Event Sourcing

What is Event Sourcing - How it works and how it can look like in TypeScript.

Transcript

1. Event Sourcing
   Why state is not a fact but a first class derivative of things that
   happened
   

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2. Who am I
   Frank Jogeleit
   @move:elevator GmbH
   Backend Engineer
   GitHub: https://github.com/fjogeleit
   Twitter: @FrankJogeleit
   

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3. Do you think your bank
   account balance is a
   column in a table?
   

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4. What is Event Sourcing?
   • Its used for
   • most financial systems like
   Banking-, Trading-, Gambling Software
   • healthcare like medical files
   • legal contracts
   For sure nothing new
   

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5. What is Event Sourcing
   • Proper Audit
   • Logging
   core idea
   

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6. „Event Sourcing says all
   state is transient and
   you only store facts“
   

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7. What is Event Sourcing?
   Traditional View
   Line Items (2)
   Purchase Order
   Shipping Informations
   

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8. What is Event Sourcing?
   Event Sourcing View
   Card
   created
   3 Items
   Added
   Item
   Removed
   Shipping
   Information
   Added
   Event Stream
   

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9. What is Event Sourcing?
   • Losing no Information
   • Explicit Time Information
   • No updates / deletes, only append
   • Who now which data will be valuable in the future?
   • State is a first class derivative
   What changes?
   

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10. What is Event Sourcing?
    • Past Data for future features
    • Reports
    • Events are use cases
    • Code reflect behavior
    • Recreate state at any point in time in the past
    New capabilities
    

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11. What is Event Sourcing?
    • Readmodels are a (persistent) representation of the current
    state of your application
    • Created as a left fold of your Event Stream in historical order
    • You can create as many Readmodels as you want for a single
    Event Stream
    • Readmodel persistence is independent from your Events, e.g.
    • InMemory for business decisions
    • Redis for short caches
    • SQL or NoSQL Databases for Views
    Readmodels
    

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12. CQRS
    Command Query Responsibility Separation
    

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13. What is CQRS?
    • Like the name already implies this pattern separate reading from
    writing operations
    • Your operation has return type void? -> Command
    • Your operation returns any value? -> Query
    • Queries must not change any state of your app
    • When you execute a query many times, the result has to be the
    same. (Assumed no other operations happened)
    Not much more as simple Pattern
    

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14. CQRS + Event Sourcing
    • Commands represent behavior and create one or many Events
    • The EventStore is the single source of truth
    • ReadModels could be used for business decisions(*)
    • Queries using Readmodels to return application state
    How they work together
    

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15. Side Effects
    • It many cases some kind of MessageBuses are used to
    implement the CQRS pattern
    • Beside a Command- and QueryBus you can use an
    EventBus to trigger some kind of Side Effect
    • An EventBus publish persisted (Domain) Events, so you
    can listen for it and execute related logic, for example
    • send a Registration E-Mail
    • trigger another related service to your event
    (Purchase- and Billing Service)
    EventBus / ProcessManager / Saga - you named it
    

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16. Trade Offs
    • Replaying many Events (really many) is time expensive,
    you can create snapshot but you also have to manage
    them
    • Creating Readmodels asynchronously can lead to
    concurrency problems
    • Events are immutable but in some cases like GDPR
    (DSVO) you have to anonymize Data
    • Event Sourcing with Files need additional work
    Event Sourcing is perfect, isn’t it? - Almost
    

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17. Bank Account API
    Event Sourcing by Example
    

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18. Event Sourcing by Example
    • Github: https://github.com/fjogeleit/event-sourcing-nodejs-example
    • NodeJS API Example with TypeScript, NestJS
    • Hosted Example http://event-sourcing-example.webdev-jogeleit.de/
    • EventStore https://github.com/fjogeleit/event-store
    Bank Account API
    

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19. Questions?
    

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