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1/101 Event Sourcing Let's write some history https://joind.in/talk/3375d

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Willem-Jan Zijderveld @willemjanz github.com/wjzijderveld  

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Rotterdam http://labs.qandidate.com

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Broadway Event Sourcing/CQRS library for PHP github.com/qandidate-labs/broadway

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Event Sourcing

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You are throwing away data!

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Only state gets stored

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Only the most recent state gets stored

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What was the previous state?

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Why did the state change?

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It's hard to keep track of all the data

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An example

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A company has one or more connected accounts

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A company has one or more enabled applications

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An account has access to one or more applications

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No content

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Complicated solutions to store and retrieve the data

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But we manage Until...

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Boss: Which accounts received access to application X in December?

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"We'll add a timestamp and we will know" From that point in time

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With CRUD you lose information You only store the last known information

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Another example

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Revoking access to an application for an account Just remove the row in our join table

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You lose when the access was revoked

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You lose why it got revoked

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You lose for which application it was revoked

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You wouldn't know anything anymore

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Let's assume you have a soft-delete in place

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Boss: I want to know for which applications access got revoked more than once in the last year

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Keep track of revocations account_id app_id datetime 75623 8 2015-01-01T00:00:00+0000 75623 8 2015-01-27T18:54:18+0000 Are you going to do this for all actions on accounts?

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How does Event Sourcing help me with that?

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Store your data in a di erent way

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Record what has happened The resulting state becomes a natural effect

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EventStream

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A serie of facts CompanyRegistered AppEnabled AccountConnected AccessGrantedToApp

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Been there, done that

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Single source of truth One source to rule all state The events cannot lie, it happened, deal with it

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You have been working with this system

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Your version control is event sourced

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Sure thing, but how? "It depends"

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Event Sourcing DDD CQRS All 3 are optional, but work very nice together

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Domain Driven Design Let the events tell the story The business should be re ected in your code

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Entity An model with an identity

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Aggregate root Responsible for keeping a group of entities consistent

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I want to create/update my model How do I do that?

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Record what happened

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You normally might do it like this < ? p h p / / C o m p a n y . p h p p u b l i c f u n c t i o n _ _ c o n s t r u c t ( $ i d , $ n a m e ) { $ t h i s - > i d = $ i d ; $ t h i s - > n a m e = $ n a m e ; }

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But we want to record an event

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< ? p h p / / C o m p a n y . p h p p u b l i c s t a t i c f u n c t i o n r e g i s t e r ( $ i d , $ n a m e ) { $ c o m p a n y = n e w C o m p a n y ( ) ; $ c o m p a n y - > a p p l y ( n e w C o m p a n y R e g i s t e r e d E v e n t ( $ i d , $ n a m e ) ) ; r e t u r n $ c o m p a n y ; }

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Record the events < p h p / / A g g r e g a t e R o o t . p h p p u b l i c f u n c t i o n a p p l y ( $ e v e n t ) { $ t h i s - > h a n d l e ( $ e v e n t ) ; $ t h i s - > u n c o m m i t e d E v e n t s [ ] = $ e v e n t ; } p r i v a t e f u n c t i o n h a n d l e ( $ e v e n t ) { $ c l a s s P a r t s = e x p l o d e ( ' \ \ ' , $ e v e n t ) ; $ m e t h o d = ' a p p l y ' . e n d ( $ c l a s s P a r t s ) ; $ t h i s - > $ m e t h o d ( $ e v e n t ) ; }

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/ / C o m p a n y . p h p p u b l i c f u n c t i o n a p p l y C o m p a n y R e g i s t e r e d E v e n t ( C o m p a n y R e g i s t e r e d E v e n t $ e v e n t ) { $ t h i s - > c o m p a n y I d = $ e v e n t - > g e t C o m p a n y I d ( ) ; $ t h i s - > c o m p a n y N a m e = $ e v e n t - > g e t C o m p a n y N a m e ( ) ; }

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Save your events in an eventstore

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Current situation

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Reloading your model

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< p h p / / C o m p a n y R e p o s i t o r y . p h p f u n c t i o n l o a d ( $ a g g r e g a t e I d ) { $ e v e n t s = $ t h i s - > e v e n t S t o r e - > l o a d ( $ a g g r e g a t e I d ) ; $ a g g r e g a t e = n e w $ t h i s - > a g g r e g a t e C l a s s ( ) ; $ a g g r e g a t e - > i n i t i a l i z e S t a t e ( $ e v e n t s ) ; r e t u r n $ a g g r e g a t e ; }

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< ? p h p / / C o m p a n y . p h p p u b l i c f u n c t i o n i n i t i a l i z e S t a t e ( a r r a y $ e v e n t s ) { f o r e a c h ( $ e v e n t s a s $ e v e n t ) { $ t h i s - > h a n d l e ( $ e v e n t ) ; } }

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Domain Message A message to tell your application what happened

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DomainMessage Identi er Sequencenumber Payload Timestamp Metadata

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Identi er + sequence number

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Payload The event itself, it tells you what happened

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Should contain everything It should only depend on previous events f i n a l c l a s s C o m p a n y R e g i s t e r e d E v e n t { p r i v a t e $ c o m p a n y I d ; p r i v a t e $ c o m p a n y N a m e ; / / c o n s t r u c t o r + g e t t e r s }

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Timestamp It tells you when it happened

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Metadata Descriptive, not structural

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Let's look at some other events CompanyRegistered AppEnabled AccountConnected AccessGrantedToApp

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CompanyRegistered / / C o m p a n y f u n c t i o n a p p l y C o m p a n y R e g i s t e r e d E v e n t ( C o m p a n y R e g i s t e r e d E v e n t $ e v e n t ) { $ t h i s - > c o m p a n y I d = $ e v e n t - > g e t C o m p a n y I d ( ) ; }

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AccountConnected / / C o m p a n y f u n c t i o n a p p l y A c c o u n t C o n n e c t e d E v e n t ( A c c o u n t C o n n e c t e d E v e n t $ e v e n t ) { $ i d = $ e v e n t - > g e t A c c o u n t I d ( ) ; $ t h i s - > a c c o u n t s [ $ i d ] = $ i d ; }

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AppEnabled / / C o m p a n y f u n c t i o n a p p l y A p p E n a b l e d E v e n t ( A p p E n a b l e d E v e n t $ e v e n t ) { $ s u b s c r i p t i o n = n e w S u b s c r i p t i o n ( $ e v e n t - > g e t C o m p a n y I d ( ) , $ e v e n t - > g e t A p p I d ( ) ) ; $ t h i s - > s u b s c r i p t i o n s [ $ e v e n t - > g e t A p p I d ( ) ] = $ s u b s c r i p t i o n ; }

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AccessGrantedToApp / / C o m p a n y p r o t e c t e d f u n c t i o n g e t C h i l d E n t i t i e s ( ) { r e t u r n $ t h i s - > s u b s c r i p t i o n s ; } / / S u b s c r i p t i o n f u n c t i o n a p p l y A c c e s s G r a n t e d T o A p p E v e n t ( A c c e s s G r a n t e d T o A p p E v e n t $ e v e n t ) { i f ( $ t h i s - > a p p I d ! = = $ e v e n t - > g e t A p p I d ( ) ) { r e t u r n ; } $ a c c o u n t I d = $ e v e n t - > g e t A c c o u n t I d ( ) ; $ t h i s - > g r a n t e d A c c o u n t s [ $ a c c o u n t I d ] = $ a c c o u n t I d ; }

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We can retrieve all AccessGranted events for December

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Won't this be slow?

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CQRS Command Query Responsibility Segregation

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Separate your writes and your reads

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Read Models Create them from events using projections Speci c read models for speci c views in your application

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EventBus All events go onto the event bus after being saved

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CompanyRegistration c l a s s C o m p a n y R e g i s t r a t i o n i m p l e m e n t s R e a d M o d e l { p u b l i c f u n c t i o n _ _ c o n s t r u c t ( $ c o m p a n y I d , $ c o m p a n y N a m e , D a t e T i m e $ r e g i s t e r e d O n ) { / / . . } }

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Company Registration Projector c l a s s C o m p a n y R e g i s t r a t i o n P r o j e c t o r i m p l e m e n t s E v e n t L i s t e n e r { p u b l i c f u n c t i o n a p p l y C o m p a n y R e g i s t e r e d E v e n t ( C o m p a n y R e g i s t e r e d E v e n t $ e v e n t , D o m a i n M e s s a g e $ d o m a i n M e s s a g e ) { $ c o m p a n y = n e w C o m p a n y R e g i s t r a t i o n ( $ e v e n t - > g e t C o m p a n y I d ( ) , $ e v e n t - > g e t C o m p a n y N a m e ( ) , $ d o m a i n M e s s a g e - > g e t R e c o r d e d O n ( ) ) ; $ t h i s - > r e p o s i t o r y - > s a v e ( $ c o m p a n y ) ; } }

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Use the right tool for the right job

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Possibilities are endless

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The ability to create multiple read models List of company registrations Graph of all connections between companies and accounts Creating reports about the amount of revocations

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So.. how does this ow work in an application?

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No content

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The C in CQRS

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The write part of CQRS

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Command & CommandHandler A CommandHandler deals with Commands and communicates with the Aggregate root

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From Controller to CommandBus c l a s s C o m p a n y C o n t r o l l e r { f u n c t i o n r e g i s t e r A c t i o n ( R e q u e s t $ r e q u e s t ) { $ t h i s - > c o m m a n d B u s - > d i s p a t c h ( n e w R e g i s t e r C o m p a n y C o m m a n d ( U u i d : : u u i d 4 ( ) , $ r e q u e s t - > r e q u e s t - > g e t ( ' c o m p a n y N a m e ' ) ) ) ; } }

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From CommandHandler to Aggregate c l a s s C o m p a n y C o m m a n d H a n d l e r { f u n c t i o n h a n d l e R e g i s t e r C o m p a n y C o m m a n d ( R e g i s t e r C o m p a n y C o m m a n d $ c o m m a n d ) { $ c o m p a n y = C o m p a n y : : r e g i s t e r ( $ c o m m a n d - > g e t C o m p a n y I d ( ) , $ c o m m a n d - > g e t C o m p a n y N a m e ( ) ) ; $ t h i s - > a g g r e g a t e R e p o s t i t o r y - > s a v e ( $ c o m p a n y ) ; } }

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From Aggregate to Event p u b l i c s t a t i c f u n c t i o n r e g i s t e r ( $ c o m p a n y I d , $ n a m e ) { $ c o m p a n y = n e w C o m p a n y ( ) ; $ c o m p a n y - > a p p l y ( n e w C o m p a n y R e g i s t e r e d E v e n t ( $ c o m p a n y I d , $ n a m e ) ) ; r e t u r n $ c o m p a n y ; } p u b l i c f u n c t i o n a p p l y C o m p a n y R e g i s t e r e d E v e n t ( C o m p a n y R e g i s t e r e d E v e n t $ e v e n t ) { $ t h i s - > c o m p a n y I d = $ e v e n t - > g e t C o m p a n y I d ( ) ; }

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From Repository to EventStore / / C o m p a n y R e g i s t r y . p h p f u n c t i o n s a v e ( $ a g g r e g a t e ) { $ e v e n t s = $ a g g r e g a t e - > g e t U n c o m m i t t e d E v e n t s ( ) ; $ t h i s - > e v e n t S t o r e - > a p p e n d ( $ a g g r e g a t e - > g e t A g g r e g a t e I d ( ) , $ e v e n t s ) ; $ t h i s - > e v e n t B u s - > p u b l i s h ( $ e v e n t s ) ; }

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From event to read model c l a s s C o m p a n y R e g i s t r a t i o n P r o j e c t o r { p u b l i c f u n c t i o n a p p l y C o m p a n y R e g i s t e r e d E v e n t ( C o m p a n y R e g i s t e r e d E v e n t $ e v e n t , D o m a i n M e s s a g e $ d o m a i n M e s s a g e ) { $ c o m p a n y = n e w C o m p a n y R e g i s t r a t i o n ( $ e v e n t - > g e t C o m p a n y I d ( ) , $ e v e n t - > g e t C o m p a n y N a m e ( ) , $ d o m a i n M e s s a g e - > g e t R e c o r d e d O n ( ) ) ; $ t h i s - > r e p o s i t o r y - > s a v e ( $ c o m p a n y ) ; } }

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Quite a bit to chew

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No content

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But we are not done yet!

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Testing

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Scenario based testing Given - When - Then

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Test your Command Handler $ t h i s - > s c e n a r i o - > g i v e n ( [ n e w C o m p a n y R e g i s t e r e d E v e n t ( 1 2 3 ) ] ) - > w h e n ( n e w E n a b l e A p p F o r C o m p a n y C o m m a n d ( 4 2 , 1 2 3 ) ) - > t h e n ( [ n e w A p p E n a b l e d E v e n t ( 4 2 , 1 2 3 ) ] ) ;

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Or your aggregate $ t h i s - > s c e n a r i o - > g i v e n ( [ n e w C o m p a n y R e g i s t e r e d E v e n t ( 1 2 3 ) ] ) - > w h e n ( f u n c t i o n ( $ c o m p a n y ) { $ c o m p a n y - > e n a b l e A p p ( 4 2 ) ; } ) - > t h e n ( [ n e w A p p E n a b l e d E v e n t ( 4 2 , 1 2 3 ) ] ) ;

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Don't forget your read models $ t h i s - > s c e n a r i o - > g i v e n ( [ ] ) - > w h e n ( n e w C o m p a n y R e g i s t e r e d E v e n t ( 1 2 3 , ' A c m e I n c ' ) ) - > t h e n ( [ n e w C o m p a n y R e g i s t r a t i o n ( 1 2 3 , ' A c m e I n c ' ) ] ) ;

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Time travel is possible! Use events you recorded to create a new report multiple years after the fact

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You made a mistake in a projection? So what? Correct your projector and recreate your read model from your event stream

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Questions? Feedback: https://joind.in/talk/3375d @willemjanz Freenode: #qandidate

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More information http://codebetter.com/gregyoung/2010/02/20/why-use- event-sourcing/ http://codebetter.com/gregyoung/2010/02/13/cqrs-and- event-sourcing/ http://martinfowler.com/eaaDev/EventSourcing.html http://martinfowler.com/bliki/CQRS.html http://www.axonframework.org/docs/2.3/domain- modeling.html http://labs.qandidate.com/