What is the general rule of public entity immunity?

general rule of public entity immunity: "Where a public entity is under a mandatory duty imposed by an enactment that is designed to protect against the risk of a particular kind of injury, the public entity is liable for an injury of that kind proximately caused by its failure to discharge the duty unless the public entity establishes that it exercised reasonable diligence to discharge the duty." (See Haggis v. City of Los Angeles (2000) 22 Cal.4th 490, 499-500 ["section 815.6 provides that the public entity 'is liable' for an injury proximately caused by its negligent failure to discharge the duty"].)