Pat. P. W. Chan, Michael R. Lyu Department of Computer Science and Engineering
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Reliable Web Services: Methodology, Experiment and Modeling
International Conference on Web Services (ICWS 2007) Pat. P. W. Chan, Michael R. LyuDepartment of ComputerScience and EngineeringThe Chinese University of Hong Kong
Miroslaw MalekDepartment of ComputerScience and EngineeringHumboldt University Berlin,Germany
Presented by Pat Chan
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Outline Introduction to Web Services Problem Statement Methodologies for Web Service Reliability New Reliable Web Service Paradigm Optimal Parameters Experimental Results and Discussion Conclusion
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Introduction Service-oriented computing is becoming a reality. The problems of service dependability, security and
timeliness are becoming critical. We propose experimental settings and offer a
roadmap to dependable Web services.
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Problem Statement Fault-tolerant techniques
Replication Diversity
Replication is one of the efficient ways for providing reliable systems by time or space redundancy. Increasing the availability of distributed systems Key components are re-executed or replicated Protect against hardware malfunctions or transient system faults
Another efficient technique is design diversity Employ independently designed software systems or services with different
programming teams, Defend against permanent software design faults.
We focus on the analysis of the replication techniques when applied to Web services.
A generic Web service system with spatial as well as temporal replication is proposed and investigated.
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Proposed Paradigm
Replication Manager
RR Algorithm / Voting
WatchDog
UDDI
Registry
WSDL
Web ServiceIIS
Application
Database
Web ServiceIIS
Application
Database
Web ServiceIIS
Application
Database
Client
Port
Application
Database Register
Look up
Get WSDL
Invoke web service
Keep check the availability of all the web.If Web service failed, update the list of availability of Web services
Update the WSDL
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Proposed Paradigm Round Robin
Parallel N-Version
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Web Service
IIS
Application
Database
Voting ClientMajority result
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Experiments A series of experiments are designed and performed for
evaluating the reliability of the Web service,
Spatial replication 0 0 0 0 1 1 1 1
Reboot 0 0 1 1 0 0 1 1
Retry 0 1 0 1 0 1 0 1
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Testing system Best Route Finding. Provide traveling suggestions for users. Starting point and destination. The system needs to provide the best route
and the price for the users.
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System Architecture
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Experimental Setup Examine the computation to communication ratio Examine the request frequency to limit the load of
the server to 75% Fix the following parameters
Computation to communication ratio (e.g 10:1) Request frequency
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Experimental Setup
Communication time: Computation time
143:14 (10:1)
Request frequency 1 request per min
Load 78.5%
Timeout period of retry 1 min
Timeout for Web service in RM 1s (web service specific)
Polling frequency 10 requests per min
Number of replicas 5
Max number of retries 5
Round-robin rate 1 s
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Experiment Parameters Fault mode
Temporary (fault probability: 0.01) Permanent (fault probability: 0.001)
Experiment time 5 days (7200 requests) Measure:
Number of failures Average response time (ms)
Failure definition: 5 retries are allowed. If there is still no correct result from
the Web service after 5 retries, it is considered as a failure.
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Experimental Result with Round-robin (failures / response time in ms)
Experiments Single server Single server with retry
Single server with reboot (continues no response for 3 requests)
Single server with retry and reboot
Spatial ReplicationRR
Hybrid approachRR+Retry
Hybrid approachRR+Reboot
All round approach RR spatial + Retry(5 times) + reboots
Normal case 0 / 183 0 / 193 0 / 190 0 / 187 0 / 188 0 / 195 0 / 193 0 / 190
Temp 705 / 190 0 / 223 723 / 231 0 / 238 711 / 187 0 / 233 726 / 188 0 / 231
Perm 6144 / -- 6337 / -- 1064 / -- 5 / 2578 5637 / -- 5532 / -- 152 / 187 0 / 191
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Experimental Result with N-Version (failures / response time in ms)
Experiments Single server Single server with retry
Single server with reboot (continues no response for 3 requests)
Single server with retry and reboot
Spatial ReplicationVoting
Hybrid approachVoting+Retry
Hybrid approachVoting +Reboot
All round approach Votingspatial + Retry(5 times) + reboots
Normal case 0 / 318 0 / 320 0 / 315 0 / 319 0 / 322 0 / 318 0 / 321 0 / 319
Temp 429 / 321 0 / 356 423 / 364 0 / 356 0 / 325 0 / 321 0 / 322 0 / 324
Perm 3861 / -- 3864 / -- 614 / -- 3 / 4027 1544 / 323 1546 / 324 63 / 324 0 / 323
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Varying the parameters Number of tries Timeout period for retry in single server Timeout period for retry in our paradigm Polling frequency Number of replicas Load of server
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Number of tries
Number of tries Number of failures in Temp
Number of failures in Perm
0 95 76
1 2 2
2 0 0
3 0 0
4 0 0
5 0 0
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Timeout period for retry in single serverTimeout period for retry (s)
Number of failures in Temp
Number of failures in Perm
0 95 7265
2 2 7156
5 0 7314
6 0 6890
7 0 189
8 0 82
9 0 11
10 0 2
12 0 0
14 0 0
16 0 0
18 0 0
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Timeout period for retry in single server
# of failure
Timeout period
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Timeout period for retry in our paradigm
Timeout period for retry (s)
Number of failures in Temp
Number of failures in Perm
0 2 81
2 0 2
5 0 0
10 0 0
20 0 0
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Polling frequency
Polling frequency (number of requests per min)
Number of failures in Temp
Number of failures in Perm
0 0 7124
1 0 811
2 0 30
5 0 12
10 0 1
15 213 254
20 1124 1023
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Polling frequency
# of failure
Polling frequency
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Number of Replicas
Number of replicas Number of failures in Temp Number of failures in Perm
No replica 91 8152
2 2 356
3 0 0
4 0 0
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Load of Web ServerLoad of the web server
(%)Number of failures in Temp
Number of failures in Perm
70 0 0
75 0 0
80 2 3
85 10 14
90 512 528
95 3214 3125
100 8792 8845
110 8997 8994
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Summary of Parameters Number of tries = 2 Timeout period for retry in single server = 10s Timeout period for retry in our paradigm = 5s Polling frequency = 10 request per min Number of replicas = 3 Load of server = < 75%
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Petri-Net (Four identical replicas)
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Petri-Net (N-version Web service with voting)
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(a)
(b)
P1
λ1
μ1c2
S-j
P2
μ2c2
λ2
S-j-1
S
S-n
F
λN
μ*c2
(1-c1)μ*
λ* S-1 S-2
λ*
μ*c2
λ*
(1-c1)μ*(1-c1)μ*
F
(1-c1)μ1
(1-c1)μ2
(1-c2)μ1
(1-c2)μ2
Reliability Model
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Reliability Model
1 2 1 2 2 2* (1 ) (1 )C C 1 1 2 2*
ID Description Value
λn Network failure rate 0.02
λ* Web service failure rate 0.025
λ1 Resource problem rate 0.142
λ2 Entry point failure rate 0.150
μ* Web service repair rate 0.286
μ1 Resource problem repair rate 0.979
μ2 Entry point failure repair rate 0.979
C1 Probability that the RM response on time 0.9
C2 Probability that the server reboot successfully 0.9
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Outcome (SHARPE)
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Conclusion Surveyed replication and design diversity
techniques for reliable services. Proposed a hybrid approach to improving the
availability of Web services. Carried out a series of experiments to evaluate
the availability and reliability of the proposed Web service system.
Optimal parameters are obtained.