Cal. Code of Civil Procedure § 763.010

(a) The form, content, and manner of the service of summons shall be the same as in civil actions generally. (b) If upon affidavit it appears to the satisfaction of the court that the plaintiff has used reasonable diligence to ascertain the identity and residence of and to serve summons on the persons named as unknown defendants and persons joined as testate or intestate successors of a person known or believed to be dead, the court shall order service by publication pursuant to Section 415.50 and the provisions of this article. The court may, in its discretion, appoint a referee to investigate whether the plaintiff has used reasonable diligence to ascertain the identity and residence of persons sought to be served by publication, and the court may rely on the report of the referee instead of the affidavit of the plaintiff in making the order for service by publication. (c) Nothing in this section authorizes service by publication upon any person named as an unknown defendant who is in open and actual possession of the property.