Replication
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Replication - Distributed Operating Systems - Lecture Slides, Slides of Computer Science
These are the Lecture Slides of Distributed Operating Systems which includes Neumann Bottleneck, Networked Information, Memory Hierarchy, Evidence, Latency, Communication, Intelligent Service, Communication Latency, Routing Path etc.Key important points are: Replication, Definition, History, Goals, Features, Structure, Applications, Benefits and Issues, Cost of Replication, Significant Points
Typology: Slides
2012/2013
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Replication
Table of Contents
Definition History Goals Features Structure How to Use Applications Benefits and Issues Cost of Replication Significant Points Summary References
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”
History
Initially, stored data back-ups (tape)
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
Goals
“to improve a service’s performance, to increase its availability, or to make it fault-tolerant.”
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)
Features
Passive
master replication manager allocates requests to slave managers
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.
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 - 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, then 2 f +1 servers would permit full function during most worst case scenarios
Applications
DNS servers
eases request bottlenecks
Applications
DNS servers
eases request bottlenecks
Community-based redundancy systems
propagated data alleviates pressure from server failures
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
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