(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.