Network management system Performance management Improving performance techniques

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. 4 © Pearson Education 2005

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. 4 © Pearson Education 2005

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: 1-0.052=1-0.0025=99.75% Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

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 FE 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. 4 © Pearson Education 2005

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

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

Caching Web cache keeps a local copy of all recently displayed pages. There are two kinds of web caching: Browser cache Proxy cache https://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_2-3/ipj_2-3.pdf

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

Web caching Advantages of web caching: Improve performance Reduce outbound traffic load Reduce network delay https://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_2-3/ipj_2-3.pdf

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 http://ptgmedia.pearsoncmg.com/images/9781587133282/downloads/9781587133282%20_chapter_2.pdf

Redundancy http://ptgmedia.pearsoncmg.com/images/9781587133282/downloads/9781587133282%20_chapter_2.pdf

Redundancy http://ptgmedia.pearsoncmg.com/images/9781587133282/downloads/9781587133282%20_chapter_2.pdf

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. https://technet.microsoft.com/en-us/library/cc738051(v=ws.10).aspx

Clustering Typical uses for server clusters include: file servers print servers database servers messaging servers. https://technet.microsoft.com/en-us/library/cc738051(v=ws.10).aspx

Clustering http://tws.trijit.uk/datacenter/clustering-servers

Clustering Advantages: Ensure high availability Provide increased scalability Reduce downtime The response time of applications can be improved http://www.tech-faq.com/server-clustering-technologies-and-concepts.html