Prescribing system acts as the FHIR server, with a persisted copy Shared Record architecture
(1) The prescribing system sends an event notification to the pharmacy system when a new MedicationRequest is ready for processing. The notification must include the external externally referencable identifer. The prescirbing system assigns the logical id.
(2) The prescribing system GETs the MedicationRequest using the identifer. The GET request may be sent at any time so the pharmacy system may choose to expose status values that represent both in-progress and end-states for the MedicationRequest record.
(3) The dispending system creates the MedicationDispense with a valid externally referencable identifer and POSTs to the presribing system which adds the logical id on receipt. The MedicationDispense is only shared once ready to share with the pharmacy system so the status will only deal with valid end-states.
(4) The prescribing system POSTs records to the shared record system. These are likely to be as MedicationStatements, which could reference the source data from MedicationRequest and MedicationDispense records. Alternatively MedicationRequest and MedicationDispense records could be shared which the shared record system amalgamates and exposes as MedicationStatements for other consuming systems.