TechDays 2017 - Event Sourcing after launch how to evolve your event store along with your application

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[email protected] @michielovereem

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400+ employees (4 locations) 10.500 customers (companies) AFAS Software

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HRM, CRM, finance, order management, project management, workflow, ... AFAS Profit

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accountId balance owner 1234567 0 Michiel Current state Event sourcing Event BankAccountCreated (accountId: 1234567, balance: 0, owner: Michiel)

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accountId balance owner 1234567 100 Michiel Current state Event sourcing Event BankAccountCreated (accountId: 1234567, balance: 0, owner: Michiel) DepositPerformed (accountId : 1234567, amount: 100, balance: 100)

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Event BankAccountCreated (accountId: 1234567, balance: 0, owner: Michiel) DepositPerformed (accountId : 1234567, amount: 100, balance: 100) WithdrawalPerformed (accountId: 1234567, amount: 50, balance: 50) accountId balance owner 1234567 50 Michiel Current state Event sourcing

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Event BankAccountCreated (accountId: 1234567, balance: 0, owner: Michiel) DepositPerformed (accountId : 1234567, amount: 100, balance: 100) WithdrawalPerformed (accountId: 1234567, amount: 50, balance: 50) OwnerChanged (accountId: 1234567, newOwner: Marten) accountId balance owner 1234567 50 Marten Current state Event sourcing

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Event ShoppingCartCreated (cartid: 1234567, user: Michiel) cartid articles user 1234567 0 Michiel Current state Event sourcing

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Event ShoppingCartCreated (cartid: 1234567, user: Michiel) ArticleAddedToCart(cartid: 1234567, article: Lumia 950) cartid articles user 1234567 1 Michiel Current state Event sourcing cartid article 1234567 Lumia 950

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cartid articles user 1234567 0 Michiel Current state Event sourcing cartid article Event ShoppingCartCreated (cartid: 1234567, user: Michiel) ArticleAddedToCart(cartid: 1234567, article: Lumia 950) ArticleRemovedFromCart(cartid: 1234567, article: Lumia 950)

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cartid articles user 1234567 1 Michiel Current state Event sourcing cartid article 1234567 iPhone X Event ShoppingCartCreated (cartid: 1234567, user: Michiel) ArticleAddedToCart(cartid: 1234567, article: Lumia 950) ArticleRemovedFromCart(cartid: 1234567, article: Lumia 950) ArticleAddedToCart(cartid: 1234567, article: iPhone X)

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A stream of events

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Multiple streams

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{ “ArticleAddedToCart”: { “cartid”: 1234567, “article”: “Lumia 950” } }

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Webshop EventStore Customer Stream ShoppingCart Stream CustomerSignedUp { … } PriviligeUpgraded { … } ShoppingCartCreated { … } ChangedQuantityOfArticle { … } ArticleAddedToChart { … }

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No content

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Customer Stream CustomerSignedUp { …, newsletter: } PriviligeUpgraded { … }

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Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … }

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Newsletter Stream CreateNewsletter { … } PublishNewsletter { … }

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Customer Stream CustomerSignedUp { …, firstName: , surName: } PriviligeUpgraded { … } CustomerSignedUp { …, fullName: }

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Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … } CustomerSignedUp { …, newsletter: }

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Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … } Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } NewsletterSubscription Stream OptInOnNewsLetter { … }

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Store level operations Add / remove streams Merge streams / split / … streams Stream level operations Add / remove events Merge events / split / … events Event level operations Add / update / remove attributes Merge attributes / split / … attributes

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Client

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Client

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Client

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Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } CustomerSignedUpV2 { …, newsletter: }

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Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … }

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Client Upcaster

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Client Upcaster

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Updater Not for the faint of heart!

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Updater

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Updater

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Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Functional suitability +/- - +/- +/- + +

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Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Maintainability - +/- +/- +/- + +

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Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Performance efficiency + + + +/- +/- -

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Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Reliability + + + - - +