Chapter Objectives
In this chapter, you will learn to:
• understand why it is important to know the basic concepts of legacy databases;
• identify the basic building blocks and limitations of the hierarchical model;
• identify the basic building blocks and limitations of the CODASYL model.
Opening Scenario
Sober purchased the customer database of Mellow Cab, who recently stepped out of the taxi business after many years at the top. Since Mellow Cab started in the mid-1970s, it was still using outdated database technology. It used CODASYL to store and manage its customer data. Sober is not familiar with this technology and wants to understand its limitations before deciding to either continue with it or invest in a modern DBMS.
In this chapter we briefly zoom into legacy database technologies and their logical data models. Although these are outdated and semantically inferior to modern database technologies, there are three reasons to review them. Firstly, many firms still struggle with legacy databases due to historical implementations and limited IT budgets – hence, knowing the basic characteristics of these models is essential to the maintenance of the corresponding database applications and the potential migration to modern DBMSs. Secondly, understanding the basics of these legacy models will contribute to better understanding the semantical richness of newer database technologies. Finally, the concept of procedural DML and navigational access originally introduced by these legacy models has also been adopted by more recent databases such as OODBMSs.
In this chapter we cover both the hierarchical and CODASYL data models. Throughout the discussion, we extensively discuss the expressive power and limitations of both models.
The Hierarchical Model
The hierarchical model is one of the first data models developed. The model originated during the Apollo moon missions program conducted by NASA. To manage the huge amount of data collected, IBMdeveloped the Information Management System or IMS DBMS (1966–1968). There is no formal description available and it has lots of structural limitations. Therefore, it is considered legacy.
The two key building blocks of the hierarchical model are record types and relationship types. A record type is a set of records describing similar entities. Think about a product record type or a supplier record type. A record type has zero, one, or more records. It consists of fields or data items.
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