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Lesson 2 The history of networking
Objective Explain the Evolution from Centralized to Decentralized Computers.

Evolution from Centralized to Decentralized Computers

While the computing environment of the 1990s is characterized by widely distributed systems, this decentralized database approach is actually a relatively new phenomenon. It is very important to understand that the client/server model does much more than distribute data across a network. One of the foremost reasons for using the client/server model is to share the processing load.

Central data, central processing, distributed data, distributed processing
Central data, central processing, distributed data, distributed processing

In the four quadrants of this graph, we see several common approaches to the distribution of Oracle data and processors:

Centralized Data, Centralized processing (no networking)

This is the traditional approach to Oracle data, where all the data resides in a common database and all the processing is performed by a centralized computer, usually a large UNIX server. This approach has the advantage of better control of the Oracle data, but has the disadvantage of being subject to widespread performance degradation when the CPU becomes overloaded. Because there is also a single point of failure , which is CPU of the Unix machine, the probability of experiencing downtime is increased. If this single CPU would fail, this problem could cripple the entire system, making access to the data impossible. For this reason it is better to focus on a solution that involves distributed networks. A distributed network would enable the data as well as the CPU to be de-centralized, thus preventing the possibility of hardware or database failure.

Centralised Database

A centralised database (sometimes abbreviated CDB) is a database that is located, stored, and maintained in a single location. This location is most often a central computer or database system, for example a desktop or server CPU, or a mainframe computer. In most cases, a centralised database would be used by an organisation (for example a business company) or an institution. Users access a centralised database through a computer network which is able to give them access to the central CPU, which in turn maintains to the database itself. All of the information stored on the CBS is accessible from a large number of different points, which in turn creates a significant amount of both advantages and disadvantages.
  1. centralized data distributed processing,
  2. distributed data centralized processing, and

Distributed Data Processing

This is the approach of many Oracle systems, whereby both
  1. the data and
  2. the processing
are distributed across a network is to use a distributed database.
The primary advantage to this approach is the ability to assign both data and processors on an as-needed basis. The main disadvantage is the problem of coordinating backup and recovery of the data across all the distributed nodes. This is implemented with Oracle Parallel Server with Oracle Net Services interfaces to remote databases.

Distributed query processing in Oracle can be described as follows. In Oracle, the query is executed at the node that the user is signed-on to, while other database's partition a distributed query into sub-queries, executing each subquery on it's host processor. In Oracle, a distributed query might query data items from widely distributed databases in a single query. Oracle implements distributed queries using database links.

Query Performance

The performance of a query should not depend on where the data resides. The optimization of distributed queries is vital because a poor execution plan can take orders of magnitude longer than the "correct" one. For example, if a query includes a large intermediate result set, that data probably should not be shipped over the network to the database with a small table that is to be joined with the result set.

All systems fall somewhere within these four domains, depending on the amount of distributed processing and number of distributed databases. The Oracle solution facilitates all of them. A distributed data store is a network in which a user stores his or her information on a number of peer network nodes. The user also usually reciprocates and allows users to use his or her computer as a storage node as well. Information may or may not be accessible to other users depending on the design of the network.

Distributed databases have become an integral part of business computing in the last 30 years. The ability to maintain the integrity of data and provide accurate and timely processing of database queries and updates across multiple sites has been an important factor in enabling businesses to utilize data in a range of different locations, sometimes on a global scale. Standardization of query languages, and of the Relational and Object models has assisted the integration of different database systems to form networks of integrated data services. The difficulties of ensuring the integrity of data, that updates are timely, and that users receive a uniform rate of response no matter where on the network they are situated remain, in many circumstances, major challenges to database vendors and users.
In this unit we shall introduce the topic of distributed database systems. We shall examine a range of approaches to distributing data across networks, and examine a range of strategies for ensuring the integrity and timeliness of the data concerned. We shall look at mechanisms for enabling transactions to be performed across different machines, and the various update strategies that can be applied when data is distributed across different sites. In the next lesson, we will take a look at the evolution of network protocols.