Event Notification

• generic code notifies specific code by events
• e.g. GUI elements

• decoupling

• inability to understand what is going on by stepping through the code

Event-Carried State Transfer

• subscribers don’t ask for additional information after an event occurred, all necessary state is given in the events
• subscribers copy whatever they need

• can greatly reduce network traffic
• better availability

• consistency lost because of replication

Event Sourcing

• ability to rebuild the full state of the system by a persisted log of events
• event sourcing works with data like version control systems work with code

• “time travel” like for debugging

• when rebuilding the state from log, can’t replay responses of external systems unless the too a recorded
• old event schema still has to work with newer versions of code
• IDs that are generated during replay are probably different than before
• asynchrony is difficult for people (but is optional, you can synchronously use event sourcing)
• versioning (snapshots can make that easier)

conclusion: replay has to work for everything, otherwise there is no point in using event sourcing

CQRS

• separate components for:
  • updating the system
  • reading the system