Event sourcing is a data persistence technique where all changes to an application's state are stored as a sequence of events. This allows the entire state of the application to be reconstructed by replaying the events in order. Some key benefits of event sourcing include capturing business intent, providing an audit log, and allowing the data to be rebuilt into any data model. However, it also has some drawbacks such as requiring multiple data sources and models, dealing with versioning issues, and potential performance impacts.

1. Event Sourcing
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2. Agenda
● Typical Data Persistence
● Overview
● Events
● Why?
● Queries
● Drawbacks

3. Typical Data Persistence
● Maintain current state
● Concurrent users, data update conflicts.
● Auditing

4. Overview
● Old concept
● Using already (transaction log)
● Capture Intent
● Event = State Change
● Sequence of Events
● Append Only

5. Events
● Describes state change
● Past tense
● Source of truth
● Audit log

6. Event Stream
Started Shopping
Added T-Shirt
Added Socks
Removed Socks
Added Coupon

7. Event
StartedShopping
{
CartId: f5a7f55d-1e33-440c-836c-ea50fc59a274,
CustomerId: f98902b3-e8cf-4b7a-b561-5e39a06b5e0c,
Date: 2014-01-03 15:36:08,
}
AddedSocksToShoppingCart
{
CartId: f5a7f55d-1e33-440c-836c-ea50fc59a274,
ProductId: ed00fa0b-6412-4cc6-bbac-14702bfeff3f,
Quantity: 1,
}

8. Why?
● Captures business intent
● Transaction & Audit log
● Build/Rebuild to any model
● Debugging
● Integration with other systems

9. Queries
● Generate Read Model from Events
● Secondary Data Source required
● Eventual Consistency
Started Shopping
Added T-Shirt
Added Socks
Removed Socks
Added Coupon
Any model you can
think of!
Cart Table
CartId
ProductId
Quantity
Denormalized Table
CartId
CustomerName
ProductId
Quantity
Price

10. CQRS/ES
Data Storage
Domain
Object
Domain
Object
Command Handlers
Client
Command
DTO
Message Queue
Read Layer
Facade
Query
DTO
Event Storage
Events
Message Queue Event Handlers
Data Storage

11. Drawbacks
● Multiple Models / Data Sources
● Versioning
● Performance
● Maturity / Public Knowledge