What is the three-tier architecture of a modern EHR system?

A modern EHR comprises a presentation layer (the user interface for clinicians — desktop, web, mobile), an application layer (business logic including clinical rules, order management, and workflow engines), and a data layer (the clinical data repository, often underpinned by relational or document-based databases). Integration engines connect the EHR to external systems using standards like HL7 FHIR.