1. Network management system
Performance management
Improving performance techniques

2. Replication
Replication of data : the maintenance of copies of data at multiple computers.
Advantages of replication:
Performance enhancement
Increased availability
Fault tolerance
Examples:
Caching
DNS
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

3. Increased availability:
The availability could be affected by:
Server failure
Network partitions and disconnected operation.
Availability calculation:
If each of n servers has an independent probability p of failing or becoming unreachable, then, the availability of object stored at each of these servers :
A=1-Probability(all managers failed or unreachable)=1-pn
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

4. Availability example: If there is a 5% probability of any individual server failure over given time period and if there are two servers then: The availability is: = =99.75%
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

5. Model components: Clients Front end Replica managers
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

6. A basic architectural model for the management of replicated dataFE
Requests and
replies C
Replica
Service
Clients
Front ends
managers RM
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

7. The passive (primary-backup) model for fault toleranceFE C RM
Primary
Backup
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

8. Active replication FE C RM
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

9. Caching Web cache keeps a local copy of all recently displayed pages.
There are two kinds of web caching:
Browser cache
Proxy cache

10. Web proxy server
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

11. Web caching Advantages of web caching: Improve performance
Reduce outbound traffic load
Reduce network delay

12. Redundancy
Multiple physical links between devices provide redundant paths
Advantages:
Network redundancy is a key maintaining network reliability ; the network can then continue to operate when a single link or port has failed
Redundant links can increase the capacity by sharing the traffic load

13. Redundancy

14. Redundancy

15. Clustering
A server cluster is a collection of servers, called nodes that communicate with each other to make a set of services highly available to clients.

16. Clustering Typical uses for server clusters include: file servers
print servers
database servers
messaging servers.

17. Clustering

18. Clustering Advantages: Ensure high availability
Provide increased scalability
Reduce downtime
The response time of applications can be improved