This book presents a coherent pattern language for event sourced applications and reliable distributed systems.

The main chapters of the book are organised in three parts. Part 1 describes how to make an event sourced domain model. Part 2 describes how to make an event sourced application. Part 3 describes how to make a reliable distributed system from a set of event sourced applications. Each part of the book has five chapters. Each chapter has complete working examples written in the Python programming language. The examples in later chapters build on the examples in previous chapters.

This book can be used as a general guide for event sourcing, event-driven systems, and the tactical patterns of Domain-Driven Design. It is an accompaniment to the popular Python eventsourcing library, and gives an overview of the design of this library. The chapter examples present a distilled and enhanced version of the code available in the Python eventsourcing library.

The book's prologue shows how Christopher Alexander's pattern language scheme was originally intended as a scheme for describing events, and is best understood as an application of the event-oriented modern process philosophy of Alfred North Whitehead. A discussion of the way pattern language was taken up in software development reveals how the event-oriented nature of Alexander's scheme was overlooked. The prevailing general understanding of things that has predominated objected-oriented analysis and design is identified as the two-hierarchies scheme, which involves the substance-quality categories that Whitehead had already shown to be inadequate. The deficiency of the two-hierarchies scheme is explained, and the detrimental consequences for software design patterns, object-oriented programming, software architecture, and the agile approach to software development are highlighted. A return to the event-oriented approach that is exhibited in Whitehead's and Alexander's schemes is proposed under the name of event-oriented analysis and design. A detailed summary of Whitehead's scheme is provided as a second introductory chapter at the start of the book.

The book's epilogue gives an overview of both the event-oriented moment-by-moment, person-centred, psychology of Carl Rogers and its application as the collaborative or nonviolent communication scheme of Marshall Rosenberg.

This book is primarily aimed at professional software developers working under deadlines to create reliable maintainable software applications and systems that support non-trivial business processes in both startups and established corporations, especially Python developers.

This book is suitable for anybody interested in the history and philosophy of technology. It will also be interesting and useful for students and teachers of computer science and software engineering.