In Smile CDR, a typical FHIR-based Clinical Data Repository has two main parts, each of which is defined in its own module through the Smile CDR module configuration.
A FHIR Storage module provides database storage and retrieval. It interfaces with the backing database and/or filesystem to store resources, update them, index them, find them, retrieve them, etc.
A FHIR REST Endpoint module is an HTTP server (i.e. a web server) that allows external clients to interact with the FHIR Storage module.
The FHIR REST Endpoint listens on a given port for incoming FHIR requests, and routes them to the FHIR Storage module.
Both modules can be clustered in an N-node cluster arrangement as needed. There is no limit to how wide a cluster can be designed (i.e. how many nodes it can have), and a single node deployment can be thought of as a cluster with only one node.
Two basic configurations are illustrated in the following diagram: