cursor = $m->foo->bar->find();
$cursor->getNext();
echo "Reading from: ", $cursor->info()["server"], "\n";
?>
Exemplul de mai sus va afișa ceva similar cu:
Reading from: australianHost
Reading from: europeanHost
Note that we have to do a query before a secondary is chosen: secondaries
are chosen lazily by the driver, and for each query separately.
You can see what the driver thinks is the current status of the set
members by running MongoClient::getHosts() or
MongoClient::getConnections().
If no secondary is readable, the driver will send reads to the
primary as we specified
MongoClient::RP_SECONDARY_PREFERRED
which will
fallback to execute a query on a primary if no secondaries are available.
A server is considered readable if its state is 2 (SECONDARY) and its
health is 1. You can check this with
MongoClient::getHosts() and
MongoClient::getConnections().
Random notes
Writes are always sent to the primary—and by default all reads are sent
to the primary too.
Querying by _id
Every object inserted is automatically assigned a unique
_id
field, which is often a useful field to use in
queries. This works similarly to "get last insert ID" functionality, except
that the _id
is chosen by the client.
Suppose that we wish to find the document we just inserted. Inserting adds
an _id
field to the document, so we can query by that:
<?php
$person = array("name" => "joe");
$people->insert($person);
// now $joe has an _id field
$joe = $people->findOne(array("_id" => $person['_id']));
?>
Unless the user has specified otherwise, the _id
field is a
MongoId. The most common mistake is attempting to use
a string to match a MongoId. Keep in mind that these
are two different datatypes, and will not match each other in the same way
that the string "array()" is not the same as an empty array. For example:
<?php
$person = array("name" => "joe");
$people->insert($person);
// convert the _id to a string
$pid = $person['_id'] . "";
// FAILS - $pid is a string, not a MongoId
$joe = $people->findOne(array("_id" => $pid));
?>
Arrays
Arrays are special in a couple ways. First, there are two types that
MongoDB uses: "normal" arrays and associative arrays. Associative arrays can
have any mix of key types and values. "Normal" arrays are defined as arrays
with ascending numeric indexes starting at 0 and increasing by one for each
element. These are, typically, just your usual PHP array.
For instance, if you want to save a list of awards in a document, you could
say:
<?php
$collection->save(array("awards" => array("gold", "silver", "bronze")));
?>
Queries can reach into arrays to search for elements. Suppose that we wish
to find all documents with an array element of a given value. For example,
documents with a "gold" award, such as:
{ "_id" : ObjectId("4b06c282edb87a281e09dad9"), "awards" : ["gold", "silver", "bronze"]}
This can be done with a simple query, ignoring the fact that "awards" is an
array:
<?php
$cursor = $collection->find(array("awards" => "gold"));
?>
Suppose we are querying for a more complex object, if each element of the
array were an object itself, such as:
{
"_id" : ObjectId("4b06c282edb87a281e09dad9"),
"awards" :
[
{
"first place" : "gold"
},
{
"second place" : "silver"
},
{
"third place" : "bronze"
}
]
}
Still ignoring that this is an array, we can use dot notation to query the
subobject:
<?php
$cursor = $collection->find(array("awards.first place" => "gold"));
?>
Notice that it doesn't matter that there is a space in the field name
(although it may be best not to use spaces, just to make things more
readable).
You can also use an array to query for a number of possible values. For
instance, if we were looking for documents "gold" or "copper", we could do:
<?php
$cursor = $collection->find(array("awards" => array('$in' => array("gold", "copper"))));
?>