Distributed Database Management Systems: Lecture 21. The main topics covered in this chapter include: types of fragmentation with horizontal fragmentation, vertical fragmentation; storing a separate copy of database at each of two or three or more sites;...
Distributed Database Management Systems Lecture # 21 In the previous lectures Fragmentation: One of the possible way we can implement a DDBS Types of Fragmentation • Horizontal Fragmentation • Vertical Fragmentation Types of Horizontal Fragmentation Primary Horizontal Fragmentation Derived Horizontal Fragmentation Vertical Fragmentation Vertical Fragmentation is relatively more complex Why ? In today’s Lecture Replication Storing a separate copy of database at each of two or three or more sites • Advantages of Replication –Reliability –Fast response –May avoid complicated distributed transaction integrity routines (if replicated data is refreshed at scheduled intervals.) –De-couples nodes (transactions proceed even if some nodes are down.) –Reduced network traffic at prime time (if updates can be delayed.) Subscriber A Subscriber is a server that receives the data replicated by the publisher • The Subscriber defines a subscription to a particular publication • A Distributor is a server that performs various tasks when moving articles from Publishers to Subscribers • The actual tasks performed depend on the type of replication performed SQL Server Replication Agents Agents are the software components used in replication Types Of SQL Server Replication Agents • Snapshot Agent • Merge Agent • Distribution Agent • Log Reader Agent • Snapshot Agent: Runs at least once in all replicas • Distribution Agent: performs different activities, mainly distributes publication to subscribers • Log reader Agent: used in transactional rep, reads log files of all servers involved • Merge Agent: Meld changes from different servers made since last snapshot Push/Pull subscriptions; depends where the distributor is running Replication Models Publ/Dist Sever Central Publisher / Distributor Remote Distr Publisher Central Publisher / Remote Distributor Replication Methods • Snapshot Replication • Transaction Replication • Merge Replication Snapshot Replication • Preferred when subscribers need readonly access • Higher latency (working without updated data) low Bandwidth • Used in all types, initially • Can be push or pull • How to it Summary .. .Lecture # 21 In the previous lectures Fragmentation: One of the possible way we can implement a DDBS Types of... In today’s Lecture Replication Storing a separate copy of database at each of two or three or more sites • Advantages of Replication –Reliability –Fast response –May avoid complicated distributed