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THIRDCHANNEL
Hi.
First, A Quick Scenario
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Balance: $100
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Balance: -$100
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Richer Data History With Event
Sourcing
Steve Pember
CTO, ThirdChannel
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
• Implementation (Reality)
• Event Storage
• Querying
• Demo
What Is Event Sourcing?
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Event Sourcing• Alternative Storage Pattern
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Instead, Store Facts
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
Event: Something Which Has
Occurred In The System
Aggregate: The Object of Events’
Affections
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
• Purely Additive
There is No Delete
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Full History of the Aggregates
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
• Purely Additive
• Optimization: Snapshots
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• snapshot photo
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
• Purely Additive
• Optimization: Snapshots
• Example: Shopping Cart
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
• Purely Additive
• Optimization: Snapshots
• Example: Shopping Cart
• Transient Objects
Objects are Transient Derivatives of Your
Events
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Event Sourcing• Alternative Storage Pattern
• Series of Facts
• Aggregates << Events
• Purely Additive
• Optimization: Snapshots
• Example: Shopping Cart
• Transient Objects
• Natural
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Questions So Far?• Events & Aggregates
• Snapshots
• Additive, loss-less model
• Transient Derivatives
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Agenda• Event Sourcing
• Event Sourcing Challenges
“I Think I Get What You’re Laying
Down, But It Still Seems Bananas”
So much work!
“That’s gotta be a ton of CPU overhead”
“Where’d my Models go? I can’t live without my
MVC!”
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Event Sourcing Challenges• Additional Work To Apply
• Additional CPU Processing
• Non-Traditional Concept of Models
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Event Sourcing Challenges• Additional Work To Apply
• Additional CPU Processing
• Non-Traditional Concept of Models
• More Storage Required VS non-ES
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Event Sourcing Challenges• Additional Work To Apply
• Additional CPU Processing
• Non-Traditional Concept of Models
• More Storage Required VS non-ES
• Reduced Database Level Constraints
Make Use of Transactional Code
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Event Sourcing Challenges• Additional Work To Apply
• Additional CPU Processing
• Non-Traditional Concept of Models
• More Storage Required VS non-ES
• Reduced Transactions / Database Level Constraints
• May Be Confusing For Junior Engineers
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So Why Use ES?
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
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Why Event Sourcing• More Than an Audit Log
Audit Logs Tell the History
Events Tell the Intent of History
ES is also a perfect Audit Log
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Why Event Sourcing• More Than an Audit Log
• Data Storage is Inexpensive
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Why Event Sourcing• More Than an Audit Log
• Data Storage is Inexpensive
• Used By All Long-Running Businesses
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All Business Problems Can Be Modeled With
Event Sourcing
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Why Event Sourcing• More Than an Audit Log
• Data Storage is Inexpensive
• Used By All Long-Running Businesses
• Only Structural Model That Does Not Lose Information
Remember: There Is No Delete
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Why Event Sourcing• More Than an Audit Log
• Data Storage is Inexpensive
• Used By All Long-Running Businesses
• Only Structural Model That Does Not Lose Information
• Simplified Testing and Debugging
Unit Test Events, Assert In Integration Tests
Time Travel Eases Debugging
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Why Event Sourcing• More Than an Audit Log
• Data Storage is Inexpensive
• Used By All Long-Running Businesses
• Only Structural Model That Does Not Lose Information
• Simplified Testing and Debugging
• Ideal for Business Analysis
Future-Proof your Data
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But With Event Sourcing…
–Johnny Appleseed
“Type a quote here.”
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Current State is a Projection
A 3C Example
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Still…
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Have you ever built?• Versioning of objects
• An ‘undo’ action
• Audit Log
• Tracking object value changes over time?
• Time series?
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Questions so far?
Next Up: Implementation
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
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Implementation (Theory)• Simple Base Objects
Three Base Objects
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Past-Tense Event Names
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Implementation (Theory)• Simple Base Objects
• Aggregate and Event SubClasses Have Transient Properties
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Implementation (Theory)• Simple Base Objects
• Aggregate and Event SubClasses Have Transient Properties
• Aggregates Receive and Play Events
Events Modify Aggregates
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Implementation (Theory)• Simple Base Objects
• Aggregate and Event SubClasses Have Transient Properties
• Aggregates Receive and Play Events
• Aggregates Require Distinction Between New and Historical Changes
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Implementation (Theory)• Simple Base Objects
• Aggregate and Event SubClasses Have Transient Properties
• Aggregates Receive and Play Events
• Aggregates Require Distinction Between New and Historical Changes
• Event Service Layer Required
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
• Implementation (Reality)
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Implementation (Reality)• Freely Add New Transient Properties
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Implementation (Reality)• Freely Add New Transient Properties
• Be Wary of Modifying Events
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Implementation (Reality)• Freely Add New Transient Properties
• Be Wary of Modifying Events
• Be Wary of Removing Aggregate Transients
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Implementation (Reality)• Freely Add New Transient Properties
• Be Wary of Modifying Events
• Be Wary of Removing Aggregate Transients
• Snapshot Rarely
Plus so much more…
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
• Implementation (Reality)
• Persistance
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Persistence & Event Storage• No Real Need for an ORM
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Persistence & Event Storage• No Real Need for an ORM
• Try a Library
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Event Sourcing Libraries• NEventStore (.NET)
• ProophEventStore (PHP)
• Akka Persistence (JVM)
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Persistence & Event Storage• No Real Need for an ORM
• Try a Library
• Database Agnostic
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Persistence & Event Storage• No Real Need for an ORM
• Database Agnostic
• Consider Using a Relational DB
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Questions?
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
• Implementation (Reality)
• Persistance
• Querying
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Querying Events• All Queries are Projections, including Current State
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Querying Events• All Queries are Projections, including Current State
• Returning Current State is Easy
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Querying Events• All Queries are Projections, including Current State
• Returning Current State is Easy
• Other Projections Can Be Tough
Query across Event Types
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Querying Events• All Queries are Projections, including Current State
• Returning Current State is Easy
• Other Projections Can Be Tough
• Try Reactive Streams
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Use Streams to Filter Events in Code
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Querying Events• All Queries are Projections, including Current State
• Returning Current State is Easy
• Other Projections Can Be Tough
• Try Reactive Streams
• Initial Projection Work May Require Dev Time
Find Someone with a Statistics
Background
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Querying Events• All Queries are Projections, including Current State
• Returning Current State is Easy
• Other Projections Can Be Tough
• Try Reactive Streams
• Initial Projection Work May Require Dev Time
• Consider Cacheable Query Layers
Separate Event Storage / Querying
Services
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Agenda• Event Sourcing
• Event Sourcing Challenges
• Don’t Worry, ES Is Worth It
• Implementation (Theory)
• Implementation (Reality)
• Persistance
• Querying
• Demo Time
Pray to the demo gods.
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In Summary
Event Sourcing -> Additive Only,
Lossless. Can Be Tricky To Work With
Not for your Mom’s Blog
Thank You!
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Image Credits• BOA: http://marketmadhouse.com/wp-content/uploads/2014/11/BofA.jpg
• Khaaaaan: http://d1oi7t5trwfj5d.cloudfront.net/38/0d9b80ae5311e1bcc4123138165f92/file/5-things-you-might-not-know-about-star-trek-ii-wrath-of-khan-30th-anniversary.jpg
• tribbles: http://de.memory-alpha.org/wiki/Tribble
• bank ledger: http://www.iowall.net/accounting-ledger-book.html
• ORM: https://guides.codepath.com/android/ActiveAndroid-Guide
• hoarders: http://www.aetv.com/hoarders
• snapshot stream: https://geteventstore.com/
• office space bobs: http://galleryhip.com/office-space-bobs.html
• Nye, Obama, Tyson photo: http://en.wikipedia.org/wiki/Selfie
• Printing Press: http://www.directindustry.com/prod/koenig-bauer-ag/offset-printing-presses-high-volume-40462-602574.html
