Advance Replication [Part-I Introduction]

Dear Friends,

We can divide advance replication in three parts for better understanding.

1. Part-I Introduction
2. Part-II Multimaster Replication
3. Part-III Materialized view replication

Part-I Introduction

What is Advance Replication?
Replication is the process of copying and maintaining database objects, such as tables, in multiple databases that make up a distributed database system. Changes applied at one site are captured and stored locally before being forwarded and applied at each of the remote locations. Advanced Replication is a fully integrated feature of the Oracle 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 employees table resides at only the ny.world database in a distributed database system that also includes the hk.world and la.world databases. Replication means that the same data is available at multiple locations. For example, the employees table is available at ny.world, hk.world, and la.world.

Some of the common reasons for using replication are:

Availability
Replication improves the availability of applications because it provides them with alternative data access options. If one site becomes unavailable, then users can continue to query or even update the remaining locations. In other words, replication provides excellent failover protection.
Performance
Replication provides fast, local access to shared data because it balances activity over multiple sites. Some users can access one server while other users access different servers, thereby reducing the load at all servers. Also, users can access data from the replication site that has the lowest access cost, which is typically the site that is geographically closest to them.
Disconnected computing
A materialized view is a complete or partial copy (replica) of a target table from a single point in time. Materialized views enable users to work on a subset of a database while disconnected from the central database server. Later, when a connection is established, users can synchronize (refresh) materialized views on demand. When users refresh materialized views, they update the central database with all of their changes, and they receive any changes that may have happened while they were disconnected.
Network load reduction
Replication can be used to distribute data over multiple regional locations. Then, applications can access various regional servers instead of accessing one central server. This configuration can reduce network load dramatically.

Architecture of advance replication


There are two types of Advance Replication

1. Multimaster Replication
2. Materialized view replication

1. Multimaster replication
Start --- > set up master site --- > Select object for replication --- > Create Group for selected objects --- > if conflict possible then configure conflict resolution ---> end

2. Materialized view replication
Start --- > set up master site -- > set up materialized view site --- > create materialized view group --- > end

Note: In Detail explanation is Part-II, III.

Replication Sites
A replication group can exist at multiple replication sites. Replication environments support two basic types of sites: master sites and materialized view sites. One site can be both a master site for one replication group and a materialized view site for a different replication group. However, one site cannot be both the master site and the materialized view site for the same replication group.

The differences between master sites and materialized view sites are the following:
Replication Group
1. A replication group at a master site is more specifically referred to as a master group.
2. A replication group at a materialized view site is based on a master group and is more specifically referred to as a materialized view group.

Replication Objects
1. A master site maintains a complete copy of all objects in a replication group,
Example: if the hr_repg master group contains the table’s employees and departments, then all of the master sites participating in a master group must maintain a complete copy of employees and departments.

2. Materialized views at a materialized view site can contain all or a subset of the table data within a master group.
Example: one materialized view site might contain only a materialized view of the employees table, while another materialized view site might contain materialized views of both the employees and departments tables.

Communication with master sites
1. All master sites in a multimaster replication environment communicate directly with one another to continually propagate data changes in the replication group
2. Materialized view sites contain an image, or materialized view, of the table data from a certain point in time.

Replication Objects
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. Advanced Replication enables you to replicate the following types of objects:
• Tables
• Indexes
• Views and Object Views
• Packages and Package Bodies
• Procedures and Functions
• User-Defined Types and Type Bodies
• Triggers
• Synonyms
• Index types
• User-Defined Operators

Replication Groups
In a replication environment, Oracle manages replication objects using replication groups. A replication group is a collection of replication objects that are logically related.
By organizing related database objects within a replication group, it is easier to administer many objects together. Typically, you create and use a replication group to organize the schema objects necessary to support a particular database application. However, replication groups and schemas do not need to correspond with one another. A replication group can contain objects from multiple schemas, and a single schema can have objects in multiple replication groups. However, each replication object can be a member of only one replication group.


Part-II