Legacy operational systems struggle to keep up with modern business needs, but rip-and-replace is often too costly. A proper integration architecture centered on business interfaces provides a way forward. Middleware such as APIs, event brokers, and integration platform as a service (iPaaS) are key tools in building an architecture that creates efficient ways to modernize legacy systems. This report gives tech leaders best practices and a variety of concrete integration patterns to breathe new life into legacy operational systems.