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Chapter 24

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

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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  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 • 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

  22. Workflow Ownership © Pearson Education Limited 1995, 2005

  23. 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

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

  25. 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

  26. 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

  27. 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

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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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

  38. Mobile DBMS © Pearson Education Limited 1995, 2005

  39. 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

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