Gr8conf US 2015 - Intro to Event Sourcing with Groovy

THIRDCHANNEL @svpember

Hi.

First, A Quick Scenario


Balance: $100


Balance: -$100






Richer Data History With Event Sourcing

Steve PemberCTO, ThirdChannelgr8Conf US 2015


Agenda• Event Sourcing

• Event Sourcing Challenges

• Don’t Worry, ES Is Worth It

• Querying

• Implementation (Theory)

• Implementation (For Real)

• Any Tools Available?

• Demo (Hopefully)

What Is Event Sourcing?


Event Sourcing• Alternative Storage Pattern


Instead, Store Facts




• Series of Facts

• Aggregates << Events

Event: Something Which Has Occurred In The System

Aggregate: The Object of Events’ Affections



• Series of Facts


• Purely Additive

There is No Delete


Full History of the Aggregates




• Series of Facts


• Purely Additive

• Optimization: Snapshots


• snapshot photo




• Series of Facts


• Purely Additive


• Example: Shopping Cart






• Series of Facts


• Purely Additive



• Transient Objects

Objects are Transient Derivatives of Your Events



• Series of Facts


• Purely Additive



• Transient Objects

• Natural




Questions So Far?• Events & Aggregates

• Snapshots

• Additive, loss-less model

• Transient Derivatives




“I Think I Get What You’re Laying Down, But It Still

Seems Bananas”

So much work!

“That’s gotta be a ton of Resource overhead”

“Where’d my Models go? I can’t live without my MVC!”


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






• Reduced Transactions / Database Level Constraints

• May Be Confusing For Junior Engineers

So Why Use ES?






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



• Data Storage is Inexpensive




• Used By All Long-Running Businesses


All Business Problems Can Be Modeled With Event Sourcing





• Only Structural Model That Does Not Lose Information

Remember: There Is No Delete






• Simplified Testing and Debugging

Unit Test Events, Assert In Integration Tests

Time Travel Eases Debugging












• Simplified Testing and Debugging

• Ideal for Business Analysis

Future-Proof your Data


But With Event Sourcing…

–Johnny Appleseed

“Type a quote here.”


Current State is a Projection

A 3C Example


Still…



Have you ever built?• Versioning of objects

• An ‘undo’ action

• Audit Log

• Tracking object value changes over time?

• Time series?





• Querying


Querying Events• All Queries are Projections, including Current State



• Returning Current State is Easy





• Other Projections Can Be Tough, use a Read Model





• Other Projections Can Be Tough

• Try Reactive Streams


Use Streams to Filter Events in Code





• Other Projections Can Be Tough

• Try Reactive Streams

• Initial Projection Work May Require Dev Time






• Querying



Implementation (Theory)• Simple Base Objects

Three Base Objects



Past-Tense Event Names




• Aggregate and Event SubClasses Have Transient Properties






• Aggregates Receive and Play Events

Events Modify Aggregates





• Aggregates Require Distinction Between New and Historical Changes






• Aggregates Require Distinction Between New and Historical Changes

• Event Service Layer Required





• Querying


• Implementation (Reality)


Implementation (Reality)• Snapshot Rarely



• ‘Pure’ Event Sourcing is tough to work with




• ‘Pure’ Event Sourcing is tough to work with

• 2 Alternatives


Sync


Plus so much more (I could talk all day about ES)





• Querying





Event Source Libraries• No Real Need for an ORM


Event Sourcing Libraries• No Real Need for an ORM

• Akka Persistence (JVM)


Persistence & Event Storage• No Real Need for an ORM

• Akka Persistence (JVM)

• Database Agnostic





Questions?





• Querying




• Demo?

Demo Time!


In Summary

Event Sourcing -> Additive Only, Lossless. Can Be Tricky To Work

With

Not for your Mom’s Blog

Thank You!


Any Questions?

Steve Pember@svpember

www.thirdchannel.com

http://www.thirdchannel.com/


More on Event Sourcing• ThirdChannel Event Source Library (Java): https://github.com/RSV2/eventsource• Demo App: https://github.com/spember/event-source-demo• “Event Sourcing” (Martin Fowler): http://martinfowler.com/eaaDev/EventSourcing.html• Event Store & “Event Sourcing Basics”: http://docs.geteventstore.com/introduction/event-sourcing-basics/• Event Sourcing presentation by Greg Young: https://www.youtube.com/watch?v=JHGkaShoyNs

https://github.com/RSV2/eventsource

https://github.com/spember/event-source-demo

http://martinfowler.com/eaaDev/EventSourcing.html

http://docs.geteventstore.com/introduction/event-sourcing-basics/

https://www.youtube.com/watch?v=JHGkaShoyNs


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

http://marketmadhouse.com/wp-content/uploads/2014/11/BofA.jpg



http://de.memory-alpha.org/wiki/Tribble

http://www.iowall.net/accounting-ledger-book.html

https://guides.codepath.com/android/ActiveAndroid-Guide

http://www.aetv.com/hoarders

https://geteventstore.com/

http://galleryhip.com/office-space-bobs.html

http://en.wikipedia.org/wiki/Selfie

http://www.directindustry.com/prod/koenig-bauer-ag/offset-printing-presses-high-volume-40462-602574.html

Gr8conf US 2015 - Intro to Event Sourcing with Groovy

Software

svpember balance

thirdchannel gr8conf

svpember richer data

store facts

tools available

quick scenario