L3 - Default Context:
In order to have a reliable way to page through the ingested events we need an immutable sequential identifier for the events. As the event time itself does not satisfy that need (past events can be reingested/updated) we need to add a ingestion timestamp field.
This way we ensure that any modifications are actually processed while paging through the events based on when they were ingested. This is inline with the `event log` concept of Even Sourcing
AT:
- a new ingestTimestamp field is added to the event index
- it's value is assigned by the event writer
- the document id is not the event id anymore but a random unique id