A year with event sourcing and CQRS
As businesses grow, so does the complexity of their software. New features, new models, and new background processes all continue to be added. . .and developers struggle to make sense of it all. Yet the end user demands a swift and functional experience when interacting with your application. It is paramount to be open to alternative patterns that help tame complex, high-demand services. Two such patterns are command-query responsibility segregation (CQRS) and event sourcing (ES). Command-query responsibility segregation is an architectural pattern for user-facing applications that extends from the now standard Model-View-Controller (MVC) pattern and is an alternative to the CRUD pattern. At its core, CQRS is about changing how we think of and work with our data by introducing two types of models: all user actions become commands, and a read-only query model powers our views. Commands and queries are logistically separated, providing additional decoupling of our application. CQRS also calls for changes in how we store and structure our data. Enter event sourcing. Instead of persisting the current state of our domain objects or entities, we record historical events about our data. The key advantage is that we can examine our application data at any point in time, rather than just the current state. This pattern changes how we persist and process our data but is surprisingly efficient. While each of the two patterns can be used exclusively, they complement each other beautifully and facilitate the construction of decoupled, scalable applications or individual services. Stephen Pember explores the fundamentals of each pattern and offers several examples and demonstration code to show how one might actually go about implementing CQRS and ES. Steve discusses task-based UIs and domain-driven design as he outlines some of the advantages—and challenges—that ThirdChannel has seen when developing systems using CQRS and ES over the past year.
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A year with event sourcing and CQRS
- 1. Hello.
- 6. You got it.
- 12. A Year With Event Sourcing and CQRS Steve Pember CTO, ThirdChannel steve@thirdchannel.com Software Architecture Conf, 2016 THIRDCHANNEL @svpember
- 13. THIRDCHANNEL @svpember Agenda • CQRS • Event Sourcing • This Tech @ ThirdChannel • Demo
- 15. Command - Query Responsibility Segregation
- 16. CRUD vs CQRS
- 17. CRUD / MVC
- 20. –Eric Evans “Some objects are not defined primarily by their attributes. They represent a thread of identity that runs through time and often across distinct representations. Sometimes such an object must be matched with another object even though attributes differ. An object must be distinguished from other objects even though they might have the same attributes.”
- 21. THIRDCHANNEL @svpember Domain Driven Design • Ubiquitous Language
- 22. THIRDCHANNEL @svpember Domain Driven Design • Ubiquitous Language • Entities / Value Objects
- 25. THIRDCHANNEL @svpember Domain Driven Design • Ubiquitous Language • Entities / Value Objects • Aggregates
- 26. Group of Entities with one Root
- 28. THIRDCHANNEL @svpember Domain Driven Design • Ubiquitous Language • Entities / Value Objects • Aggregates • Bounded Context
- 30. Objects inside an Aggregate may hold refs to other Roots
- 31. Deleting an Aggregate also deletes dependents
- 32. Isolate related objects into Modules
- 33. Furthermore, No direct comms across boundaries
- 34. THIRDCHANNEL @svpember Domain Driven Design • Ubiquitous Language • Entities / Value Objects • Aggregates • Bounded Context • Domain Events
- 36. Events are Transactionally Safe
- 37. THIRDCHANNEL @svpember Back to CQRS…
- 38. CQRS is an Evolution on DDD
- 39. Basis: Two Objects where there was One
- 41. But allows for Interesting and efficient Architectures
- 44. Multiple Query Caches derived from events
- 45. –Johnny Appleseed “Type a quote here.”
- 46. THIRDCHANNEL @svpember When should you use CQRS?
- 48. Complex Domains
- 50. Task-Based UIs
- 52. –Johnny Appleseed “Type a quote here.”
- 53. However…
- 55. THIRDCHANNEL @svpember Ok, so how about Event Sourcing?
- 56. What do we do with our Domain Events?
- 57. THIRDCHANNEL @svpember Event Sourcing • Alternative Storage Pattern - Save Deltas
- 59. Store Deltas instead of Current State
- 61. Objects are backed - ‘sourced’ - by events
- 62. THIRDCHANNEL @svpember Event Sourcing • Alternative Storage Pattern - Save Deltas • Event vs Aggregate
- 63. Event: Something which has occurred in the system
- 65. THIRDCHANNEL @svpember Event Sourcing • Alternative Storage Pattern - Save Deltas • Event vs Aggregate • Purely Additive
- 66. There is no Delete
- 67. THIRDCHANNEL @svpember Event Sourcing • Alternative Storage Pattern - Save Deltas • Event vs Aggregate • Purely Additive • It’s Old
- 70. THIRDCHANNEL @svpember Event Sourcing • Alternative Storage Pattern - Save Deltas • Event vs Aggregate • Purely Additive • It’s Old • Event Stream -> Transient Models
- 72. All Objects are Transient Derivatives of the Event Stream
- 74. I’m sure you have some doubts.
- 75. Wouldn’t it take up a large amount of storage space?
- 77. Wouldn’t ES add overhead?
- 80. Try to avoid ‘heavy’ persistence mechanisms
- 81. The toughest thing, though:
- 82. ES Can be difficult for junior developers
- 83. THIRDCHANNEL @svpember So Why Would I Want Event Sourcing?
- 84. THIRDCHANNEL @svpember So Why ES? • More than the Perfect Audit Log
- 85. Audit logs tell the History Events tell the intent of History
- 87. Also, You’re now a Time Traveller
- 88. THIRDCHANNEL @svpember So Why ES? • More than the Perfect Audit Log • Only Structural Model That Does Not Lose Information
- 89. NEVER DELETE!!!
- 90. THIRDCHANNEL @svpember So Why ES? • More than the Perfect Audit Log • Only Structural Model That Does Not Lose Information • Ideal for Business Analysis
- 91. Analysis of the Event Stream is a Projection
- 93. THIRDCHANNEL @svpember Current State is Boring • Mine ALL ProductRemoved events • Find ALL ProductAdded events followed by RemovedEvents for same product within 5 minutes • Find Average Duration between ProductAdd and OrderPlaced
- 95. Future Proof your Data!
- 100. Still…
- 101. Businesses depend on Time Series Data
- 103. THIRDCHANNEL @svpember Mechanics and Demo
- 104. Pure Event Sourcing
- 105. 3 Base Objects
- 109. Successful Commands Result in new Events on the Aggregate
- 110. –Johnny Appleseed “Type a quote here.”
- 111. Loading a Query builds up Aggregates from the Events
- 114. THIRDCHANNEL @svpember Tying It All Together
- 115. CQRS & ES
- 116. –Johnny Appleseed “Type a quote here.”
- 117. The Event Stream is the Source of Truth.
- 118. Pre-Build Query Objects from the Event Stream
- 119. THIRDCHANNEL @svpember Questions So Far?
- 120. THIRDCHANNEL @svpember What have we learned @ThirdChannel?
- 122. There are few available tools …
- 124. THIRDCHANNEL @svpember Akka & Akka Persistance
- 126. …and we used none of them
- 127. –Johnny Appleseed “Type a quote here.”
- 129. Commands & Queries immediately felt like a perfect fit
- 130. …and make sure you append your class names with ‘Command’ and ‘Query’
- 131. Query - based Views are an efficiency gain
- 132. Query Read Models should be able to be freely trashed and rebuilt
- 133. We’ve still not run into Aggregate write contention
- 135. Save Every Event
- 136. You really don’t need snapshots (at first)
- 137. Be Wary of Editing an Event
- 138. Small Events > Large Events
- 139. This is Not… Great.
- 140. A Bit Better
- 141. This next bit is important
- 142. Pure Event Sourcing Is Not Always The Best Choice
- 143. Include a ‘dateEffective’ field on the Event table
- 144. Add Current-State, Indexed, Columns to your Aggregates
- 145. And Make Many of Them
- 149. THIRDCHANNEL @svpember Image Credits • Mad Developers: Last Week Tonight • CQRS Diagram: http://udidahan.com/2009/12/09/clarified-cqrs/ • Happy Developers: http://www.businessinsider.com/programming-languages-in-highest-demand-2015-6 • Obama Thumbs up: http://amiloszportraits.com/barack-obama-thumbs-up-meme • Who am i?: http://www.catholicismusa.com/want-joy-seek-god-find-your-true-identity/ • Star Trek face palm: http://tvtropes.org/pmwiki/pmwiki.php/FacePalm/LiveActionTV • Sauron’s Eye: http://www.independent.co.uk/arts-entertainment/architecture/russian-orthodox-church-condemns- moscows-eye-of-sauron-art-installation-celebrating-the-hobbit-the-9915050.html • Grain of Salt: http://www.pxleyes.com/photography-picture/4c3004291f903/Grain-of-Salt.html • Obama, Nye, and Tyson: en.wikipedia.org/wiki/Selfie