System Design for .NET Engineers: From Senior Developer to System Design Expert

Title page....11

Copyright....12

Preface....13

How the book is organized....14

How each idea is taught....15

The seven-step framework....16

The code....17

How to use it....18

I Foundations....19

1. Thinking in Systems: The Design Process and Estimation....20

1.1 Learning objectives....20

1.2 Why senior developers struggle with system design....20

1.3 Functional and non-functional requirements....21

1.4 The seven-step framework....22

1.5 Back-of-envelope estimation....23

1.6 Latency numbers every engineer should know....25

1.7 Worked example: scoping Design Twitter....27

1.8 Summary....29

1.9 Review questions....29

1.10 Further reading....30

2. How Requests Travel: Networking and API Styles....31

2.1 Learning objectives....31

2.2 What happens when you press Enter....31

2.3 TCP, UDP and QUIC....33

2.4 HTTP versions and what changes for your design....34

2.5 API styles compared....35

2.6 Server push: WebSockets, SSE and long polling....35

2.7 Designing a good API contract....36

2.8 Worked example: orders and a live trade dashboard....39

2.9 Summary....40

2.10 Review questions....41

2.11 Further reading....41

3. Scaling Out: Stateless Services, Load Balancers, Gateways and CDNs....43

3.1 Learning objectives....43

3.2 Vertical vs. horizontal scaling....43

3.3 Stateless services....44

3.4 Load balancing....46

3.5 Reverse proxies and API gateways....49

3.6 CDNs and edge caching....51

3.7 Mapping to Azure....51

3.8 Worked example: scaling out Contoso Shop....52

3.9 Summary....53

3.10 Review questions....54

3.11 Further reading....54

4. Availability and Reliability by the Numbers....55

4.1 Learning objectives....55

4.2 SLIs, SLOs, SLAs and error budgets....55

4.3 The nines....56

4.4 Composite availability....57

4.5 Redundancy and failover....58

4.6 Worked example: order API on App Service, SQL and Redis....59

4.7 Summary....60

4.8 Review questions....61

4.9 Further reading....61

II Data....62

5. Indexes, Transactions and Isolation Through a Design Lens....63

5.1 Learning objectives....63

5.2 Indexes as a design decision....63

5.3 Designing indexes from access patterns....64

5.4 ACID....65

5.5 Isolation levels and anomalies....66

5.6 MVCC and SQL Servers row versioning....69

5.7 Worked example: a funds transfer....71

5.8 Summary....73

5.9 Review questions....74

5.10 Further reading....74

6. Beyond Relational: Storage Engines and NoSQL Families....75

6.1 Learning objectives....75

6.2 B-trees vs. LSM trees....75

6.3 Write amplification and compaction....77

6.4 The four NoSQL families....78

6.5 Choosing a store....80

6.6 Worked example: storing ticks for a brokerage....81

6.7 Summary....82

6.8 Review questions....83

6.9 Further reading....83

7. Replication and Partitioning....84

7.1 Learning objectives....84

7.2 Leader-follower replication....84

7.3 Multi-leader and leaderless replication....85

7.4 Replication lag in practice....86

7.5 Partitioning....88

7.6 Consistent hashing and resharding....89

7.7 Worked example: sharding a SaaS by tenant....91

7.8 Summary....93

7.9 Review questions....93

7.10 Further reading....94

8. CAP, PACELC and Consistency Models....95

8.1 Learning objectives....95

8.2 CAP, stated precisely....95

8.3 PACELC....97

8.4 A ladder of consistency models....98

8.5 Consistency in Cosmos DB....99

8.6 Worked example: choosing per feature....101

8.7 Summary....102

8.8 Review questions....102

8.9 Further reading....103

9. Caching: Patterns, Pitfalls and the .NET Cache Stack....104

9.1 Learning objectives....104

9.2 Why cache, and where....104

9.3 Caching patterns....105

9.4 TTL, eviction and sizing....107

9.5 The .NET cache stack....108

9.6 Invalidation and consistency....109

9.7 Cache failure modes....110

9.8 Worked example: caching a product catalog API....113

9.9 Summary....115

9.10 Review questions....115

9.11 Further reading....115

10. Blobs, Search and Data Modeling for Scale....117

10.1 Learning objectives....117

10.2 Object storage....117

10.3 Search engines....120

10.4 Denormalization and materialized views....121

10.5 CQRS....122

10.6 Event sourcing....123

10.7 Worked example: a ledger audit trail....123

10.8 Summary....124

10.9 Review questions....125

10.10 Further reading....125

III Asynchronous and Distributed Systems....126

11. Messaging: Queues, PubSub, Streams and Delivery Guarantees....127

11.1 Learning objectives....127

11.2 Why asynchronous messaging....127

11.3 Queues vs. pubsub....128

11.4 Logs and Kafka....129

11.5 Delivery semantics....131

11.6 Idempotency....132

11.7 Poison messages and DLQs....135

11.8 Worked example: trade events to several consumers....136

11.9 Summary....137

11.10 Review questions....138

11.11 Further reading....138

12. Distributed Transactions: 2PC, Sagas and the Outbox....139

12.1 Learning objectives....139

12.2 The dual-write problem....139

12.3 Two-phase commit....140

12.4 Sagas....141

12.5 The transactional outbox....143

12.6 Worked example: an orchestrated trip saga....145

12.7 Summary....147

12.8 Review questions....147

12.9 Further reading....147

13. Agreement and Order: Consensus, Time and IDs....149

13.1 Learning objectives....149

13.2 Why consensus....149

13.3 Raft in one diagram....150

13.4 Split brain, leases and fencing tokens....151

13.5 Clocks and ordering....152

13.6 Generating IDs....153

13.7 Worked example: sortable order IDs....156

13.8 Summary....157

13.9 Review questions....157

13.10 Further reading....158

14. Protecting the System: Rate Limiting and Resilience....159

14.1 Learning objectives....159

14.2 Rate-limiting algorithms....159

14.3 Rate limiting in ASP.NET Core....160

14.4 Distributed rate limiting....162

14.5 Resilience patterns....162

14.6 Polly v8 and Microsoft.Extensions.Http.Resilience....165

14.7 Retry storms and load shedding....166

14.8 Worked example: the quotes API....167

14.9 Summary....168

14.10 Review questions....168

14.11 Further reading....169

15. Architecture Styles and Service Infrastructure....170

15.1 Learning objectives....170

15.2 The costs of distribution....170

15.3 Modular monolith first....171

15.4 Bounded contexts....172

15.5 Service infrastructure....174

15.6 Aspire, Dapr and Istio....176

15.7 Worked example: cutting a fintech platform into modules....177

15.8 Summary....178

15.9 Review questions....179

15.10 Further reading....179

16. Batch, Streams and Observability....180

16.1 Learning objectives....180

16.2 Batch processing....180

16.3 Stream processing....181

16.4 Lambda and Kappa architectures....183

16.5 Observability....184

16.6 RED and USE metrics....186

16.7 Worked example: tracing an order through three services....188

16.8 Summary....188

16.9 Review questions....189

16.10 Further reading....189

IV Cross-Cutting Concerns....191

17. Identity and Security by Design....192

17.1 Learning objectives....192

17.2 Authentication vs. authorization....192

17.3 OAuth 2.0 and OpenID Connect....193

17.4 JWTs and refresh tokens....195

17.5 RBAC vs. ABAC....196

17.6 Encryption and secrets....197

17.7 Multi-tenant isolation....199

17.8 Worked example: a multi-tenant invoicing SaaS....200

17.9 Summary....201

17.10 Review questions....202

17.11 Further reading....202

18. Shipping and Paying: Deployment, Cloud Building Blocks and Cost....204

18.1 Learning objectives....204

18.2 Containers and Kubernetes essentials....204

18.3 Release strategies....207

18.4 Cloud building blocks....209

18.5 Cost as a design constraint....210

18.6 Worked example: pricing the orders platform....211

18.7 Summary....213

18.8 Review questions....213

18.9 Further reading....214

19. Surviving Disaster and Load: Multi-Region, DR and Capacity Planning....215

19.1 Learning objectives....215

19.2 RPO and RTO....215

19.3 Geo-replication and residency....217

19.4 Tail latency....218

19.5 Littles Law and capacity....219

19.6 Load testing....220

19.7 Worked example: DR plan for a payments ledger....221

19.8 Summary....223

19.9 Review questions....223

19.10 Further reading....224

V Case Studies....225

20. Case Studies I: Core Building Blocks....226

20.1 Learning objectives....226

20.2 URL shortener....226

20.3 Distributed rate limiter....229

20.4 Key-value store (Dynamo-style)....232

20.5 Notification system....234

20.6 Summary....236

20.7 Review questions....237

20.8 Further reading....237

21. Case Studies II: Social and Real-Time Systems....239

21.1 Learning objectives....239

21.2 News feed....239

21.3 Chat....242

21.4 Typeahead....244

21.5 Web crawler....246

21.6 Summary....249

21.7 Review questions....249

21.8 Further reading....250

22. Case Studies III: Media, Files, Location and Scheduling....251

22.1 Learning objectives....251

22.2 Video streaming....251

22.3 File sync....253

22.4 Ride-hailing proximity....255

22.5 Distributed job scheduler....258

22.6 Summary....261

22.7 Review questions....261

22.8 Further reading....262

23. Case Studies IV: Money and Contention....263

23.1 Learning objectives....263

23.2 Payment system....263

23.3 Stock exchange....266

23.4 Ticket booking....268

23.5 Leaderboard and metrics....271

23.6 Summary....273

23.7 Review questions....274

23.8 Further reading....274

VI Mastery....275

24. Trade-offs, Design Docs and the Classic Papers....276

24.1 Learning objectives....276

24.2 Design docs and ADRs....276

24.3 Articulating trade-offs....278

24.4 Four classic papers....279

24.5 Worked example: an ADR for the catalog cache....280

24.6 Summary....281

24.7 Review questions....282

24.8 Further reading....282

25. Designing AI Systems....283

25.1 Learning objectives....283

25.2 The RAG pipeline....283

25.3 Vector databases....285

25.4 Serving and cost....286

25.5 Evals and guardrails....287

25.6 Agents....288

25.7 Worked example: internal document Q&A....289

25.8 Summary....290

25.9 Review questions....290

25.10 Further reading....291

26. Capstone: Modernizing a Legacy Fintech Platform....292

26.1 Learning objectives....292

26.2 Current state....292

26.3 Target architecture....293

26.4 Migration plan....294

26.5 Risks and governance....296

26.6 Worked example: switching payments....297

26.7 Summary....298

26.8 Review questions....298

26.9 Further reading....299

27. The Design Interview and Your Path Forward....300

27.1 Learning objectives....300

27.2 The 45-minute structure....300

27.3 Two mock prompts....301

27.4 Self-review rubric....302

27.5 Where to go next....302

27.6 Summary....303

27.7 Review questions....303

27.8 Further reading....304

Glossary....305

A....306

B....308

C....309

D....311

E....312

F....313

G....314

H....315

I....316

J....317

K....318

L....319

M....321

N....322

O....323

P....324

Q....326

R....327

S....329

T....331

U....333

V....334

W....335

Index....336

You already know how to build software. Learn to decide what should exist, how it should scale, and what happens when it fails.

System Design for .NET Engineers is a practical guide for experienced developers preparing for architecture decisions, design reviews, and system design interviews. It connects distributed systems principles to familiar C#, ASP.NET Core, SQL Server, and cloud examples.

Follow a repeatable seven-step framework: requirements, estimates, API, data model, high-level design, deep dive, and trade-offs. Across 27 chapters, explore scaling, storage, consistency, caching, messaging, resilience, security, deployment, cost, and disaster recovery.

Apply those ideas to 16 case studies, from URL shorteners and chat to payments and ticketing. Then work through AI system design and a legacy fintech modernization capstone. Diagrams, calculations, code examples, review questions, and further reading support a structured 60-hour study plan.

Build the judgment to explain your choices, recognize failure modes, and defend an architecture with clear trade-offs.