This article presents and discusses the computation and communication model used by R*, a prototype distributed database management system.An R* computation consists of a tree of processes connected by virtual circuit communication paths.The process management and communication protocols used by R* enable the system to provide reliable, distributed transactions while maintaining adequate levels of performance.Of particular interest is the use of processes in R* to retain user context from one transaction to another, in order to improve the system performance and recovery characteristics.

R* is an experimental prototype distributed database management system. The computation needed to perform a sequence of multisite user transactions in R* is structured as a tree of processes communicating over virtual circuit communication links. Distributed computation can be supported by providing a server process per site which performs requests on behalf of remote users. Alternatively, a new process could be created to service each incoming request. Instead of using a shared server process or using the process per request approach, R* creates a process associated with the computation of the user on the first request to the remote site. This process is incorporated into the tree of processes serving a single user and is retained for the duration of the user computation. This approach allows R* to factor some of the request execution overhead into the process creation phase, and simplifies the retention of user and transaction context at the multiple sites of the distributed computation.

System R supports a high-level relational user language called SQL which may be used by ad hoc users at terminals or as an embedded data sublanguage in PL/I or COBOL. Host-language programs with embedded SQL statements are processed by the System R precompiler which replaces the SQL statements by calls to a machine-language access module. The precompilation approach removes much of the work of parsing, name binding, and access path selection from the path of a running program, enabling highly efficient support for repetitive transactions. Ad hoc queries are processed by a similar approach of name binding and access path selection which takes place on-line when the query is specified. By providing a flexible spectrum of binding times, System R permits transaction-oriented programs and ad hoc query users to share a database without loss of efficiency. System R is an experimental database management system designed and built by members of the IBM San Jose Research Laboratory as part of a research program on the relational model of data. This paper describes the architecture of System R, and gives some preliminary measurements of system performance in both the ad hoc query and the “canned program” environments.