1. Distributed Database Management Systems

2. Lecture # 21

3. Fragmentation: One of the possible way we can implement a DDBS
In the previous lectures
Fragmentation: One of the possible way we can implement a DDBS

4. Types of Fragmentation
Horizontal Fragmentation
Vertical Fragmentation

5. Types of Horizontal Fragmentation
Primary Horizontal Fragmentation
Derived Horizontal Fragmentation

6. Vertical Fragmentation
Vertical Fragmentation is relatively more complex.
Why ?

7. In today’s Lecture
Replication

8. Storing a separate copy of database at each of two or three or more sites

9. Advantages of Replication
Reliability.
Fast response.
May avoid complicated distributed transaction integrity routines (if replicated data is refreshed at scheduled intervals.)

10. De-couples nodes (transactions proceed even if some nodes are down.)
Reduced network traffic at prime time (if updates can be delayed.)

11. Disadvantages of Replication
Additional requirements for storage space.
Additional time for update operations.

12. Therefore, better when used for non-volatile data.
Complexity and cost of updating.
Integrity exposure of getting incorrect data if replicated data is not updated simultaneously.
Therefore, better when used for non-volatile data.

13. Replication Architecture in SQL Server

14. Replication uses a publish-subscribe model for distributing data

15. Publication
A Publication is group of related data and objects that we want to replicate together.

16. Publisher
A Publisher is a server that is the source of data to be replicated.

17. Subscriber
A Subscriber is a server that receives the data replicated by the publisher.

18. The Subscriber defines a subscription to a particular publication

19. 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.

20. SQL Server Replication Agents
Agents are the software components used in replication

21. Types Of SQL Server Replication Agents
Snapshot Agent
Merge Agent
Distribution Agent
Log Reader Agent

22. 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

23. Push/Pull subscriptions; depends where the distributor is running.

24. Replication Models

25. Publ/Dist Sever
Central Publisher / Distributor

26. Remote
Distr
Publisher
Central Publisher / Remote Distributor

27. Replication Methods Snapshot Replication Transaction Replication.
Merge Replication.

28. Snapshot Replication

29. Preferred when subscribers need read-only access
Higher latency (working without updated data) low Bandwidth

30. Used in all types, initially
Can be push or pull
How to do it.

31. Summary