Distributed Databases Overview

Other capabilities of Streams include the following:

See Also: Oracle9i Streams

Advanced Queuing Overview

Oracle Advanced Queuing provides an infrastructure for distributed applications to communicate asynchronously using messages. Oracle Advanced Queuing stores messages in queues for deferred retrieval and processing by the Oracle server. This provides a reliable and efficient queuing system without additional software such as transaction processing monitors or message-oriented middleware.

Messages pass between clients and servers, as well as between processes on different servers. An effective messaging system implements content-based routing, subscription, and querying.

A messaging system can be classified into one of two types:

Synchronous Communication Synchronous communication is based on the request/reply paradigm—a program sends a request to another program and waits until the reply arrives.

This model of communication (also called online or connected) is suitable for programs that need to get the reply before they can proceed with their work. Traditional client/server architectures are based on this model. The major drawback of this model is that the programs where the request is sent must be available and running for the calling application to work.

Asynchronous Communication In the disconnected or deferred model, programs communicate asynchronously, placing requests in a queue and then proceeding with their work.

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Distributed Databases Overview

For example, an application might require entry of data or execution of an operation after specific conditions are met. The recipient program retrieves the request from the queue and acts on it. This model is suitable for applications that can continue with their work after placing a request in the queue — they are not blocked waiting for a reply.

For deferred execution to work correctly in the presence of network, machine, and application failures, the requests must be stored persistently and processed exactly once. This is achieved by combining persistent queuing with transaction protection.

See Also: Oracle9i Application Developer’s Guide - Advanced

Queuing

Heterogeneous Services Overview

Heterogeneous Services is necessary for accessing a non-Oracle database system. The term "non-Oracle database system" refers to the following:

Any system accessed by PL/SQL procedures written in C (that is, by external procedures)

Any system accessed through SQL (that is, by Oracle Transparent Gateways and Generic Connectivity)

Any system accessed procedurally (that is, by procedural gateways) Heterogeneous Services makes it possible for users to do the following:

Use Oracle SQL statements to retrieve data stored in non-Oracle systems.

Use Oracle procedure calls to access non-Oracle systems, services, or application programming interfaces (APIs) from within an Oracle distributed environment.

Heterogeneous Services is generally applied in one of two ways:

Oracle Transparent Gateway is used in conjunction with Heterogeneous Services to access a particular, vendor-specific, non-Oracle system for which an Oracle Transparent Gateways is designed. For example, you would use the Oracle Transparent Gateway for Sybase on Solaris to access a Sybase database system that was operating on a Solaris platform.

Heterogeneous Services’ generic connectivity is used to access non-Oracle databases through ODBC or OLE DB interfaces.

See Also: Oracle9i Heterogeneous Connectivity Administrator’s Guide

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