Beyond Redundancy How Geographic Redundancy Can Improve Service Availability and Reliability of Computer-Based Systems

Randee Adams is a Consulting Member of Technical Staff in the Applications Group of Alcatel-Lucent. Currently, she is focusing on reliability for Alcatel-Lucent's software applications.

Daniel Eustace is a Distinguished Member of Technical Staff in the IMS Solutions Organization of Alcatel-Lucent. Currently, he is a solution architect focusing on reliability, key quality indicators, geographical redundancy, and call processing.

Tables xix

Equations xxi

Preface and Acknowledgments xxiii

Audience xxiv

Organization xxiv

Acknowledgments xxvi

PART 1 BASICS 1

1 SERVICE, RISK, AND BUSINESS CONTINUITY 3

1.1 Service Criticality and Availability Expectations 3

1.2 The Eight-Ingredient Model 4

1.3 Catastrophic Failures and Geographic Redundancy 7

1.4 Geographically Separated Recovery Site 11

1.5 Managing Risk 12

1.6 Business Continuity Planning 14

1.7 Disaster Recovery Planning 15

1.8 Human Factors 17

1.9 Recovery Objectives 17

1.10 Disaster Recovery Strategies 18

2 SERVICE AVAILABILITY AND SERVICE RELIABILITY 20

2.1 Availability and Reliability 20

2.2 Measuring Service Availability 25

2.3 Measuring Service Reliability 33

PART 2 MODELING AND ANALYSIS OF REDUNDANCY 35

3 UNDERSTANDING REDUNDANCY 37

3.1 Types of Redundancy 37

3.2 Modeling Availability of Internal Redundancy 44

3.3 Evaluating High-Availability Mechanisms 52

4 OVERVIEW OF EXTERNAL REDUNDANCY 59

4.1 Generic External Redundancy Model 59

4.2 Technical Distinctions between Georedundancy and Co-Located Redundancy 74

4.3 Manual Graceful Switchover and Switchback 75

5 EXTERNAL REDUNDANCY STRATEGY OPTIONS 77

5.1 Redundancy Strategies 77

5.2 Data Recovery Strategies 79

5.3 External Recovery Strategies 80

5.4 Manually Controlled Recovery 81

5.5 System-Driven Recovery 83

5.6 Client-Initiated Recovery 85

6 MODELING SERVICE AVAILABILITY WITH EXTERNAL SYSTEM REDUNDANCY 98

6.1 The Simplistic Answer 98

6.2 Framing Service Availability of Standalone Systems 99

6.3 Generic Markov Availability Model of Georedundant Recovery 103

6.4 Solving the Generic Georedundancy Model 115

6.5 Practical Modeling of Georedundancy 121

6.6 Estimating Availability Benefit for Planned Activities 130

6.7 Estimating Availability Benefit for Disasters 131

7 UNDERSTANDING RECOVERY TIMING PARAMETERS 133

7.1 Detecting Implicit Failures 134

7.2 Understanding and Optimizing RTO 141

8 CASE STUDY OF CLIENT-INITIATED RECOVERY 147

8.1 Overview of DNS 147

8.2 Mapping DNS onto Practical Client-Initiated Recovery Model 148

8.3 Estimating Input Parameters 154

8.4 Predicted Results 165

8.5 Discussion of Predicted Results 172

9 SOLUTION AND CLUSTER RECOVERY 174

9.1 Understanding Solutions 174

9.2 Estimating Solution Availability 177

9.3 Cluster versus Element Recovery 179

9.4 Element Failure and Cluster Recovery Case Study 182

9.5 Comparing Element and Cluster Recovery 186

9.6 Modeling Cluster Recovery 187

PART 3 RECOMMENDATIONS 201

10 GEOREDUNDANCY STRATEGY 203

10.1 Why Support Multiple Sites? 203

10.2 Recovery Realms 204

10.3 Recovery Strategies 206

10.4 Limp-Along Architectures 207

10.5 Site Redundancy Options 208

10.6 Virtualization, Cloud Computing, and Standby Sites 216

10.7 Recommended Design Methodology 217

11 MAXIMIZING SERVICE AVAILABILITY VIA GEOREDUNDANCY 219

11.1 Theoretically Optimal External Redundancy 219

11.2 Practically Optimal Recovery Strategies 220

11.3 Other Considerations 228

12 GEOREDUNDANCY REQUIREMENTS 230

12.1 Internal Redundancy Requirements 230

12.2 External Redundancy Requirements 233

12.3 Manually Controlled Redundancy Requirements 235

12.4 Automatic External Recovery Requirements 237

12.5 Operational Requirements 242

13 GEOREDUNDANCY TESTING 243

13.1 Georedundancy Testing Strategy 243

13.2 Test Cases for External Redundancy 246

13.3 Verifying Georedundancy Requirements 247

13.4 Summary 254

14 SOLUTION GEOREDUNDANCY CASE STUDY 256

14.1 The Hypothetical Solution 256

14.2 Standalone Solution Analysis 259

14.3 Georedundant Solution Analysis 263

14.4 Availability of the Georedundant Solution 269

14.5 Requirements of Hypothetical Solution 269

14.6 Testing of Hypothetical Solution 277

Summary 285

Appendix: Markov Modeling of Service Availability 292

Acronyms 296

References 298

About the Authors 300

Index 302

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