On Immutability of events

Israel is the land of milk and honey, but not the land of snow - snow is quite rare in Israel. This week the rare event of snow occurred in high mountains across the country - in the picture snow near one of the gates of the old city of Jerusalem. In the Carmel mountain ridge, where I live there was a little bit of snow, higher in the ridge, and some people went there and brought to the office bags full of snow... As I lived several years in the Philadelphia area in USA, I am personally less excited from snow - but it is a notable event here.

Today's topic relates to a question that Marco, fellow EP blogger, and the person behind rulecore, has asked about the previous posting, his question related to enrichment, but I'll extend it to the rest of the transformations : "event is something that is happened, and as such it is immutable, cannot be changed; Do transformations and enrichments break this model?".

The answer for this question is - no. Transformations do not break the model, and events are still immutable. According to the pure model - events cannot be altered or deleted, and when represented in an event store, it has to be an "append only" type of store.

Enrichment and transformations are, in fact, creation of new derived events, as a function of raw events. Thus, according to the pure model, transformed or enriched event is not the same event:

• It has a different type of event - with different structure.
• It has a different event-id.

An Enrichment example maybe - the event is: order, and it has an attribute that refers to customer. The enrichment function looks at the customer in some database, and fetches the values of : customer type (platinum, gold, silver, nobody) and customer_credit_limit.

The fact that there is a different event-id for the raw event and transformed event allows also the traceability to trace the transformation (maybe the problem is in wrong identification of the customer?).

There are some considerations that may, in practice, push towards not obeying to the pure model, and I'll talk about them some other time.