# Holoscan Core Concepts

An Application is composed of Fragments, each of which runs a graph of Operators. The implementation of that graph is sometimes referred to as a pipeline, or workflow, which can be visualized below:

Fig. 3 Core concepts: Application

Fig. 4 Core concepts: Port

The core concepts of the Holoscan API are:

• Application: An application acquires and processes streaming data. An application is a collection of fragments where each fragment can be allocated to execute on a physical node of a Holoscan cluster.

• Fragment: A fragment is a building block of the Application. It is a Directed Acyclic Graph (DAG) of operators. A fragment can be assigned to a physical node of a Holoscan cluster during execution. The run-time execution manages communication across fragments. In a Fragment, Operators (Graph Nodes) are connected to each other by flows (Graph Edges).

• Operator: An operator is the most basic unit of work in this framework. An Operator receives streaming data at an input port, processes it, and publishes it to one of its output ports. A Codelet in GXF would be replaced by an Operator in the Holoscan SDK. An Operator encapsulates Receivers and Transmitters of a GXF Entity as Input/Output Ports of the Operator.

• (Operator) Resource: Resources such as system memory or a GPU memory pool that an Operator needs to perform its job. Resources are allocated during the initialization phase of the application. This matches the semantics of GXF’s Memory Allocator or any other components derived from the Component class in GXF.

• Condition: A condition is a predicate that can be evaluated at runtime to determine if an operator should execute. This matches the semantics of GXF’s Scheduling Term.

• Port: An interaction point between two operators. Operators ingest data at Input ports and publish data at Output ports. Receiver, Transmitter, and MessageRouter in GXF would be replaced with the concept of Input/Output Port of the Operator and the Flow (Edge) of the Application Workflow (DAG) in the Framework.

• Message: A generic data object used by operators to communicate information.

• Executor: An Executor that manages the execution of a Fragment on a physical node. The framework provides a default Executor that uses a GXF Scheduler to execute an Application.