(mongodb >=1.13.0)
MongoDB\Driver\TopologyDescription::hasReadableServer — Returns whether the topology has a readable server
$readPreference = null): bool Returns whether the topology has a readable server or, if readPreference is specified, a server matching the specified read preference.
此函数没有参数。
Returns whether the topology has a readable server or, if readPreference is specified, a server matching the specified read preference.