DATABASE REPLICATION DISTRIBUTED DATABASE

O VERVIEW Replication : process of copying and maintaining database object, in multiple database that make up database system (Oracle docs) Changes applied at one site are captured and stored locally before being forwarded and applied at each of the remote locations The goal of replication is database consistency, scalability, and availability

M OST OF THE REASON USING REPLICATION Availability Performance Disconnected computing Network load reduction Mass deployment

M UTUAL CONSISTENCY OVERVIEW

M UTUAL CONSISTENCY OVERVIEW ( CONT ’ D )

T WO APPROACHES OF REPLICATION CONTROL Synchronous replication control – replica are kept in sync at all times Asynchronous replication control – replica are not kept in sync at all times

S YNCHRONOUS REPLICATION All copies of the same data item must show the same value when transaction access them

S YNCHRONOUS REPLICATION ( CONT ’ D ) Solution : Transaction is split into three transaction (T11, T12, T13) When T1 commits, all three copies will have update accordingly

A SYNCHRONOUS REPLICATION Copies don’t have to be kept sync at all times One of the approaches to implementing sync is primary copy approach / stored and forward approach. Stored and forward approach : Primary sites (publisher) : sites were updated first Secondary sites (subscriber) All transaction are run against first Then queued for transaction at secondary sites.

O RACLE R EPLICATION Type of oracle replication Multimaster replication Materialized view replication Hybrid configuration (multimaster and materialized view)

O RACLE REPLICATION ( CONT ’ D ) Multimaster replication Peer-to-peer or n-way replication – enable multiple site as equal peers Each node is master site Each site communicate with the other master site Use Asynchronous replication method, but did not rule to use synchronous

O RACLE REPLICATION ( CONT ’ D ) Materialized view replication Contains complete or partial copy at of target master from a single point at time. Target master is master table or master site or master materialized view The benefits : Enable local access Offload queries Increase data security – allow replicate only a selected subset of data

O RACLE REPLICATION ( CONT ’ D ) Read only materialized view replication Benefits : Eliminate possibility of conflict Support complex materialized view

O RACLE REPLICATION ( CONT ’ D ) Updatable materialized view Allow insert, update, delete rows of target master Benefits : Allow user to query and update local replicated data Require fewer resource than multimaster replication

O RACLE REPLICATION ( CONT ’ D ) Writeable materialized view Users can perform DML changes on materialized view, but the changes cannot be pushed back to the master, and lost if the materialized view refreshes. Row and column subsetting materialized view Enable to perform materialized view that contain a partial copy of data at master table or master materialized view.