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Chapter 24. Replication and Mobile Databases Transparencies. © Pearson Education Limited 1995, 2005. Chapter 24 - Objectives. How a replicated database differs from a distributed database. The benefits of database replication. Distributed deadlock detection.

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chapter 24

Chapter 24

Replication and Mobile Databases

Transparencies

© Pearson Education Limited 1995, 2005

chapter 24 objectives
Chapter 24 - Objectives
  • How a replicated database differs from a distributed database.
  • The benefits of database replication.Distributed deadlock detection.
  • Examples of applications that use database replication.
  • Basic components of a replication system.

© Pearson Education Limited 1995, 2005

chapter 24 objectives1
Chapter 24 - Objectives
  • How synchronous replication differs from asynchronous replication.
  • The main types of data ownership are master/salve, workflow, and update-anywhere.
  • The functionality of a database replication server.
  • Main implementation issues associated with database replication.

© Pearson Education Limited 1995, 2005

chapter 24 objectives2
Chapter 24 - Objectives
  • How mobile computing supports the mobile worker.
  • Functionality of a mobile DBMS.
  • How Oracle DBMS supports database replication.

© Pearson Education Limited 1995, 2005

introduction to database replication
Introduction to Database Replication
  • Functionality of DDBMS is attractive. However, implementations of required protocols and algorithms are complex and can cause problems that may outweigh advantages.
  • Alternative and more simplify approach to data distribution is provided by a replication server.
  • Every major database vendor has replication solution.

© Pearson Education Limited 1995, 2005

introduction to database replication1
Introduction to Database Replication
  • Database Replication is the process of copying and maintaining database objects, such as relations, in multiple databases that make up a distributed database system.

© Pearson Education Limited 1995, 2005

benefits of database replication
Benefits of Database Replication

© Pearson Education Limited 1995, 2005

applications of replication
Applications of Replication
  • Replication supports a variety of applications that have very different requirements.
  • Some applications are supported with only limited synchronization between the copies of the database and the central database system.
  • Other applications demand continuous synchronization between all copies of the database.

© Pearson Education Limited 1995, 2005

basic components of database replication
Basic Components of Database Replication
  • Replication object is a database object such as a relation, index, view, procedure, or function 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.

© Pearson Education Limited 1995, 2005

basic components of database replication1
Basic Components of Database Replication
  • Replication objects are managed using replication groups.
  • A replication group is a collection of replication objects that are logically related.
  • A replication group can exist at multiple replication sites.

© Pearson Education Limited 1995, 2005

basic components of database replication2
Basic Components of Database Replication
  • Replication environments support two basic types of sites: master sites and slave sites.
  • A replication group can be associated with one or more master sites and with one or more slave sites.

© Pearson Education Limited 1995, 2005

basic components of database replication3
Basic Components of Database Replication
  • One site can be both master site for one replication group and slave site for different replication group.
  • However, one site cannot be both the master site and slave site for same replication group.

© Pearson Education Limited 1995, 2005

basic components of database replication4
Basic Components of Database Replication
  • A master site controls a replication group and the objects in that group.
  • This is achieved by maintaining a complete copy of all objects in a replication group and by propagating any changes to a replication group to any slave sites.

© Pearson Education Limited 1995, 2005

basic components of database replication5
Basic Components of Database Replication
  • A slave site can contain all or a subset of objects from a replication group. However, slave sites only contain a snapshot of a replication group.
  • Typically, a snapshot site is refreshed periodically to synchronize it with its master site.

© Pearson Education Limited 1995, 2005

basic components of database replication6
Basic Components of Database Replication
  • For a replication environment with many master sites, all of those sites communicate directly with one another to continually propagate data changes in the replication group.

© Pearson Education Limited 1995, 2005

synchronous versus asynchronous replication
Synchronous Versus Asynchronous Replication
  • Synchronous – updates to replicated data are part of enclosing transaction.
    • If one or more sites that hold replicas are unavailable transaction cannot complete.
    • Large number of messages required to coordinate synchronization.
  • Asynchronous - target database updated after source database modified. Delay in regaining consistency may range from few seconds to several hours or even days.

© Pearson Education Limited 1995, 2005

data ownership
Data Ownership
  • Ownership relates to which site has privilege to update the data.
  • Main types of ownership are:
    • Master/slave (or asymmetric replication),
    • Workflow,
    • Update-anywhere (or peer-to-peer or symmetric replication).

© Pearson Education Limited 1995, 2005

master slave ownership
Master/Slave Ownership
  • Asynchronously replicated data is owned by one (master) site, and can be updated by only that site.
  • Using ‘publish-and-subscribe’ metaphor, master site makes data available.
  • Other sites ‘subscribe’ to data owned by master site, receiving read-only copies.
  • Potentially, each site can be master site for non-overlapping data sets, but update conflicts cannot occur.

© Pearson Education Limited 1995, 2005

master slave ownership data dissemination
Master/Slave Ownership – Data Dissemination

© Pearson Education Limited 1995, 2005

master slave ownership data consolidation
Master/Slave Ownership – Data Consolidation

© Pearson Education Limited 1995, 2005

workflow ownership
Workflow Ownership
  • Avoids update conflicts, while providing more dynamic ownership model.
  • Allows right to update replicated data to move from site to site.
  • However, at any one moment, only ever one site that may update that particular data set.
  • Example is order processing system, which follows series of steps, such as order entry, credit approval, invoicing, shipping, and so on.

© Pearson Education Limited 1995, 2005

workflow ownership1
Workflow Ownership

© Pearson Education Limited 1995, 2005

update anywhere ownership
Update-Anywhere Ownership
  • Creates peer-to-peer environment where multiple sites have equal rights to update replicated data.
  • Allows local sites to function autonomously, even when other sites are not available.
  • Shared ownership can lead to conflict scenarios and have to employ methodology for conflict detection and resolution.

© Pearson Education Limited 1995, 2005

update anywhere ownership1
Update-Anywhere Ownership

© Pearson Education Limited 1995, 2005

replication servers functionality
Replication Servers Functionality
  • Basic function is copy data from one database to another (using synch. or asynch. replication).
  • Other functions include:
    • Scalability
    • Mapping and Transformation
    • Object Replication
    • Specification of Replication Schema
    • Subscription mechanism
    • Initialization mechanism
    • Easy Administration

© Pearson Education Limited 1995, 2005

implementation issues
Implementation Issues
  • Issues associated with the provision of data replication by the replication server include:
    • transactional updates;
    • snapshots and database triggers;
    • conflict detection and resolution.

© Pearson Education Limited 1995, 2005

non transactional versus transactional update
Non-Transactional versus Transactional Update
  • Early replication mechanisms were non-transactional.
  • Data was copied without maintaining atomicity of transaction.
  • With transactional-based mechanism, structure of original transaction on source database is also maintained at target site.

© Pearson Education Limited 1995, 2005

non transactional versus transactional update1
Non-Transactional versus Transactional Update

© Pearson Education Limited 1995, 2005

snapshots
Snapshots
  • Allow asynchronous distribution of changes to individual tables, collections of tables, views, or partitions of tables according to pre-defined schedule.
  • Common approach for snapshots uses the recovery log, minimizing the extra overhead to the system.

© Pearson Education Limited 1995, 2005

snapshots1
Snapshots
  • In some DBMSs, process is part of server, while in others it runs as separate external server.
  • In event of network or site failure, need queue to hold updates until connection is restored.
  • To ensure integrity, order of updates must be maintained during delivery.

© Pearson Education Limited 1995, 2005

database triggers
Database Triggers
  • Could allow users to build their own replication applications using database triggers.
  • Users’ responsibility to create code within trigger that will execute whenever appropriate event occurs.

© Pearson Education Limited 1995, 2005

database triggers1
Database Triggers

CREATE TRIGGER StaffAfterInsRow

BEFORE INSERT ON Staff

FOR EACH ROW

BEGIN

INSERT INTO StaffDuplicate@Rentals.Glasgow.North.Com

VALUES (:new.staffNo, :new:fName, :new:lName, :new.position, :new:sex, :new.DOB, :new:salary, :new:branchNo);

END;

© Pearson Education Limited 1995, 2005

database triggers drawbacks
Database Triggers - Drawbacks
  • Management and execution of triggers have a performance overhead.
  • Burden on application/network if master table updated frequently.
  • Triggers cannot be scheduled.
  • Difficult to synchronize replication of multiple related tables.
  • Activation of triggers cannot be easily undone in event of abort or rollback.

© Pearson Education Limited 1995, 2005

conflict detection and resolution
Conflict Detection and Resolution
  • When multiple sites are allowed to update replicated data, need to detect conflicting updates and restore data consistency.
  • For a single table, source site could send both old and new values for any rows updated since last refresh.
  • At target site, replication server can check each row in target database that has also been updated against these values.

© Pearson Education Limited 1995, 2005

conflict detection and resolution1
Conflict Detection and Resolution
  • Also want to detect other types of conflict such as violation of referential integrity.
  • Some of most common mechanisms are:
    • Earliest and latest timestamps.
    • Site Priority.
    • Additive and average updates.
    • Minimum and maximum values.
    • User-defined.
    • Hold for manual resolution.

© Pearson Education Limited 1995, 2005

mobile databases
Mobile Databases
  • Increasing demands on mobile computing to provide types of support required by growing number of mobile workers.
  • Work as if in the office but in reality working from remote locations.
  • ‘Office’ may accompany remote worker in form of laptop, PDA (Personal Digital Assistant), or other Internet access device.

© Pearson Education Limited 1995, 2005

mobile database
Mobile Database
  • Database that is portable and physically separate from a centralized database server but is capable of communicating with server from remote sites allowing the sharing of corporate data.

© Pearson Education Limited 1995, 2005

mobile dbms
Mobile DBMS

© Pearson Education Limited 1995, 2005

mobile dbms1
Mobile DBMS
  • Functionality required of mobile DBMSs includes ability to:
    • communicate with centralized database server through modes such as wireless or Internet access;
    • replicate data on centralized database server and mobile device;
    • synchronize data on centralized database server and mobile device;
    • capture data from various sources such as Internet;
    • manage/analyze data on the mobile device;
    • create customized mobile applications.

© Pearson Education Limited 1995, 2005