Caching aggregates with Axon Framework and Spring Boot 2

This requires the following setup:

• @EnableCaching annotation
• org.axonframework.common.caching.Cache bean
  • we use an implementation of type org.axonframework.common.caching.EhCacheAdapter. This requires:
    • net.sf.ehcache:ehcache dependency (EhCache v2)
    • ehcache.xml
• @Aggregate(cache = "axonCache") with axonCache the name of the org.axonframework.common.caching.Cache bean
  • SpringAggregateLookup.postProcessBeanFactory() uses this to configure the BeanDefinition used to instantiate CachingEventSourcingRepository

ExampleIT demonstrates that:

1. caching aggregates may result in stale aggregates when writes are done by multiple nodes (SimpleCommandBus)
   1. aggregates may reject valid commands (testStaleAggregateRefusesValidCommand())
   2. aggregates may accept invalid commands and publish events, that are in turn rejected by axon server (testAxonServerRefusesEventFromStaleAggregate())
2. caching aggregates never results in stale aggregates when using the single writer principle (AxonServerCommandBus = default command bus when using axon server), as demonstrated by testNoStaleAggregatesWithAxonServerCommandBus(). Axon reference states that:

   Commands are always sent to exactly one application. 
   Commands for the same aggregate are always sent to the same application instance, 
   to avoid problems with concurrent updates of the aggregate.
   

   https://docs.axoniq.io/reference-guide/v/4.5/axon-server-introduction#message-patterns