Ghasi e-Prescribing Gateway Service — Domain Model

Status: populated Owner: TBD Last updated: 2026-04-18 Companion: Service Template · 03 platform-services · 02 DDD

Aggregates

PrescriptionRecord (Root Aggregate)

Represents the gateway's authoritative interop copy of a MedicationRequest. The EHR is the clinical author; the gateway mediates and persists the interop copy.

Invariants:

• tenantId is extracted from JWT — never client-supplied.
• prescriberId must be a valid EHR actor (enforced by ehr-backend persona scope).
• Status transitions strictly follow the FHIR MedicationRequest state machine.
• ETag is issued on create and required on update (If-Match); stale writes return 412.
• Idempotent create: same Idempotency-Key + payload fingerprint → same resource ID, no duplicate.
• A MedicationDispense cannot reference a prescriptionId unknown to this tenant (BR-RX-001).

FHIR resource: MedicationRequest

Status state machine:

---

DispenseRecord (Root Aggregate)

Represents the gateway's interop copy of a MedicationDispense. The Pharmacy is the author.

Invariants:

• Must reference a known PrescriptionRecord for the same tenant (BR-RX-001).
• Only pharmacy-backend persona may create.
• ETag issued; If-Match required on update.
• Partial fills allowed when tenant policy permits; quantity.value < prescribed.quantity.value.

FHIR resource: MedicationDispense

---

SubscriptionChannel (Aggregate)

Represents a registered HTTPS notification channel for a consumer (EHR or Pharmacy per tenant).

Invariants:

• Must have a valid endpointUrl (HTTPS, verified on registration).
• Signing key pair rotates per tenant policy.
• Cursor tracks last delivered event sequence; replay from cursor on reconnect.

FHIR resource: Subscription

---

WorkflowTask (Aggregate)

Represents a renewal or clarification task (P1+).

FHIR resource: Task

Invariants:

• requester is the pharmacy or patient proxy.
• owner is the prescriber queue (EHR).
• Terminal states: completed, rejected, cancelled.

---

Domain Events

Event type (NATS subject)	Version	Trigger
eprescribing.medication_request.created.v1	v1	MR successfully persisted and routed
eprescribing.medication_request.updated.v1	v1	MR status changed or revised
eprescribing.medication_request.cancelled.v1	v1	MR cancelled by EHR actor
eprescribing.medication_dispense.created.v1	v1	MD accepted by gateway
eprescribing.medication_dispense.updated.v1	v1	MD status/quantity updated
eprescribing.task.status_changed.v1	v1	Renewal/clarification task state change (P1)
eprescribing.subscription.delivery_failed.v1	v1	After retry exhaustion → DLQ
eprescribing.subscription.replayed.v1	v1	Manual replay triggered

---

Value Objects

Value object	Description
PrescriptionBusinessId	Platform-issued correlation ID spanning EHR–Gateway–Pharmacy; format prx_<ULID>
ETag	Opaque version token; format W/"<sha256-of-resource>"
IdempotencyKey	Client-supplied string + payload fingerprint hash
TenantId	Branded string Branded<string, 'TenantId'>
PrescriptionRecordId	ULID with prefix mr_
DispenseRecordId	ULID with prefix md_
SubscriptionId	ULID with prefix sub_
WorkflowTaskId	ULID with prefix wtk_
RoutingEnvelope	{ targetOrgId, targetEndpointId, region, fallbackEndpointId? }
ValidationOutcome	{ valid: boolean, issues: OperationOutcomeIssue[] }
PersonaType	enum { ehr_backend, pharmacy_backend, b2b_external }

---

Ubiquitous Language

Term	Definition
Gateway	The ghasi-eprescribing-gateway-service as a platform component
PrescriptionRecord	Gateway's authoritative interop copy of a MedicationRequest
DispenseRecord	Gateway's interop copy of a MedicationDispense
Prescription business ID	Platform-level correlation identifier (prx_) spanning all systems
ETag	Opaque concurrency token issued per resource version; required on updates
Idempotency-Key	Client-supplied header to deduplicate creates on retry
IG package	FHIR Implementation Guide package (StructureDefinition + ValueSets) pinned per tenant
Subscription	FHIR R4 Subscription: durable HTTPS notification channel
DLQ	Dead-letter queue for failed notification deliveries
Persona type	ehr_backend, pharmacy_backend, or b2b_external — dictates which resources the actor may write
Routing envelope	Resolved pharmacy target (Organization + Endpoint) for a prescription
Store-and-forward	Offline-tolerant mode: gateway accepts and queues prescriptions when pharmacy endpoint is unreachable