Replication.ppt
- 1. Replication
- 2. What is replication? ?& why replication? • Replication is having multiple copies of data and services in a distributed system • Reasons: – Reliability of the system – Better protection against corrupted data – Improved Performance and faster response time – Facilitates scaling in numbers and geographical area.
- 3. Key Issues: • Where, when and by whom replicas should be placed. • Mechanisms to keep them consistent. • Two main sub-problems: – Replica-server Placement » Finding best location or placed where a server can be placed. – Content Placement » Finding out which server is best for storing a particular content.
- 4. Content Replication and Placement • Permanent Replicas – Geographically distributed - Mirroring – Same location – Round Robin • Server Initiated Replicas • Client Initiated Replicas
- 5. Server Initiated Replicas • Initiative of owner of data store • Enhance performance P C1 C2 Server without copy of F Server with copy of F Q
- 6. Client Initiated Replicas • Client caches • Managing is entirely by client • Improve access time • Placement – Same machine – LAN – WAN
- 7. Content Distribution • Propagation of Updated content – Propagate only notification of an update » Invalidation Protocols – Transfer data from one copy to another – Propagate the update operation to other copies
- 8. Push Vs Pull Protocols • Push – Server based – Read to update ratio is high – High degree of consistency – Multicasting • Pull – Client based – Read to update ratio is low – Unicasting • Lease
- 9. Why Use Replication Enhances a service (object/data/service) Increased Availability Of service. When servers fail or when the network is partitioned, service still available at at least once server. Fault Tolerance Under the fail-stop model, if up to f of f+1 servers crash, at least one is alive. Load Balancing One approach: Multiple server IPs can be assigned to the same name in DNS, which returns answers/IPs round-robin. P: probability that one server fails= 1 – P= availability of service. e.g. P = 5% => service is available 95% of the time. Pn: probability that n servers fail= 1 – Pn= availability of replicated service. e.g. P = 5%, n = 3 => service available 99.875% of the time
- 10. Goals of Replication Replication Transparency User/client need not know that multiple physical copies of data exist. Replication Consistency Data is consistent on all of the replicas of an object (or is converging towards becoming consistent). Client Front End (FE) RM RM RM Client Front End (FE) Client Front End (FE) Service server server server Replica Manager
- 11. Types of data replication • 1. Synchronous Replication: In synchronous replication, the replica will be modified immediately after some changes are made in the relation table. So there is no difference between original data and replica. 2. Asynchronous replication: In asynchronous replication, the replica will be modified after commit is fired on to the database.
- 12. Replication Schemes Full Replication • In full replication scheme, the database is available to almost every location or user in communication network. Advantages of full replication • High availability of data, as database is available to almost every location. • Faster execution of queries. Disadvantages of full replication • Concurrency control is difficult to achieve in full replication. • Update operation is slower.
- 13. Replication Schemes No Replication • No replication means, each fragment is stored exactly at one location. Advantages of no replication • Concurrency can be minimized. • Easy recovery of data. Disadvantages of no replication • Poor availability of data. • Slows down the query execution process, as multiple clients are accessing the same server .
- 14. Replication Schemes Partial replication • Partial replication means only some fragments are replicated from the database. Advantages of partial replication • The number of replicas created for fragments depend upon the importance of data in that fragment. Disadvantages of partial replication • Identification of critical & non critical data