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Oracle Distributed Systems by Charles Dye

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Benefits of Distributed Databases

The separation of the various system components, especially the separation of application servers from database servers, yields tremendous benefits in terms of cost, management, and performance.

Tunability

A machine’s optimal configuration is a function of its workload. Machines that house web servers, for example, need to service a high volume of small transactions, whereas a database server with a data warehouse has to service a relatively low volume of large transactions (i.e., complex queries). Separating the web server from the database server in this example allows the system administrators to optimize these machines without compromise. A machine configured as a web server will differ from a machine configured as a data warehouse database server. If performance problems arise in a distributed architecture, it is much easier not only to identify problems but also to solve them without the risk of compromising other components.

Platform Autonomy

Since applications and databases do not reside on the same machines, there is no particular reason why they even need to reside on the same type of machine. SQL*Net and Net8 provide a protocol-independent network interface allowing connectivity among disparate platforms and even disparate database engines. This openness allows DBAs, developers, and desktop users to choose their platforms without being restricted by anybody else’s preferences or requirements. Whether you perform a major platform change such as moving from VMS to Unix or a minor upgrade such as from Solaris 2.5 to Solaris 2.6, you can make these changes without risking functionality changes in the Oracle database engine.

Fault Tolerance

The failure of a single component in a distributed architecture is much less drastic than in an environment in which databases and applications are housed on the same machine. Administrators can design failover methodologies that are appropriate to each component’s functionality. For example, database machines might implement parallel server or synchronous replication to protect against failure of a database machine, whereas application servers may have backup hardware available so that the application can run on a new machine if an application server fails. Protecting against failure of machines that house data is generally much more complicated than protecting against failure of machines that simply run applications.

Scalability

A server that houses nothing other than an Oracle database scales very predictably; sites taking advantage of the parallel query option (and/or parallel DML in Oracle8) can expect performance to be a nearly linear function of the number of processors (up to the point of at least 30 processors on Solaris). Other applications may or not scale this way, but if the applications have their own host, system administrators can understand their requirements and allocate hardware resources appropriately.

Location Transparency

Location transparency means that neither applications nor users need to be concerned with the logistics of where data actually resides or how it is distributed. Needless to say, being shielded from these specifics enhances the usability of a database because developers and users do not need to consider such details as connect strings. Moreover, data can be relocated from one database instance to another with minimal impact on users and applications.

Site Autonomy

Distributed databases allow various locations to share their data without conceding administrative control. If a database instance at headquarters contains particularly sensitive information or has high availability requirements, it can still share data without compromising its security or availability. In addition, any given site in a distributed database environment can follow its own administrative procedures and upgrade paths, within reason. Of course, we hope that administrators from various sites are in communication with one another and that they coordinate their activities, but they are in no way handcuffed to one another.

Enhanced Security

The components of the distributed architecture are completely independent of one another, which means that every site can be maintained independently. You can share data without sharing accounts and passwords. Each site can have its own administrators and its own sets of accounts, and private data can be kept private.

As an example, you can implement a replicated environment with updateable snapshots that would allow users at a branch office to update something as sensitive as the salary table without having any access to the salary data for headquarters (horizontal partitioning) . As another example, you can use workflow partitioning (discussed in Chapter 15) in a multi-master replicated environment to limit the set of rows that can be updated at any given site.

You also can configure a distributed environment to provide security in the sense of survivability—that is, you can maintain two or more versions of entire schema by replicating them to different machines at different locations.

There is no reason for developers or end users to have accounts on a database server, because all database access is through network APIs (Application Programming Interfaces). The database server’s exposure to malicious intruders and careless users is minimal. In fact, it is not uncommon for users to have no idea whatsoever where the database resides!

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