Chapter 24 Replication and Mobile Databases Transparencies © Pearson Education Limited 1995, 2005

2 Chapter 24 - Objectives u How a replicated database differs from a distributed database. u The benefits of database replication. Distributed deadlock detection. u Examples of applications that use database replication. u Basic components of a replication system. © Pearson Education Limited 1995, 2005

3 Chapter 24 - Objectives u How synchronous replication differs from asynchronous replication. u The main types of data ownership are master/salve, workflow, and update-anywhere. u The functionality of a database replication server. u Main implementation issues associated with database replication. © Pearson Education Limited 1995, 2005

4 Chapter 24 - Objectives u How mobile computing supports the mobile worker. u Functionality of a mobile DBMS. u How Oracle DBMS supports database replication. © Pearson Education Limited 1995, 2005

5 Introduction to Database Replication u Functionality of DDBMS is attractive. However, implementations of required protocols and algorithms are complex and can cause problems that may outweigh advantages. u Alternative and more simplify approach to data distribution is provided by a replication server. u Every major database vendor has replication solution. © Pearson Education Limited 1995, 2005

6 Introduction to Database Replication u 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

7 Benefits of Database Replication © Pearson Education Limited 1995, 2005

8 Applications of Replication u Replication supports a variety of applications that have very different requirements. u Some applications are supported with only limited synchronization between the copies of the database and the central database system. u Other applications demand continuous synchronization between all copies of the database. © Pearson Education Limited 1995, 2005

9 Basic Components of Database Replication u Replication object is a database object such as a relation, index, view, procedure, or function existing on multiple servers in a distributed database system. u 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

10 Basic Components of Database Replication u Replication objects are managed using replication groups. u A replication group is a collection of replication objects that are logically related. u A replication group can exist at multiple replication sites. © Pearson Education Limited 1995, 2005

11 Basic Components of Database Replication u Replication environments support two basic types of sites: master sites and slave sites. u A replication group can be associated with one or more master sites and with one or more slave sites. © Pearson Education Limited 1995, 2005

12 Basic Components of Database Replication u One site can be both master site for one replication group and slave site for different replication group. u However, one site cannot be both the master site and slave site for same replication group. © Pearson Education Limited 1995, 2005

13 Basic Components of Database Replication u A master site controls a replication group and the objects in that group. u 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

14 Basic Components of Database Replication u 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. u Typically, a snapshot site is refreshed periodically to synchronize it with its master site. © Pearson Education Limited 1995, 2005

15 Basic Components of Database Replication u 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

16 Synchronous Versus Asynchronous Replication u 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. u 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

17 Data Ownership u Ownership relates to which site has privilege to update the data. u 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

18 Master/Slave Ownership u Asynchronously replicated data is owned by one (master) site, and can be updated by only that site. u Using ‘publish-and-subscribe’ metaphor, master site makes data available. u Other sites ‘subscribe’ to data owned by master site, receiving read-only copies. u Potentially, each site can be master site for non- overlapping data sets, but update conflicts cannot occur. © Pearson Education Limited 1995, 2005

19 Master/Slave Ownership – Data Dissemination © Pearson Education Limited 1995, 2005

20 Master/Slave Ownership – Data Consolidation © Pearson Education Limited 1995, 2005

21 Workflow Ownership u Avoids update conflicts, while providing more dynamic ownership model. u Allows right to update replicated data to move from site to site. u However, at any one moment, only ever one site that may update that particular data set. u 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

22 Workflow Ownership © Pearson Education Limited 1995, 2005

23 Update-Anywhere Ownership u Creates peer-to-peer environment where multiple sites have equal rights to update replicated data. u Allows local sites to function autonomously, even when other sites are not available. u Shared ownership can lead to conflict scenarios and have to employ methodology for conflict detection and resolution. © Pearson Education Limited 1995, 2005

24 Update-Anywhere Ownership © Pearson Education Limited 1995, 2005

25 Replication Servers Functionality u Basic function is copy data from one database to another (using synch. or asynch. replication). u Other functions include: –Scalability –Mapping and Transformation –Object Replication –Specification of Replication Schema –Subscription mechanism –Initialization mechanism –Easy Administration © Pearson Education Limited 1995, 2005

26 Implementation Issues u 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

27 Non-Transactional versus Transactional Update u Early replication mechanisms were non- transactional. u Data was copied without maintaining atomicity of transaction. u With transactional-based mechanism, structure of original transaction on source database is also maintained at target site. © Pearson Education Limited 1995, 2005

28 Non-Transactional versus Transactional Update © Pearson Education Limited 1995, 2005

29 Snapshots u Allow asynchronous distribution of changes to individual tables, collections of tables, views, or partitions of tables according to pre-defined schedule. u Common approach for snapshots uses the recovery log, minimizing the extra overhead to the system. © Pearson Education Limited 1995, 2005

30 Snapshots u In some DBMSs, process is part of server, while in others it runs as separate external server. u In event of network or site failure, need queue to hold updates until connection is restored. u To ensure integrity, order of updates must be maintained during delivery. © Pearson Education Limited 1995, 2005

31 Database Triggers u Could allow users to build their own replication applications using database triggers. u Users’ responsibility to create code within trigger that will execute whenever appropriate event occurs. © Pearson Education Limited 1995, 2005

32 Database Triggers CREATE TRIGGER StaffAfterInsRow BEFORE INSERT ON Staff FOR EACH ROW BEGIN INSERT INTO VALUES (:new.staffNo, :new:fName, :new:lName, :new.position, :new:sex, :new.DOB, :new:salary, :new:branchNo); END; © Pearson Education Limited 1995, 2005

33 Database Triggers - Drawbacks u Management and execution of triggers have a performance overhead. u Burden on application/network if master table updated frequently. u Triggers cannot be scheduled. u Difficult to synchronize replication of multiple related tables. u Activation of triggers cannot be easily undone in event of abort or rollback. © Pearson Education Limited 1995, 2005

34 Conflict Detection and Resolution u When multiple sites are allowed to update replicated data, need to detect conflicting updates and restore data consistency. u For a single table, source site could send both old and new values for any rows updated since last refresh. u 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

35 Conflict Detection and Resolution u Also want to detect other types of conflict such as violation of referential integrity. u 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

36 Mobile Databases u Increasing demands on mobile computing to provide types of support required by growing number of mobile workers. u Work as if in the office but in reality working from remote locations. u ‘Office’ may accompany remote worker in form of laptop, PDA (Personal Digital Assistant), or other Internet access device. © Pearson Education Limited 1995, 2005

37 Mobile Database u 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

38 Mobile DBMS © Pearson Education Limited 1995, 2005

39 Mobile DBMS u 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