Use conditional update (PUT with If-None-Match or search parameters) to create-or-update a FHIR resource without prior knowledge of its server id

In-depth guide

Steps

1. Identify a business identifier or set of search parameters that uniquely identifies the resource (e.g., Patient?identifier=system|value)
2. Send PUT [base]/[ResourceType]?[search parameters] with the full resource body; do not include an id in the URL path — include the business identifier in the resource body
3. If the server finds exactly one matching resource it performs an update (200 or 204); if it finds zero matching resources it creates a new one and returns 201 Created
4. Inspect the response Location header on 201 to obtain the server-assigned logical id and record it for future direct access
5. If the server finds more than one matching resource it returns 412 Precondition Failed or 422; refine the search parameters to ensure uniqueness
6. Use the ETag from a prior read in If-Match for concurrent update safety if the resource may have been updated since last read