|Oracle8i Integration Server Overview
Release 3 (8.1.7)
Part Number A83729-01
In a distributed database system, Oracle replication is a process that maintains multiple copies of the same data in separate Oracle databases. Replication captures and stores changes made to the data at one location before forwarding and applying them at each remote location. Users at each location see the same consistent view of the data. They need not access data remotely, as they would in a standard distributed database system. This chapter contains:
Replication copies and maintains database objects, such as tables, in the multiple databases that make up a distributed database system. Oracle replication is a fully integrated feature of the Oracle database server; it is not a separate server.
Replication uses distributed database technology to share data between multiple sites, but a replicated database and a distributed database are not the same. In a distributed database, data is available at many locations, but a particular table resides at only one location.
For example, the
emp table resides only at the DB1 database in a distributed database system that also includes the DB2 and DB3 databases. Replication means that the same data is available at multiple locations. For example, the
emp table is available at DB1, DB2, and DB3.
This section contains:
Some common reasons for using replication are:
Replication is ideally suited to synchronize data between systems in different locations so that each has a consistent view of the data without the necessity of accessing the data remotely.
Replication enables you to replicate tables and other supporting objects such as views, database triggers, packages, indexes, and synonyms by organizing logically related objects into replication groups. A replication object is a database object existing on multiple servers in a distributed database system. In a replication environment, any updates made to a replication object at one site are applied to the copies at all other sites. Use replication to:
You can ensure transactional integrity by choosing particular replication features and setting up the replication to occur synchronously. (Asynchronous replication is the norm.) Replication has its own set of data dictionary tables and views called the replication catalog. A replication management API consists of PL/SQL packages that can be used to administer the replication objects.
The Oracle database supports three types of replication:
Multimaster replication (also called peer-to-peer or n-way replication) enables multiple sites, acting as equal peers, to manage groups of replicated database objects. Each site in a multimaster replication environment is a master site.
Applications can update any replicated table at any site in a multimaster configuration. Oracle database servers operating as master sites in a multimaster environment automatically converge the data of all table replicas and ensure global transaction consistency and data integrity.
A snapshot contains a complete or partial copy of a target master table from a single point in time. A snapshot may be read-only or updatable. All snapshots:
Multimaster replication and snapshots can be combined in hybrid or mixed configurations to meet different application requirements. Mixed configurations can have any number of master sites and multiple snapshot sites for each master.
For example, multimaster (or n-way) replication between two masters can support full-table replication between the databases that support two geographic regions. Snapshots can be defined on the masters to replicate full tables or table subsets to sites within each region.
Oracle Corporation offers a number of products that you can use to facilitate communication with non-Oracle products and technologies. These are grouped under Data Access Gateways and fall into three broad categories: Oracle Transparent Gateways, Oracle Procedural Gateways, and Oracle Access Managers. Both the Transparent Gateways and the Procedural Gateways enable you to access non-Oracle technology from an Oracle environment. The Oracle Access Managers enable you to access Oracle technology from a non-Oracle environment.
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Oracle Transparent Gateways gives applications access through transactional SQL to non-Oracle data stores such as DB2, IMS, and DB2/400.
Oracle Transparent Gateways provides:
OTG provides two major IBM-oriented transparent gateways for DRDA and EDA/SQL, a transparent gateway for ODBC, and transparent gateways for Sybase, Infomix, Ingres, Microsoft SQL Server, HP IMAGESQL, Digital RMS, and Rdb.
The Oracle Transparent Gateway for IBM DRDA is the Oracle implementation of the IBM DRDA architecture. It provides Oracle applications with read and write access to DRDA server databases, including DB2 OS/390, SQL/DS, DB2/400, and DB2/UDB.
With the Transparent Gateway for EDA/SQL running on OS/390, you can access the majority of non-Oracle databases and file systems, including VSAM, IMS, ISAM, Teradata, ADABAS and DB2.
The Procedural Gateways offer programmatic access to non-Oracle transactions from within the context of an Oracle transaction. They are IBM-oriented and fall into two types: Procedural Gateway for APPC and Procedural Gateway for MQSeries.
Because the Procedural Gateway for MQSeries is message-oriented, we cover it in Chapter #, "Advanced Queueing" (hotlink).
The Oracle Procedural Gateway for APPC uses native IBM APPC / LU6.2 mechanisms to execute mainframe transactions through a transaction monitor such as IBM CICS, IBM IMS/TM, or CA-IDMS/DC.
These transactions access a variety of databases and file systems, including:
Procedural Gateway for APPC enables:
Used in combination with the Transparent Gateways, Access Managers enable mainframe applications to access virtually any data store. The Access Managers let you use existing OS/390 and AS/400 applications by providing them with access through SQL to data in the Oracle Server. The Access Manager for AS/400 enables AS/400 applications written in RPG, COBOL, and C to access data stored in Oracle.
The Oracle Access Managers for CICS and IMS/TM are part of the Oracle for MVS Client Solution, which enables you to access data in Oracle from TSO, batch, CICS, and IMS/TM programs:
An original method of integrating applications was to use data level integration to maintain multiple copies of the data in the different applications and to periodically synchronize the copies. If the requirements for integration are relatively basic, then data level integration provides a simple, cost-effective, and manageable solution.
Solutions that successfully employ data level integration have most or all of the following characteristics:
transactiontables) are broadly similar in all applications.
Classic solutions that use this style of integration include:
The Oracle products best suited to providing this level of integration are Oracle Replication and Oracle Data Access Gateways.
Oracle Replication enables asynchronous synchronization of data across distributed Oracle databases using journal logs to capture both changes made to the master copy of table structures and row-level data changes. The logs then apply the changes to the snapshot copies of the tables.
Oracle Replication is a mature product that contains numerous advanced features that extend this basic functionality to:
In a data-level integration solution, Oracle Replication is the mainstay of integration between applications based on the Oracle database.
Oracle Corporation provides a number of transparent gateways that enable seamless integration to other databases in Oracle-controlled transactions. For transactions that are not controlled by the Oracle transaction manager, use the Access Managers to enable data in Oracle databases to be accessed by non-Oracle-controlled transactions.