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Introduction to Event Sourcing … and CQRS

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Vladik Khononov Chief Architect at Internovus http://il.linkedin.com/in/vladikkhononov/ vladikk vladikk http://vladikk.com

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Information Systems

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HTML5 AngularJS WPF IOS Flash ExtJS Android Silverlight Responsive webdesign SPA CSS3 User Interface

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User Interface MySQL MongoDB DynamoDB Neo4j RavenDB PostgreSQL HBase Cassandra Sql Server MariaDB Redis CouchDB Data Access

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User Interface Data Access “Verbs & Nouns” Domain Driven Design Business Logic

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Domain Driven Design?

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User Interface Business Logic Data Access

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– www.dictionary.com Model: A simplified representation of a system or phenomenon. – www.wikipedia.org לדומ: גוציי יטרואת לש תכרעמ תבכרומ , ותרטמש תוקחל תא תכרעמה םיטביהב םייתוהמ . לדומה וניא ראתמ לכ העפות תכרעמב , אלא סחייתמ םיטביהל םירדגומ םימצמוצמו הלש . לדומה ססובמ לע בוריק לש תואיצמה ךרדב לש הטשפה , דוחיא תויושי תומלעתהו םימרוגמ םתעפשהש הניא תיתוהמ .

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User Interface Business Logic Data Access

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Domain Model

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Good Domain Model • Not too much • Not too less • Sweet spot • The information we need • The information we will need

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Why domain models fail • We absolutely suck at predicting the future • No single model can suit all the use cases • Model transformations are painful

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Case Study: Customers Management

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• A customer has customer id number and a name • A customer has contact information: email, phone number • A customer can be in on of the following states: New, CallLater, Converted, NotInterested • A seller has seller id number, name and password • The selling home page contains a list of the customers in the following statuses: • New • CallLater (when scheduled call date is met)

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• Seller chooses a customer to call from the home page • Seller can change the customer’s status to “Converted”, “Not Interested” • Seller can schedule a future call by setting the future call date. The Customer’s status will change to “CallLater” • Seller can change name, email, and phone number

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enum Status { New, CallLater, Converted, NotInterested } class Customer { int Id; string Name; Status Status; DateTime? ScheduledCall; string Email; string PhoneNumber; } CREATE TABLE Customers ( ID INTEGER, Name CHAR(40), Email CHAR(40), PhoneNumber CHAR(40), Status INTEGER, ScheduledCall DATETIME, PRIMARY KEY (ID) ) class Seller { int Id; string Name; string Password; } CREATE TABLE Sellers ( ID INTEGER, Name CHAR(40), Password CHAR(40) PRIMARY KEY (ID) )

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• Seller can find customers by name, email, and/or phone number in the search page • The customers should be found by the current and the past values

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• If no new customers are available on the home page, it should display customers in the NotInterested status, if it was set more than a 30 days ago

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• Analysts should be able to review status changes history for each customer - change date and the new status

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class Seller { int Id; string Name; string Password; } CREATE TABLE Sellers ( ID INTEGER, Name CHAR(40), Password CHAR(40) PRIMARY KEY (ID) ) enum Status { New, CallLater, Converted, NotInterested } class Customer { int Id; string Name; Status Status; DateTime? ScheduledCall; string Email; string PhoneNumber; } CREATE TABLE Customers ( ID INTEGER, Name CHAR(40), Email CHAR(40), PhoneNumber CHAR(40), Status INTEGER, ScheduledCall DATETIME, PRIMARY KEY (ID) )

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 04-2342343 New

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 04-2342343 CallLater 27/10/2014

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 08-9876653 CallLater 27/10/2014

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 08-9876653 Converted

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 08-9876653 Converted

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Event Sourcing Capture all changes to an application state as a sequence of events ~ Martin Fowler

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class StatusChanged : IEvent { Status NewStatus; DateTime CreatedOn; int CreatedBy; } class FutureCallScheduled : IEvent { DateTime ScheduledCallTime; DateTime CreatedOn; int CreatedBy; } class ContactDetailsWereUpdated : IEvent { string NewName; string NewEmail; string NewPhone; DateTime CreatedOn; int CreatedBy; }

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class Customer { int Id; string Name; string Email; string PhoneNumber; Status Status; DateTime? ScheduledCall; List Events; … void Apply(StatusChanged e) { Status = e.NewStatus; Events.Add(e); } …. …. }

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class Customer { int Id; string Name; string Email; string PhoneNumber; Status Status; DateTime? ScheduledCall; List Events; … … void Apply(FutureCallScheduled e) { ScheduledCall = e.ScheduledCallTime; Events.Add(e); } … }

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class Customer { int Id; string Name; string Email; string PhoneNumber; Status Status; DateTime? ScheduledCall; List Events; … … … void Apply(ContactDetailsWereUpdated e) { Name = e.NewName; Email = e.NewEmail; PhoneNumber = e.NewPhoneNumber; Events.Add(e); } }

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Id Name Email Phone number Status Scheduled Call 10 John john@gmail.com 08-9876653 Converted

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1. new StatusChanged(Status.CallLater) 2. new FutureCallScheduled(’27/10/2014’) 3. new ContactDetailsWereUpdated( NewPhoneNumber=’08-9876653’ ) 4. new StatusChanged(Status.Converted)

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Event Storage Entity Id + New events Entity Id Event1, Event2, Event3, ….

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class CustomerSearchModel { int Id; List Names; List Emails; List PhoneNumbers; Status Status; DateTime? ScheduledCall; … … … void Apply(ContactDetailsWereUpdated e) { Names.Add(e.NewName); Emails.Add(e.NewEmail); PhoneNumbers.Add(e.NewPhoneNumber); } }

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class CustomerAnalysisModel { int Id; string Name; string Email; string PhoneNumber; List StatusChanges; DateTime? ScheduledCall; … void Apply(StatusChanged e) { StatusChanges.Add( new StatusChange(e.CreatedOn, e.NewStatus) ); } …. …. }

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class Customer { int Id; string Name; string Email; string PhoneNumber; Status Status; DateTime? ScheduledCall; List Events; }

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class CustomerSearchModel { int Id; List Names; List Emails; List PhoneNumbers; Status Status; DateTime? ScheduledCall; }

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class CustomerAnalysisModel { int Id; string Name; string Email; string PhoneNumber; List StatusChanges; DateTime? ScheduledCall; }

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Good Domain Model • Not too much • Not too less • Sweet spot • The information we need • The information we might need

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Why domain models fail • We absolutely suck at predicting the future • No single model can suit all the use cases (e.g.: transactional processing, business intelligence, search) • Model transformations are painful

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Event Storage Entity Id + New events Entity Id Event1, Event2, Event3, ….

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CQRS Command Query Responsibility Segregation

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• Event based models • Effective modeling • No future predicting required • Time machine • Retroactive debugging • Infrastructural freedom • Infinite scalability • Easy to scale writes • Easy to scale reads Conclusions

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Bounded Contexts

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Domain Types • Core Domain • Subdomain • Generic Subdomain

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Plexop

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Campaign Management (Core Domain) Creative Catalog (Subdomain) User Management (Generic Subdomain) Billing (Generic Subdomain)

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Before you try this at home • Read “Implementing Domain Driven Design” by Vaughn Vernon • Read “Domain Driven Design” by Eric Evans • Follow Greg Young, Udi Dahan, DDD community • Read DDD/CQRS on Google Groups • Join http://meetup.com/DDD-IL :)

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Questions?

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http://il.linkedin.com/in/vladikkhononov http://twitter.com/vladikk http://vladikk.com

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