L 119/114 EN Official Journal of the European Union 4.5.2016 2. Member States shall provide for the controller to implement appropriate technical and organisational measures ensuring that, by default, only personal data which are necessary for each specific purpose of the processing are processed. That obligation applies to the amount of personal data collected, the extent of their processing, the period of their storage and their accessibility. In particular, such measures shall ensure that by default personal data are not made accessible without the individual's intervention to an indefinite number of natural persons. Article 21 Joint controllers 1. Member States shall, where two or more controllers jointly determine the purposes and means of processing, provide for them to be joint controllers. They shall, in a transparent manner, determine their respective responsibilities for compliance with this Directive, in particular as regards the exercise of the rights of the data subject and their respective duties to provide the information referred to in Article 13, by means of an arrangement between them unless, and in so far as, the respective responsibilities of the controllers are determined by Union or Member State law to which the controllers are subject. The arrangement shall designate the contact point for data subjects. Member States may designate which of the joint controllers can act as a single contact point for data subjects to exercise their rights. 2. Irrespective of the terms of the arrangement referred to in paragraph 1, Member States may provide for the data subject to exercise his or her rights under the provisions adopted pursuant to this Directive in respect of and against each of the controllers. Article 22 Processor 1. Member States shall, where processing is to be carried out on behalf of a controller, provide for the controller to use only processors providing sufficient guarantees to implement appropriate technical and organisational measures in such a manner that the processing will meet the requirements of this Directive and ensure the protection of the rights of the data subject. 2. Member States shall provide for the processor not to engage another processor without prior specific or general written authorisation by the controller. In the case of general written authorisation, the processor shall inform the controller of any intended changes concerning the addition or replacement of other processors, thereby giving the controller the opportunity to object to such changes. 3. Member States shall provide for the processing by a processor to be governed by a contract or other legal act under Union or Member State law, that is binding on the processor with regard to the controller and that sets out the subject-matter and duration of the processing, the nature and purpose of the processing, the type of personal data and categories of data subjects and the obligations and rights of the controller. That contract or other legal act shall stipulate, in particular, that the processor: (a) acts only on instructions from the controller; (b) ensures that persons authorised to process the personal data have committed themselves to confidentiality or are under an appropriate statutory obligation of confidentiality; (c) assists the controller by any appropriate means to ensure compliance with the provisions on the data subject's rights; (d) at the choice of the controller, deletes or returns all the personal data to the controller after the end of the provision of data processing services, and deletes existing copies unless Union or Member State law requires storage of the personal data;