Researching FHIR bundle transactions.
The idea is that you can post a bundle to a FHIR server with a set of resources. And you can send it as a transaction, so only all the resources will be published or neither.
E.g. I send a bundle of two resources: patient and its observation.
The observation resource should reference an existing patient. But it probably doesn't exist yet on the server and goes within the same bundle.
For this purpose there is a way to have a temporary id in the bundle, to let resources reference ids.