PHP Manual

Replica Sets

To connect to a replica set, specify one or more members of the set and use the "replicaSet" option. Multiple servers may be delimited by a comma.

Example #1 ReplicaSet seed list

// Using multiple servers as the seed list (prefered)
$m = new MongoClient("mongodb://,");

// Using one server as the seed list
$m = new MongoClient("mongodb://", array("replicaSet" => "myReplSetName"));

// Using multiple servers as the seed list
$m = new MongoClient("mongodb://,", array("replicaSet" => "myReplSetName"));

The PHP driver will query the database server(s) listed to determine the primary. So long as it can connect to at least one host listed and find a primary, the connection will succeed. If it cannot make a connection to any servers listed or cannot find a primary, a MongoConnectionException will be thrown.


You should always provide a seed list with more than one member of the ReplicaSet. For highest availability you should seed with at least one server from each of your datacenters.


The host names that you specify in the seed list must match the host names in the replica set configuration. This is because the driver only uses the host names in the replica set configuration to create the hash for its persistent connections.

For example, if an IP address is used in the seed list and the replica set is configured with host names, the driver will discard any seed list connection(s) that differ from the canonical host names reported by the replica set. Effectively, these non-canonical seed list connections will be recreated on each request, greatly reducing the benefit of using persistent connections.


The driver does not support connecting to different replica sets with the same name. This extends beyond one script so make sure to have separate names for each of your replica sets. That also means that you can not do this:

Example #2 Wrong replica set name duplication

= new MongoClient("mongodb://devserver1,devserver2,devserver3/?replicaSet=repset");
$m = new MongoClient("mongodb://prodserver1,prodserver2,prodserver3/?replicaSet=repset");

Instead, you need to have two different names for your replica sets:

Example #3 Correct use of two replica set names

= new MongoClient("mongodb://devserver1,devserver2,devserver3/?replicaSet=devset");
$m = new MongoClient("mongodb://prodserver1,prodserver2,prodserver3/?replicaSet=prodset");

If the primary becomes unavailable, an election will take place and a secondary will be promoted to the role of primary (unless a majority vote cannot be established). During this time (» 20-60 seconds), the connection will not be able to perform any write operations and attempts to do so will result in an exception. Connections to secondaries will still be able to perform reads.


The default Read Preference is to only read from the primary. During the election process there is no primary, and all read will therefore fail.

It is recommended to use MongoClient::RP_PRIMARY_PREFERRED Read Preference for applications that require high availability for reads, as reads will only be executed on the secondaries when there simply isn't a primary available.

Once a primary is elected, attempting to perform a read or write will allow the driver to detect the new primary. The driver will make this its primary database connection and continue operating normally.

The health and state of a secondary is checked every 5 seconds (configurable with mongo.ping_interval) or when the next operation occurs after 5 seconds. It will also recheck the configuration when the driver has a problem reaching a server.

Replica set failovers are checked every 60 seconds (configurable with mongo.is_master_interval), and whenever a write operation fails when using acknowledged writes.


Secondaries may be behind the primary in operations, so your application must be able to handle out-of-date data when using Read Preferences other then MongoClient::RP_PRIMARY.

For more information on replica sets, see the » core documentation.

See Also


Version Description
1.0.9 Added support for connecting to ReplicaSet and automatic failover.

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