CQRS Journey

July 2012

Exploring CQRS and Event Sourcing

The project is focused on building highly scalable, highly available and maintainable applications with the Command & Query Responsibility Segregation and the Event Sourcing patterns.

The project was positioned as a learning journey. This guidance is designed to help you get started with the CQRS pattern and event sourcing. It is not intended to be the definitive guide to the CQRS pattern and event sourcing. Instead, it's a journal that describes the experiences of a development team with no prior CQRS proficiency in building, deploying (to Microsoft Azure), and maintaining a sample real-world complex enterprise system as a reference implementation (RI) to showcase various CQRS and ES concepts & techniques. The development team did not work in isolation; we actively sought input from industry experts and from a wider group of advisors to ensure that the guidance is both detailed and practical.

The CQRS pattern and event sourcing are not mere simplistic solutions to the problems associated with large-scale, distributed systems. By providing you with both a working application and written guidance, we expect you'll be well prepared to embark on your own CQRS journey.

This written guidance is itself split into three distinct sections that you can read independently: a description of the journey we took as we learned about CQRS, a collection of CQRS reference materials, and a collection of case studies that describe the experiences other teams have had with the CQRS pattern. The map below illustrates the relationship between the first two sections and our main milestones.

Download Book	Download Book: Exploring CQRS and Event Sourcing
Order Book	Exploring CQRS and Event Sourcing is available in paperback from Amazon.com Exploring CQRS and Event Sourcing (Kindle) is available in mobi format from Amazon.com
Download Code	Download Reference Implementation Code
CQRS Journey - Reference Implementation Evolution	CQRS Journey – Reference Implementation Evolution
Release Notes	Release Notes, Migration Notes, Known Issues
Deployed Live Reference Implementation	Registrant View, Business Customer View
	What other readers are saying about the guide Foreword by Greg Young Preface A CQRS journey Chapter 1, "Our Domain: The Contoso Conference Management System," introduces our sample application and our team of (fictional) experts. Chapter 2, "Decomposing the Domain," provides a high-level view of the sample application and describes the bounded contexts that make up the application. Chapter 3, "Orders and Registrations Bounded Context," introduces our first bounded context, explores some CQRS concepts, and describes some elements of our infrastructure. Chapter 4, "Extending and Enhancing the Orders and Registrations Bounded Context," describes adding new features to the bounded context and discusses our testing approach. Chapter 5, "Preparing for the V1 Release," describes adding two new bounded contexts and handling integration issues between them, and introduces our event-sourcing implementation. This is our first pseudo-production release. Chapter 6, "Versioning Our System," discusses how to version the system and handle upgrades with minimal down time. Chapter 7, "Adding Resiliency and Optimizing Performance," describes what we did to make the system more resilient to failure scenarios and how we optimized the performance of the system. This was the last release of the system in our journey. Chapter 8, "Lessons Learned," collects the key lessons we learned from our journey and suggests how you might continue the journey. CQRS reference Chapter 1, "CQRS in Context," provides some context for CQRS, especially in relation to the domain-driven design approach. Chapter 2, "Introducing the Command Query Responsibility Segregation Pattern," provides a conceptual overview of the CQRS pattern. Chapter 3, "Introducing Event Sourcing," provides a conceptual overview of event sourcing. Chapter 4, "A CQRS and ES Deep Dive," describes the CQRS pattern and event sourcing in more depth. Chapter 5, "Communicating between Bounded Contexts," describes some options for communicating between bounded contexts. Chapter 6, "A Saga on Sagas," explains our choice of terminology: process manager instead of saga. It also describes the role of process managers. Chapter 7, "Technologies Used in the Reference Implementation," provides a brief overview of some of the other technologies we used, such as the Azure Service Bus. Tales from the trenches Case study 1, "Twilio" Case study 2, "Lokad Hub" Case study 3, "DDD/CQRS for large financial company" Case study 4, "Digital Marketing" Case study 5, "Topaz Technologies" Case study 6, "eMoney Nexus"
Videos	A Journey into CQRS (Channel9)
Feedback and Issue Reporting	https://go.microsoft.com/fwlink/?LinkID=259598
License	©2012 Microsoft. All rights reserved. Certain content used with permission from contributors. Licensed under the Apache License, Version 2.0

Authors and Contributors

This guide was produced by the following individuals:

• Program and Product Management: Grigori Melnik (Microsoft Corporation)
• Development: Julian Dominguez (Microsoft Corporation), Daniel Cazzulino and Fernando Simonazzi (Clarius Consulting)
• Testing: Mani Subramanian (Microsoft Corporation), Hernan de Lahitte (Digit Factory), Rathi Velusamy (Infosys Technologies Ltd.)
• Documentation: Dominic Betts (Content Master Ltd), Julian Dominguez, Grigori Melnik, and Mani Subramanian (Microsoft Corporation), and Fernando Simonazzi (Clarius Consulting)
• Graphic design: Alexander Ustinov and Anton Rusecki (JetStyle)
• Editing and production: RoAnn Corbisier and Nelly Delgado (Microsoft Corporation) and Nancy Michell (Content Master Ltd.).

We want to thank the DDD/CQRS community members who have accompanied us on this journey and patiently reviewed our early content and drafts. Suggestions, insights, critiques, challenges, and reviews by our advisory board members are appreciated. Among those, we would like to highlight the exceptional contributions of Greg Young, Kelly Sommers, Clemens Vasters, Glenn Block, Udi Dahan, Adam Dymitruk, Bruno Terkaly, Cesar De la Torre Llorente, Ricardo Villalobos, Christopher Bennage, Eugenio Pace, Josh Elster, Mark Seemann, Matias Woloski, Peter Ritchie, Bill Wilder, and Tim Shakarian.

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