Oracle's Distributed Database System (DDBS) Overview

One, what is a distributed database

The Distributed Database System (ddbs:distributed) is developed on the basis of centralized database system. is a combination of database technology and network technology products.

There are two kinds of distributed database systems: One is physically distributed, but logically it is centralized. This distributed database is only suitable for the use of a single, small unit or department. Another kind of distributed database system is distributed in both physics and logic, which is called Federated distributed Database System. Because each sub database system that makes up the federation is relatively "autonomous", this system can accommodate a variety of different uses of the larger database, more suitable for large-scale database integration.

Distributed Database Systems (DDBS) contain distributed database management systems (DDBMS) and distributed Databases (DDB).

In the distributed database system, an application can transparently operate the database, the data in the database are stored in different local databases, managed by different DBMS, run on different machines, supported by different operating systems, and connected by different communication networks.

A distributed database is logically a unified whole: a single logical database in front of the user and physically stored on separate physical nodes. An application can access a geographically dispersed database through a network connection. The distribution of the data in the database is not stored in the same site. More specifically, it is not stored on the same computer's storage device. This is the difference from a centralized database. From the user's point of view, a distributed database system is logically the same as a centralized database system, and users can perform global applications on any site. Just fine. The data is stored on the same computer, with a single database management system (DBMS) management, and the user does not feel the same.

Each database server in a distributed database cooperates to maintain the consistency of the global database.

A distributed database system is a customer/server architecture.

Each computer in the system is called a node. If a node has a management database software, the node is called the database server. If a node is an application of the requesting server's information, the node is called the customer. In Oracle customers, perform database applications, Access data information, and interact with users. On the server, execute Oracle software to handle concurrent, shared data access to Oracle databases. Oracle allows the above two parts to be on the same computer, but is more efficient when the customer part and the server portion are on different computers connected by the network.

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Distributed processing is the process of sharing a single task by multiple processors. Examples of distributed processing in Oracle database systems are as follows:

Customers and servers are on different computers that are located on a network connection.

There are multiple processors on a single computer, and different processors perform customer applications separately.

Each server participating in a distributed database manages the database independently, as if each database is not a networked database. Each database is managed independently, called site autonomy. The advantages of site autonomy are as follows:

The node of the system can reflect the logical organization of the company.

Local data is controlled by local database administrators so that each database administrator responsibility domain is smaller and more manageable.

As long as a database and network are available, the global database can be partially available. Does not stop all operations or cause performance bottlenecks due to a database failure.

Recovery is usually performed at a single node.

There is a data dictionary for each local database.

Nodes can be upgraded independently of the software.

You can access schema objects from all nodes in a distributed database, so just as a partially distributed DBMS, you must provide a mechanism to reference an object in a local database. A distributed DBMS must provide a naming pattern so that an object in a distributed database can be uniquely identified and referenced in the application. Typically, uniqueness is implemented at each layer of the hierarchy. The distributed DBMS simply expands the hierarchical naming model and implements the unique database naming on the network. Therefore, the global object name of an object is guaranteed to be unique within the distributed database.

Oracle allows the use of global object names in SQL statements to refer to Schema objects (tables, views, and procedures) in a distributed database. In Oracle, the global name of a schema object consists of three parts: the schema name, object name, and database name of the containing object, in the form of:

SCOTT. Emp@sales. DIVISION3. Acme.com

A remote query is a query that selects information from one or more remote tables that reside at the same remote node.

A distributed query can retrieve data from two or more nodes. A distributed update can modify data in two or more than two nodes.

A remote transaction is a transaction that contains one or more remote statements, all of which refer to the same remote node. A transaction in a distributed transaction that contains one or more statements that modify data in two or more different nodes of a distributed database.

In the distributed database, transaction control must be in the net and ensure data consistency. The two-phase commit mechanism guarantees that all database servers participating in the distributed transaction are the statements in all or all rollback transactions.

Oracle Distributed Database system architecture allows Oracle database administrators to provide location transparency for end users and applications, using views, synonyms, and procedures to provide location transparency in Oracle distributed database systems.

Oracle provides two mechanisms to achieve the transparency of table duplication in distributed databases: Table snapshots provide asynchronous table duplicates; In both cases, the transparency of table duplication is achieved.

In a single site or distributed database, all transactions are aborted with a commit or ROLLBACK statement.

II. classification of distributed database systems:

(1) Homogeneous homogenous type Ddbs: Each site uses the same type of data model (for example, all relational) and is the same model DBMS.

(2) Homogeneous heterogeneous type Ddbs: Each site uses the same type of data model, but the DBMS models are different, such as DB2, ORACLE, SYBASE, SQL Server and so on.

(3) Heterogeneous type Ddbs: The data model of each site is different, even the type is different. With the development of computer network technology, the problem of heterogeneous machine networking has been better solved, at this time, depending on heterogeneous type Ddbs can access the data in various heterogeneous local libraries in the whole network.