2 Architecture of Active Database Systems 2.1 Introduction

The architecture of an active DBMS determines both its functionality and the components that are required for its implementation. This chapter addresses rst some issues that have an impact on the architecture of an active database system, and presents the various architectural alternatives. The basic functions of an ADBMS are identiied and then related to the architectural alternatives. This chapter discusses rule speciication and registration , and rule execution. Special attention is devoted to the execution of transactions and its relationship to the execution of ECA rules. The architecture of an active DBMS determines both its functionality and the components that are required to realize it. Since an ADBMS by deeni-tion must provide the active capabilities in addition to full DBMS features, it can be viewed as an extension of a passive DBMS. There are various properties of the underlying DBMS and of the architectural strategy used for implementing the active extensions that will have an impact on the func-tionality and the performance of the active DBMS. The major dimensions that need to be considered are: the degree of integration between the underlying DBMS and the active capabilities, the system architecture of the underlying DBMS, and the data model of the DBMS and the programming language used for the active extensions. In this chapter we will brieey discuss the various architectural alternatives. We will then identify the basic functions and components of an ADBMS and will relate them to the various architectural alternatives and will point out the eeect of an architecture on the implementation of a given feature. We will address rst the rule speciication and registration func-tionality and then the rule execution subsystem. A central aspect of rule