The key representing this connection.
The key is a unique string which identifies a database connection. A connection can be a single server or a cluster of primary and replicas (use target to pick between primary and replica).
Type: string|null
protected $key = NULL;
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Database!Connection.php/property/Connection::key/8.1.x