• User instance: A user instance is an object that is created by the master node for a user who has active jobs – the job is initialized or in execution. The user instance is deleted by the master node, when the user does not have active jobs any more. A user instance is an important but dynamic element of the security framework. It is related with other information and data of the corresponding user as well as the authentication process on slave nodes. User instances are independent between any two different users. Another property of user instance is sharing. If there are more than one active jobs of a user, these jobs share a common user instance. In case that the user is logged out but there are still active jobs for him, the user instance will not be deleted, thus to implement an “off-line execution” mode by using the user instance approach.