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Distributed queries The user should be able to join data from any of the tables in the (distributed) database, even if the tables are located on different physical systems. Distributed updates The user should be able to update data in any table for which the user has the necessary privileges, whether that table is on the local system or on a remote system. Distributed transactions The DBMS should support distributed transactions (using COMMIT and ROLLBACK) across system boundaries, maintaining the integrity of the (distributed) database even in the face of network failures and failures of individual systems. Security The DBMS must provide a security scheme adequate to protect the entire (distributed) database from unauthorized forms of access. Universal access The DBMS should provide universal, uniform access to all of the organization s data. No current distributed DBMS product even comes close to meeting this ideal, and it s unlikely that any product ever will. In practice, formidable obstacles make it difficult to provide even simple forms of distributed database management. These obstacles include Performance In a centralized database, the path from the DBMS to the data has an access speed of a few milliseconds and a data transfer rate of millions of characters per second. Even on a fast local area network, access speeds lengthen to hundredths or tenths of a second, and transfer rates can fall to 100,000 characters per second or less. On a modem link, data access may take seconds or minutes, and a few thousand characters per second may be the maximum effective throughput. This vast difference in speeds can dramatically slow the performance of remote data access. Integrity Distributed transactions require active cooperation by two or more independent copies of the DBMS software running on different computer systems if the transactions are to remain all-or-nothing propositions. Special two-phase commit transaction protocols must be used. These protocols generate a great deal of network traffic and lock parts of the databases that are participating in the distributed transaction for long periods. Static SQL A static embedded SQL statement is compiled and stored in the database as an application plan. When a query combines data from two or more databases, where should its application plan be stored Must there be two or more cooperating plans If there is a change in the structure of one database, how are the application plans in the other databases to be notified Using dynamic SQL to solve these problems in a networked database environment almost always leads to unacceptably slow application performance, due to network overhead and delays. Optimization When data is accessed across a network, the normal rules for SQL optimization don t apply. For example, it may be more efficient to sequentially scan an entire local table than to use an index search on a remote table. The optimization software must know about the network(s) and their speeds. Generally speaking, optimization becomes both more critical and more difficult.
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Data compatibility Different computer systems support different data types, and even when two systems offer the same data type, they often use different formats. For example, a Windows PC and an Apple Mac store 16-bit integers differently. IBM mainframes store data using the EBCDIC character set, while UNIX and Linuxbased servers and PCs use ASCII. A distributed DBMS must mask these differences. System catalogs As a DBMS carries out its tasks, it frequently accesses its system catalogs. Where should the catalog be kept in a distributed database If it is centralized on one system, remote access to it will be slow, bogging down the DBMS. If it is distributed across many different systems, changes must be propagated around the network and synchronized. Mixed-vendor environment It s highly unlikely that all the data in an organization will be managed by a single brand of DBMS, so distributed database access will cross DBMS brand boundaries. This requires active cooperation between DBMS products from highly competitive vendors an unlikely prospect. As the DBMS vendors scramble to extend the capabilities of their products with new features, capabilities, and data types, the ability to sustain a cross-vendor standard is even less likely. Distributed deadlocks When transactions on two different systems each try to access locked data on the other system, a deadlock can occur in the distributed database, even though the deadlock is not visible on either of the two systems. The DBMS must provide global deadlock detection for a distributed database. Again, this requires coordination of processing across a network and will typically lead to unacceptably slow application performance. Recovery If one of the systems running a distributed DBMS fails, the operator of that system must be able to run its recovery procedures independently of the other systems in the network, and the recovered state of the database must be consistent with that of the other systems.
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