서버 선택 및 모니터링

<?php
/**
 * When constructing a Client, the library creates a MongoDB\Driver\Manager
 * object from the extension. In turn, the extension will either create a
 * libmongoc client object (and persist it according to the constructor
 * parameters) or re-use a previously persisted client.
 *
 * Assuming a new libmongoc client was created, the host name(s) in the
 * connection string must be resolved via DNS. Likewise, if the connection
 * string includes a mongodb+srv scheme, SRV/TXT records must be resolved.
 * Following DNS resolution, the driver should then have a list of one or
 * more hosts to which it can connect. This is referred to as the seed list.
 *
 * If a previously persisted client was re-used, no DNS resolution is needed
 * and there will likely already be connections and topology state associated
 * with the client.
 *
 * Drivers perform no further IO when constructing a client, so control is
 * returned the the PHP script.
 */
$client = new MongoDB\Client('mongodb://a.example.com:27017/?replicaSet=rs0');
/**
 * The library creates a MongoDB\Database object from the Client. This does
 * not entail any IO, as the Database and Collection objects only associate
 * a database or namespace with a Client object, respectively.
 */
$database = $client->test;
/**
 * The library creates an internal object for this operation and must select
 * a server to use for executing that operation.
 *
 * If this is the first operation on the underlying libmongoc client, it must
 * first discover the topology. It does so by establishing connections to any
 * host(s) in the seed list (this may entail TLS and OCSP verification) and
 * issuing "hello" commands.
 *
 * In the case of a replica set, connecting to a single host in the seed list
 * should allow the driver to discover all other members in the replica set.
 * In the case of a sharded cluster, the driver will start with an initial
 * seed list of mongos hosts and, if SRV polling is utilized, may discover
 * additional mongos hosts over time.
 *
 * If the topology was already initialized (i.e. this is not the first
 * operation on the client), the driver may still need to perform monitoring
 * (i.e. "hello" commands) and refresh its view of the topology. This process
 * may entail adding or removing hosts from the topology.
 *
 * Once the topology has been discovered and any necessary monitoring has
 * been performed, the driver may select a server according to the rules
 * outlined in the server selection specification (e.g. applying a read
 * preference, filtering hosts by latency).
 */
$database->command(['ping' => 1]);