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

Event sourcing can be a solution for certain problems in your application, but it also comes with its own set of challenges.
One of those is the evolution of your event schema. How should you handle changes to events?
We found many approaches while doing our research (we’ve published earlier results in our paper “The Dark Side of Event Sourcing”) and we gained a lot of experienced from building a full-size ERP application using CQRS and event sourcing.
In this talk we will show what kind of approaches we’ve found and what we use to deal with our specific challenges. We will also share early research results from our interviews with over 20 different companies using event sourcing.
Every request for advice can be answered with ‘it depends’, and this talk will not solve that. But we will tell you what your options are, what we think works, and what many others have done.

Transcript

1. michiel.overeem@afas.nl @michielovereem
2. 400+ employees (4 locations) 10.500 customers (companies) AFAS Software
3. HRM, CRM, finance, order management, project management, workflow, ... AFAS Profit
5. accountId balance owner 1234567 0 Michiel Current state Event sourcing Event BankAccountCreated (accountId: 1234567, balance: 0, owner: Michiel)
6. 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)
7. 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
8. 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
10. Event ShoppingCartCreated (cartid: 1234567, user: Michiel) cartid articles user 1234567 0 Michiel Current state Event sourcing
11. 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
12. 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)
13. 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)
14. 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)
20. Copyright Nasa Goddard But there is a dark side… How can we handle changes? What is the best way for our context?
21. A stream of events
23. Multiple streams
24. { “ArticleAddedToCart”: { “cartid”: 1234567, “article”: “Lumia 950” } }
26. Webshop EventStore Customer Stream ShoppingCart Stream CustomerSignedUp { … } PriviligeUpgraded { … } ShoppingCartCreated { … } ChangedQuantityOfArticle { … } ArticleAddedToChart { … }
28. Customer Stream CustomerSignedUp { …, newsletter: <bool> } PriviligeUpgraded { … }
29. Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … }
30. Newsletter Stream CreateNewsletter { … } PublishNewsletter { … }
31. Customer Stream CustomerSignedUp { …, firstName: <string>, surName: <string> } PriviligeUpgraded { … } CustomerSignedUp { …, fullName: <string> }
32. Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … } CustomerSignedUp { …, newsletter: <bool> }
33. Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } OptInOnNewsLetter { … } Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } NewsletterSubscription Stream OptInOnNewsLetter { … }
34. 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
36. Copyright Stephen Cairns
37. Client
38. Client
39. Client
41. Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … } CustomerSignedUpV2 { …, newsletter: <bool> }
42. Customer Stream CustomerSignedUp { … } PriviligeUpgraded { … }
43. Client Upcaster
44. Client Upcaster
45. Updater Not for the faint of heart!
46. Updater
47. Updater
48. Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Functional suitability +/- - +/- +/- + +
49. Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Maintainability - +/- +/- +/- + +
50. Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Performance efficiency + + + +/- +/- -
51. Multiple versions Weak schema Upcasters Lazy transformation In place transformation Copy and transformation Reliability + + + - - +
61. michiel.overeem@afas.nl @michielovereem https://dev.afas.nl