Which design pattern is commonly used for integrating legacy systems with modern applications?

The Adapter pattern is particularly effective for integrating legacy systems with modern applications because it acts as a middle layer that allows two incompatible interfaces to work together. When dealing with legacy systems, which often have outdated interfaces or protocols, the Adapter pattern enables modern applications to communicate seamlessly with these older systems without needing to alter their internal implementations.

By employing the Adapter pattern, developers can create a new interface that translates calls from the modern application into the format that the legacy system understands. This not only preserves the functionality of the legacy system but also ensures that the modern application can utilize its capabilities effectively. This results in improved flexibility and maintainability, allowing for smoother transitions during system integrations or upgrades.

In contrast, the other design patterns mentioned serve different purposes. The Facade pattern simplifies interactions with a complex subsystem but does not specifically address the integration challenges posed by legacy systems. The Bridge pattern is focused on decoupling an abstraction from its implementation, facilitating more flexible variations of those entities. The Composite pattern, on the other hand, deals primarily with tree-like structures of objects, allowing clients to treat individual objects and compositions uniformly; it does not inherently address integrating disparate systems. Thus, the Adapter pattern stands out as the most relevant choice for this scenario.