2.3. Example: System R

The main goals of the System R developers were the following:

• provide a high-level, user-friendly interface with the system, allowing to achieve data independence and enable users to work as efficiently as possible;
• provide a variety of acceptable ways to use the DBMS, including programmable transactions, online transactions and report generation;
• maintain a dynamically changing database environment in which relationships, indexes, views, transactions, and other objects can be easily added and destroyed without interrupting the normal functioning of the system;
• to provide the possibility of parallel work with one database of many users with the assumption of parallel modification of database objects in the presence of the necessary means of protecting the integrity of the database;
• provide a means of restoring a consistent state of databases after various kinds of hardware or software failures;
• provide a flexible mechanism that allows you to define different representations of stored data, and limit these views to user access to the database by sampling and modification based on an authorization mechanism;
• ensure system performance when performing the above functions, comparable to the performance of existing low-level DBMS.

The structural organization of System R is quite consistent with the goals set during its development. The main structural components of System R are the relational memory management system (RSS) and the SQL query compiler. RSS provides an interface that is rather low, but sufficient to implement the SQL level for accessing the data stored in the database. Transaction synchronization, change logging and database recovery after failures are also among the RSS functions. The query compiler uses the RSS interface to access a variety of reference information (relationship catalogs, indexes, access rights, integrity conditions, conditional effects, etc.) and produces work programs that are later executed using the RSS interface. Thus, the system is naturally divided into two levels - the level of memory management and synchronization, in fact, independent of the basic query language of the system, and the language level (SQL level), which solves most of the System R. problems. Note that this independence is rather conditional, than absolute: the SQL language can be replaced by another language, but it should have about the same semantics.