This guide is focused on building highly scalable, highly available, and maintainable applications with the Command & Query Responsibility Segregation and the Event Sourcing architectural patterns. It presents a learning journey, not definitive guidance. It describes the experiences of a development team with no prior CQRS proficiency in building, deploying (to Windows Azure), and maintaining a sample real-world, complex, enterprise system to showcase various CQRS and ES concepts, challenges, and techniques.

The development team did not work in isolation; we actively sought input from industry experts and from a wide 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.

Table of Contents

Journey 1: Our Domain: Conference Management System

Reference 2: Introducing the Command Query Responsibility Segregation Pattern

Reference 3: Introducing Event Sourcing

Reference 4: A CQRS and ES Deep Dive

Reference 5: Communicating Between Bounded Contexts

Reference 6: A Saga on Sagas

Reference 7: Technologies Used in the Reference Implementation

Journey 8: Epilogue: Lessons Learned

Reference 1: CQRS in Context

Reference 2: Introducing the Command Query Responsibility Segregation Pattern

Reference 3: Introducing Event Sourcing

Reference 4: A CQRS and ES Deep Dive

Reference 5: Communicating Between Bounded Contexts

Reference 6: A Saga on Sagas

Reference 7: Technologies Used in the Reference Implementation

Review

"The CQRS journey release by patterns & practices provides real world insight into the increasingly popular CQRS pattern used in distributed systems that rely upon asynchronous, message based approaches to achieve very large scale. The exploration of the issues the team faced throughout the implementation of the pattern is extremely useful for organizations considering CQRS, both to determine where the pattern is appropriate for them, and to go into the design and implementation with a baseline understanding of the complexity it will introduce. I really enjoyed the candor around the approach taken, the issues encountered, and the early design choices that the team would change in hindsight. This is a must read for any organization embarking upon CQRS, regardless of what platform they are using. - Chris Keyser, VP Engineering, CaseNetwork

This is another excellent guide from the patterns & practices team-real software engineering with no comforting illusions taken or offered. This guide provides a detailed journal of the practitioners implementing a real production system using the CQRS and Event Sourcing patterns, and also highlights the tradeoffs and teaches the principles that underlie them. The topics presented are relevant and useful, especially if you are building highly scalable Windows Azure applications. You'll be both challenged and inspired! -Scott Guthrie, Corporate Vice-President, Azure App Platform, Microsoft

CQRS is as much about architecture community as it is about concrete patterns-thus the project is aptly named "CQRS Journey." The community involvement and engagement in this project is unprecedented for Microsoft and reflects the enthusiasm amongst the many (if may say: young) software architects from across the industry who are rediscovering proven architecture patterns and are recomposing them in new ways to solve today's challenges. For me, one takeaway from this project is that the recipes developed here need to be carefully weighed against their alternatives. As with any software architecture approaches that promise easy scalability or evolvability of solutions, the proof will be in concrete, larger production implementations and how they hold up to changing needs over time. Thus, the results of this Journey project mark a start and not a finish line. -Clemens Vasters, Principal Technical Lead, Microsoft Corporation

Having participated and co-authored various guides from patterns & practices, the "CQRS Journey" follows the same walkthrough, scenario-based style, but adding even more fresh empirical content. It's a true testament of a skilled development team without previous CQRS experience, going through the journey of implementing a complex system and documenting their adventures and lessons learnt in this diary. If I had to recommend to someone where to start with CQRS, I would definitely point them to this guide. -Matias Woloski, CTO, Auth10 LLC"

CQRS is a very important pattern, and a tool that any cloud developer should have in his or her tool-belt. It is particularly well-suited for the cloud since it allows for the implementation of massively scalable solutions based on simple, common patterns (like queues, event handlers, and view models, to name a few). Like all patterns, there are several concrete, correct ways of implementing CQRS. A journey of the type undertaken by Microsoft's patterns & practices team is a great way to explore the different options, tradeoffs, and even possible mistakes one can make along the way, and accelerate one's learning of the CQRS pattern. -Shy Cohen, Principal, Shy Cohen Consulting

Perhaps the best lessons out of this guidance will be just how easy it is to work with Microsoft now that they are embracing more community and open source. -Adam Dymitruk, Systems Architect

"The "CQRS Journey" guide is an excellent resource for developers who want to begin developing a CQRS system or convert their current system. It's a true "trial by fire" approach to the concepts and implementation hurdles that a team would encounter when adopting CQRS. I would recommend reading it twice as I picked up even more lessons the second time through. -Dan Piessens, Lead Software Architect, Zywave

Patterns & practices assembled many of the active and key people in the CQRS community to join them on the their journey with CQRS and along the way discovered confusing terminology and concepts that created opportunities for leaders in the community to bring clarity to a broad audience. The material produced is influenced from the results of building a real world application and expresses the experiences from advisors and the patterns & practices team during the development process. By request from the community to allow outside contributions, everything has been open sourced on GitHub. Anyone interested is encouraged to take a look at the guide or implementation. The patterns & practices team has been very welcoming to anyone who wants to collaborate on covering additional areas, alternative implementations or further extending what is currently in place. -Kelly Sommers, Developer

The experiences and conclusions of the p&p team match up very well with our own real-world experiences. Their conclusions in Chapter 8 are spot on. One of the best aspects of this guidance is that the p&p team exposes more of their thought processes and learning throughout the Journey than most write-ups that you may read. From arguments between Developer 1 and Developer 2 on the team, to discussions with experts such as Greg Young and Udi Dahan, to an excellent post-project review in Chapter 8, the thought process is out there for you to learn from. Thanks for this great work, guys. I hope you keep this style with your upcoming guidance pieces. -Jon Wagner, SVP & Chief Architect, eMoney Advisor

It is a great resource on tactical and technical aspects of building a distributed system. -Rinat Abdullin, Technology Leader, Lokad"