Replication Libby Rasnick Christopher Newport University CPSC 550 Spring 2003

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University2 Table of Contents DefinitionHistoryGoalsFeaturesStructure How to Use Applications Benefits and Issues Cost of Replication Significant Points SummaryReferences

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University3 Defining replication “the maintenance of copies of data at multiple computers as a technique for automatically maintaining the availability of data despite server failures”

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University4 Defining replication “the maintenance of copies of data at multiple computers as a technique for automatically maintaining the availability of data despite server failures” “a key to the effectiveness of distributed systems in that it provides enhanced performance, high availability and fault tolerance”

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University5 History Initially, stored data back-ups (tape)

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University6 History Initially, stored data back-ups (tape) With distributed systems, partial or entire copies of a node’s data are stored by another node

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University7 History Initially, stored data back-ups (tape) With distributed systems, partial or entire copies of a node’s data are stored by another node Recent development, peer-to-peer resource trading (community-based redundancy system) ~ Napster

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University8 Goals “to improve a service’s performance, to increase its availability, or to make it fault-tolerant.”

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University9 Goals “to improve a service’s performance, to increase its availability, or to make it fault-tolerant.” enhances distributed systems by restricting or eliminating downtime

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University10 Goals “to improve a service’s performance, to increase its availability, or to make it fault-tolerant.” enhances distributed systems by restricting or eliminating downtime fundamental trade-off in fault-tolerance: consistency v. availability (and performance)

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University11 Features Passive master replication manager allocates requests to slave managers

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University12 Features Passive master replication manager allocates requests to slave managers Active requests are multicast to all replication managers and each replies independently

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University13 Structure Replication system transaction steps [1] Step Name Description 1 request FE issues a request for service to one or more RMs 2 coordination RMs communicate together to deliver consistent results An ordering scheme must be applied to incoming requests. Common ordering schemes used: FIFO, casual, and total. 3 execution RM executes the request in a reversible manner 4 agreement RMs agree on the effects of the request 5 response RM(s) response sent to FE FE must consolidated and forward to client.

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University14 How to Use (server/RM) Determine the number of servers & availability of data Number of RMs (servers): n servers, probability p of server access failure 1-probability(all servers fail) = 1 - p n Data availability (objects): If f of f+1 servers crash, then 1 server is still operating & if up to f servers can exhibit Byzantine failures, then 2f+1 servers would permit full function during most worst case scenarios

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University15 How to Use (server/RM) add RMs (as calculated) ensure communication between RMs install FEs to accommodate number of clients follow five transaction steps: request, coordination, execution, agreement, and response

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University16 Applications DNS servers eases request bottlenecks

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University17 Applications DNS servers eases request bottlenecks Community-based redundancy systems propagated data alleviates pressure from server failures

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University18 Applications DNS servers eases request bottlenecks Community-based redundancy systems propagated data alleviates pressure from server failures gossip, Bayou, Coda

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University19 Benefits and Issues Benefits “selecting a nearby replica to serve an access request results in a shorter service time” [2] reduced or eliminated down-time due to server failure

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University20 Benefits and Issues Benefits “selecting a nearby replica to serve an access request results in a shorter service time” [2] reduced or eliminated down-time due to server failure Issues location dependent replica updating cost of replication

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University21 Cost of Replication What to charge for and when

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University22 Cost of Replication What to charge for and when Calculated per transaction

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University23 Cost of Replication What to charge for and when Calculated per transaction Cost modeling

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University24 Cost of Replication What to charge for and when Calculated per transaction Cost modeling Cost Model Analysis V – set of sites in a network S – set in question R – resident set R = S  V of copy-bearing sites

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University25 Significant Points Proportional cost based on number of accesses

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University26 Significant Points Proportional cost based on number of accesses fee paid by the server for every access past a set maximum allotment of accesses

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University27 Significant Points Proportional cost based on number of accesses fee paid by the server for every access past a set maximum allotment of accesses rewards servers that perform good file maintenance (requesting a replica of a frequently accessed file)

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University28 Significant Points Proportional cost based on number of accesses fee paid by the server for every access past a set maximum allotment of accesses rewards servers that perform good file maintenance (requesting a replica of a frequently accessed file) more of the cost of the replication system is absorbed by those servers that do not use it properly

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University29 Summary Definition: maintaining copies of data in fault- independent location Goals: accessibility, fault-tolerance, consistency Features: passive and active models Structure: clients, front ends, replication managers

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University30 Summary How to Use: request, coordination, execution, agreement, response Applications: DNS servers, Napster, gossip, Bayou, Coda Benefits: increased data accessibility and fault- tolerance Issues: data consistency and transparency, cost of replication

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University31 References [1] Coulouris, George, Dollimore, Jean, and Kindberg, Tim. Distributed Systems: Concepts and Design. Third Ed. New York: Addison Wesley [2] Levy, Eliezer and Silberschatz, Abraham. Distributed File Systems: Concepts and Examples. ACM Computing Surveys, 22(4): , December [3] Cooper, Brian F. and Garcia-Molina, Hector. Peer-to-peer resource trading in a reliable distributed system. 1 st International Workshop on Peer- to-Peer Systems, 11 November [4] Goldweber, Michael and Johnson, Donald B. Minimizing Access Cost in Replicated Distributed Systems. Symposium on Principles of Distributed Computing: 56, [5] Comer, Douglas E. Computer Networks and Internets. Second Ed. Upper Saddle River: Prentice Hall