Adaptability Using Reeection

Adaptability, i.e. the ability of a system to adapt dynamically to changes in its execution environment, is considered as an important property of computer systems. Scaling directory replication in name servers and load balancing are well known examples. However , adaptability in today's systems, if present at all, generally is burnt in and dedicated to singular properties of the execution environment. This paper discusses a more universal approach to adaptability. The approach is based on the | in some cases artiicial | separation of nonfunctional properties from the functionality of application systems. The scheme provides full control over non functional properties while preserving transparency for the application programs. To study that approach, reeection has been incorporated into the BirliX object model and its implementation. It has been used to experiment with some non functional properties such as migration and protection. The paper motivates the need for a universal adaptability scheme, revisits reeection as used in programming language systems and discusses design decisions for reeective systems, describes the resulting reeective BirliX model and its implementation and nally describes applications. 1 The need for adaptability The starting point for our work is the observation that many of today's more sophisticated applications have a common problem exploiting the power of distributed systems. They need to dynamically adapt 1 the underlying system to their special needs to meet requirements in performance, security, or fault tolerance. A few examples are: 1 There are other systems which are adaptable to special hardware requirements during connguring the system 1, 2]. Mobile computers using phone lines, radio, or ber optics for communication need to adapt to various bandwidths using appropriate protocols 3]. Frequently used services (e. g. le services, name services) have to scale to the number of their clients. These services can be adapted to growing networks by a growing scale of service replication 4]. Recent work 5] indicates that migration can be a very powerful mechanism if it is not restricted to just processes or les. It has to be easily adaptable to various types of objects and to a wide variety of purposes. Auditing is employed in some systems for intrusion detection. The detection of suspect events causes systems to switch to a security-alert state. Here adaptation can be used to tailor the system entities to provide more detailed auditing information , e. g. by activating several additional auditing sensors. 6] gives more examples for …