DB2's use of the coupling facility for data sharing by J. W. Josten C. Mohan I. Narang J. Z. Teng We examine the problems encountered in extending DATABASE 2 rM (DB2®) for Multiple Virtual Storage/Enterprise Systems Architecture (MVS/ESA rM ), also called DB2 for OS/390 rM , an industrial-strength relational database management system originally designed for a single-system environment, to support the multisystem shared-data architecture. The multisystem data sharing function was delivered in DB2 VersiJm 4. DB2 data sharing requires a System/390 Parallel Sysplex rM environment because DB2's use of the coupling facility technology plays a central role in delivering highly efficient and scalable data sharing functions. We call this the shared-data architecture because the coupling facility is a unique feature that it employs. O ne approach to improving the capacity and availability characteristics of a single-system da- tabase management system (DBMS) is to use mul- tiple systems. Before the introduction of the System/390* (S/39o*) Parallel Sysplex*, there were two major architectures in use in the multisystem environment: the shared-disk (SDi) architecture, also called data sharing, 1 and the shared-nothing (SN) or partitioned architecture." The S/390 Parallel Sysplex introduces a third multisystem architecture called the shared-data (SDa) architecture. With SDi, all the disks containing the databases are accessible from all the sharing systems and each sys- tem has its own buffer pool (BP)to cache data in pro- cessor storage for fast reference. Every system that has an instance of the DBMS executing on it may ac- cess and modify any portion of the database on the shared disks. Because each instance has its own buffer pool and because conflicting accesses to the same data may be made from different systems, the interactions among the systems must be controlled, using various synchronization protocols. This neces- sitates global locking and protocols for the mainte- nance of buffer coherency. SDi is the approach used in IBM's Information Management System/Virtual Storage (IMS*/VS) data sharing product.>" and Amoeba project," and DEC's V AX* * DBMS and VAX Rdb/VMS**.7-9 With SN, each system owns a portion of the data- base and only that portion may be directly read or modified by that system. That is, the database is par- titioned among the multiple systems. The kind of syn- chronization protocols needed for SDi are not needed for SN. But a transaction that accesses data in mul- tiple systems would need a form of two-phase com- mit protocol 10,1 J to coordinate its activities. This is the approach taken in Tandem's NonStop** SQL 1Z (System Query Language), Teradata's DBC/1012,13 and the University of Wisconsin's Gamma. 14 ©Copyright 1997 by International Business Machines Corpora- tion. Copying in printed form for private use is permitted with- out payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copy- right notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free without further permission by computer-based and other information-service systems. Permission to republish any other portion of this paper must be obtained from the Editor. IBM SYSTEMS JOURNAL, VOL 36, NO 2, 1997 0018-8670/97/$5.00 © 1997 IBM JOSTEN ET Al. 327