State or Events?
Speaker: Kenny Bastani & Jakub Pilimon
- Showed Groovy code using Spock for tests. [didn’t actually mention it was Spock, but easy enough to recognize]
- Wrote tests first for a CreditCard object [nice advantage of tests in Groovy. The methods don’t need to exist and you don’t get red errors as you type so not tempted to write the methods]
- The CreditCard implementation was logical with instance variables.
- Noted that the method to assign the limit is a command with an invariant.
- CQS – command query segregation
Refactoring to events
- Create domain events
- For name, pick a past tense description of what it does. For example “limitAssigned”. Used same name for method and domain event object (which is passed to method as parameter).
- Add timestamp to event so know order. That way can determine current state of object.
- Also need former instance variables (limit) and parameters (amount) as attributes of the new domain event.
- Similarly refactored to have withdraw(CardWithdraw c) and repay(CardRepay c)
- Keep track of changes and store
- Dirty context – when change object but haven’t saved yet.
- Add instance variable for list of domain events. This keeps track of the pending/dirty events
- Create Repository object
- instance variable with map of unique credit card ids to a list of domain events
- method to save to map
- method to load credit cards by replaying all events. Used left fold. “It’s so fancy that Java 8 doesn’t support it out of the box”. Used javaslang.collection.List API to get it.
- Update CreditCard three handler methods to return a references to “this” so can chain
- Added Kafka so could store events (via Topics)
- Added spring-kafka and kafka-streams to pom
- Created config class with @EnableKafka
- Update save method to use Kafka class
- Then switched to Kafka streams
- Created new bean for stream configuration
- return KTable – key table – aggregate of events from object
- Create controller
- method to get list of credit cards
- Created scheduler for testing a bunch of credit card transactions
[this was a full house; I barely got a seat. Pivotal handed that well. They found all the empty seats and directed people do them. Even so, there are a good number of people in the back.]