The patients keep living. New encounters, observations, and claims land on a schedule — so delta polling, subscriptions, and demos behave like a production integration instead of a frozen snapshot.
A sandbox that never changes cannot test the half of your integration that deals with change. Incremental sync, cursor handling, duplicate suppression, out-of-order arrival, and empty-delta behavior are all untestable against a fixed dataset — so they get written, never exercised, and discovered in production at a customer site.
Resources carry honest meta.lastUpdated stamps, so _lastUpdated=gt{timestamp} on a type-level search and Patient/{id}/$everything?_since={timestamp} both return real deltas. The $everything delta is assembled per resource type and returns a Bundle whose meta.lastUpdated is the cursor for your next poll — it deliberately does not advance past data it could not return, so "always use the returned cursor" is safe unconditionally.
The same clinical events are emitted as HL7v2 messages and C-CDA documents, not just FHIR — because most real integrations still meet at least one of those on the wire. See inbound and subscriptions for the receiving direction: POST HL7v2, C-CDA, or a Redox message in and watch it land as FHIR.