Node graph architecture is a software design structured around the notion of a node graph. Both the source code and the user interface are designed around the editing and composition (or linking) of atomic functional units. Node graphs are a type of visual programming language. The source code for the software application is organized into atomic functional units called nodes. This is typically done using classes derived from a base class for all nodes. Each node can have inputs and outputs, which are typically also implemented using classes derived from base classes for all inputs and all outputs. Outputs and inputs can refer to each other, typically by holding pointers to instances of other outputs or inputs. When a node executes its functionality, it retrieves its inputs by following the pointers stored in its inputs to retrieve data output by other nodes. The node then executes its operation on these inputs to produce its own outputs. The ability to link nodes together in this way allows complex tasks or problems to be broken down into atomic nodal units that are easier to understand. The user interface of the software application will often visually display the node graph to the user. Nodes are often drawn as rectangles, and connections between nodes are drawn with lines or splines. The use of node graph architecture started in the 1960s. Today the use of node graphs has exploded. The fields of graphics, games, and machine learning are the main adopters of this software design with the majority of tools using node graph architecture. To this day, there is some debate as to the benefits of visual programming and node graph architecture. Advocates highlight how the abstraction that node graphs provide makes the tool easier to use. Critics highlight how visual programming is too restrictive and how they must resort to modifying source code or scripts to accomplish their tasks.
History There is an ongoing effort by Eric Hosick on X to collect snapshots of all node graph user interfaces in most software applications. The effort attempts to document the evolution and explosion of node graph user interfaces starting from their initial roots. This visual history is hosted on a blog page called Visual Programming Languages - Snapshots. Work leading to node graph architectures and visual programming seems to have started in the 1960s, in the area known as "man-machine communications". In William Robert Sutherland's MIT thesis (1966) "Online Graphical Specification of Procedures", he describes and analyses topics around a 2D pictorial language. This is one of the first investigations in dataflow-based workflows or programs. Since then his thesis has been used as "prior art" in order to quash lawsuits about dataflow ideas today. His work is often thought to have led the way to what is known as computer-aided design (CAD) today.
A pictorial program is a natural way of expressing parallel processes. The two-dimensional nature of the language helps in visualizing many things happening at once. The ease of debugging programs, particularly parallel ones, will be enhanced by a pictorial language form. Being able to attach data probes and to see a program run gives one a grasp of detail that is hard to obtain in any other way. A program's execution need not be controlled by the usual explicit sequential flow conventions. The movement of data through a program may determine its operation. A data controlled convention corresponds closely to our intuitive ideas of how a graphical program should operate and also allows parallel programming without explicit flow designations. In 1969, T. O. Ellis, J. F. Heafner, and W. L. Sibley published a paper concerning a Graphical Input Language (GRAIL). Their work was related to the RAND Tablet which began with research on Sketchpad, a system where users could write computer commands directly on a tablet, conducted by Ivan Sutherland. The GRAIL system used a flowchart-based graphical programming language and could recognize handwritten letters and gestures. Alan Kay has given a number of demos of the GRAIL system, however, he was not involved with the creation of the system.
Important organizational concepts in the GRAIL system are the sequential flow of control, the hierarchy of subroutines, and the language (flow diagrams) for pictorially relating the organization within the concepts of the first two. The sequential nature of control allows the man to envision isolated processes that are adapted to specific functions--which, in turn, allow the organizer to think of the total program in terms of manageable subparts. The subroutine hierarchy emphasizes the notion of isolated processes even more strongly. Flow diagrams help the man to picture his control options and the relationship between processes by expressing these interrelationships in two dimensions.
Some of the more recent uses of node graph architectures started around 2005. Node graphs in this time frame start to develop paradigms to deal with complexity in the node graph. The complexity arose as the number of nodes and links in the graph increased. One of the main ideas dealing with complexity was the concept of a group or package node which hid nodes inside of itself, only exposing the inputs and outputs of the group.
Katana, Foundry Houdini, SideFX Nuke, Foundry Mari, Foundry Maya, Autodesk Blender (software) § Geometry nodes — Official Documentation Grasshopper, McNeel & Associates Dynamo, Autodesk
Abstraction and complexity In the paper Hierarchical Small Worlds in Software Architecture author Sergi Valverde argues that most large software systems are built in a modular and hierarchical fashion, and that node graphs can be used to analyze large software systems. Many other software analysis papers often use node graphs to analyze large software systems suggesting that node graphs are good models of the internal structure and operation of the software.
Visual programming debate Node graphs are a subset of the broader class of visual programming languages. Node graphs allow you to design programs in a visual and structured way instead of through the authoring of source code. In the film and video game industries node graphs are synonymous with visual programming. There is currently some debate on the power, abstraction, and need of node graphs and visual programming languages.
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