Message- and Event-Driven Systems with CQRS and Event Sourcing

Alex Lawrence is a software developer, architect, technical lead, author, and educator. He started his journey in software development at an early age, driven by a general fascination for computers. His most notable technical expertise lies in message-driven systems, event-driven architecture, CQRS, and Event Sourcing. Over the years, he has contributed to multiple companies building large-scale message-driven systems and gained deep, hands-on experience. During his work, Alex focuses on understanding business and product challenges to design software for effective and lasting solutions. His approach goes beyond technical implementations, always ensuring that software serves real-world needs and drives meaningful outcomes. Sharing knowledge has always been a core part of his work. Over the years, Alex has lectured and given guest lectures at multiple universities.

• Series Editor Foreword xviiPreface xxiChapter 1: Business Strategy Meets Message-Driven Systems 1Why Businesses Benefit 2How Domain-Driven Scenarios Are Solved 3How Businesses Can Thrive in the Face of Constant, Rapid Change 5Message-Driven Architecture 6Loose Coupling 9Relationship to Reactive Architecture 10Relation to Monoliths and Microservices 10Domain-Driven Running Example 12Modeling Messaging Systems 15Summary 22References 23Chapter 2: Anatomy of Message-Driven Systems 25Message Broker 26Synchronous Communication 34The Structure of Messages 36Message Categories 39Serialization, Schemas, and Types in Code 48Guarantees in Distribution 57Ordering of Messages 65Dealing with Duplicates 67Error Handling 71Summary 76References 76Chapter 3: In-Context Architectures 79The Purpose of Layers 80Dependency Inversion 82Ports and Adapters 89Generic Layouts and Guided Designs 92The Four Standard Layers 95Architecture for the Running Example 106Summary 107References 108Chapter 4: Tactical Building Blocks 109Value Objects 110Identities 116Entities 119Domain Services 124Repositories 126Domain Events 136Aggregates and Eventual Consistency 147Summary 162References 163Chapter 5: CQRS 165When Business Software Should Segregate Responsibilities 166Commands and Queries 170Command Model and Query Models 175Model Synchronization 180Technical Segregation 190Initializing and Evolving Query Models 195Eventual Consistency in the User Interface 201Dos and Don’ts 203Summary 205References 206Chapter 6: Event Sourcing 207Your Business as Auditable History 208Modeling State as Events 211Event-Sourced Command Model 215Event Store 222Comparison of Store Technologies 234Query Model Projections 246Processing and Publishing Events 259Data Deletion 265Summary 266References 267Chapter 7: Long-Running Processes 269What Is a Process? 270Message-Driven Processes 278Choreography 284Orchestration 290Correlation of Messages 305Process as a Domain Model Component 308Evolution of Long-Running Processes 315Summary 318References 319Chapter 8: Evolution of Messages and Message Streams 321Purpose and Scope Determine Life Cycle 322Relation of Data, Serialization, Schemas, and Types 327Types of Message Changes 332Adaptation Techniques 338Evolution of Persistent Message Streams 347Schema Registries 354Summary 357References 358Chapter 9: Composing Data from Different Sources 359Global Message Journal 360Replicating Query Models 370Synchronous On-Demand Composition 389The "Right Way" to Compose Query Models 399Summary 399References 400Chapter 10: Message-Driven User Interfaces 401Frontend Architecture 402Messaging Technologies and Protocols 406Executing Commands and Queries 418Reacting to Events 424Building Reactive Query Models 428Client-Side Projections of Change Events 432Summary 434References 434Index 435