replication libby rasnick christopher newport university cpsc 550 spring 2003

ReplicationReplication

Libby RasnickLibby Rasnick

Christopher Newport UniversityChristopher Newport University

CPSC 550CPSC 550

Spring 2003Spring 2003

Libby Rasnick CPSC 550 Replication Case Study Christopher Newport University

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Table of ContentsTable of ContentsDefinitionDefinitionHistoryHistoryGoalsGoalsFeaturesFeaturesStructureStructureHow to UseHow to UseApplicationsApplicationsBenefits and IssuesBenefits and IssuesCost of ReplicationCost of ReplicationSignificant PointsSignificant PointsSummarySummaryReferencesReferences


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Defining replicationDefining replication

“the maintenance of copies of data at multiple computers as a technique for automatically maintaining the availability of data despite server failures”


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Defining replicationDefining 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”


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HistoryHistory

Initially, stored data back-ups (tape)


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HistoryHistory


With distributed systems, partial or entire copies of a node’s data are stored by another node


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HistoryHistory


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


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GoalsGoals

“to improve a service’s performance, to increase its availability, or to make it fault-tolerant.”


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GoalsGoals


enhances distributed systems by restricting or eliminating downtime


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GoalsGoals


enhances distributed systems by restricting or eliminating downtime

fundamental trade-off in fault-tolerance: consistency v. availability (and performance)


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FeaturesFeatures

Passive

master replication manager allocates requests to slave managers


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FeaturesFeatures

Passive

master replication manager allocates requests to slave managers

Active

requests are multicast to all replication managers and each replies independently


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StructureStructureReplication system transaction steps [1]Step Name Description1 request FE issues a request for service to one or more

RMs2 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 manner4 agreement RMs agree on the effects of the request5 response RM(s) response sent to FE

FE must consolidated and forward to client.


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How to Use (server/RM)How to Use (server/RM) Determine the number of servers & availability of data

Number of RMs (servers):

n servers, probability p of server access failure1-probability(all servers fail) = 1 - pn

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, then2f+1 servers would permit full function during most worst case scenarios


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How to Use (server/RM)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


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ApplicationsApplications

DNS servers

eases request bottlenecks


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DNS servers


Community-based redundancy systems

propagated data alleviates pressure from server failures


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DNS servers


Community-based redundancy systems

propagated data alleviates pressure from server failures

gossip, Bayou, Coda


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Benefits and IssuesBenefits 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


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Benefits and IssuesBenefits and IssuesBenefits

“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


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Cost of Replication

What to charge for and when


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Cost of Replication


Calculated per transaction


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Cost of Replication



Cost modeling


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Cost of Replication



Cost modeling

Cost Model AnalysisV – set of sites in a networkS – set in questionR – resident set

R = S V of copy-bearing sites


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Significant PointsSignificant Points

Proportional cost based on number of accesses


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fee paid by the server for every access past a set maximum allotment of accesses


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rewards servers that perform good file maintenance (requesting a replica of a frequently accessed file)


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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


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SummarySummary 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


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SummarySummaryHow 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


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ReferencesReferences[1] Coulouris, George, Dollimore, Jean, and Kindberg, Tim. Distributed Systems: Concepts and Design. Third Ed. New York: Addison Wesley. 2001.

[2] Levy, Eliezer and Silberschatz, Abraham. Distributed File Systems: Concepts and Examples. ACM Computing Surveys, 22(4): 321-374, December 1990.

[3] Cooper, Brian F. and Garcia-Molina, Hector. Peer-to-peer resource trading in a reliable distributed system. 1st International Workshop on Peer-to-Peer Systems, 11 November 2002.

[4] Goldweber, Michael and Johnson, Donald B. Minimizing Access Cost in Replicated Distributed Systems. Symposium on Principles of Distributed Computing: 56, 1996.

[5] Comer, Douglas E. Computer Networks and Internets. Second Ed. Upper Saddle River: Prentice Hall. 1999.

replication libby rasnick christopher newport university cpsc 550 spring 2003

Documents

libby rasnick cpsc

replication managers

performance slide

availability of data

downtime slide

node slide

napster slide

fault tolerance slide