PERFORMANCE MANAGEMENT IMPROVING PERFORMANCE TECHNIQUES Network management system 1

Improving performance techniques 2 Replication Caching Redundancy Clustering

3 Replication of data : the maintenance of copies of data at multiple computers. Advantages of replication: 1. Performance enhancement 2. Increased availability 3. Fault tolerance Examples: 1. Caching 2. DNS Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Replication

4 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-p n Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

5 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. 4 © Pearson Education 2005

6 Model components: Clients Front end Replica managers Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

A basic architectural model for the management of replicated data Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 FE Requests and replies C Replica C Service Clients Front ends managers RM FE RM

The passive (primary-backup) model for fault tolerance Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 FE C C RM Primary Backup RM

Active replication Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 FEC CRM

Caching 10 Web cache keeps a local copy of all recently displayed pages. There are two kinds of web caching: Browser cache Proxy cache

Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Web proxy server

Web caching 12 Advantages of web caching: Improve performance Reduce outbound traffic load Reduce network delay

Redundancy 13 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

14 Redundancy

15 Redundancy

Clustering 16 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.

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

18 Clustering

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