This example visualizes a hierarchical network of servers and their applications using a force-directed graph. It shows four main server nodes, each connected to child application nodes, with some child nodes having leaf nodes attached. The visualization demonstrates bidirectional links between paired servers and uses node size and color to distinguish hierarchy levels. The code implements this using D3 v4's `forceSimulation`, `forceLink`, `forceManyBody`, and `forceCenter` APIs, with drag interaction handling and SVG rendering of circles, lines, and text labels.