Modern enterprise applications have evolved over the years to become more than just functional software systems. They are now complex, distributed interconnected ecosystems that need to be resilient, scalable, and adaptable to changing business needs and consumer expectations. Jakarta EE, as we saw in the previous chapters, is the open source evolution of Java EE (and J2EE before that) that provides a rich set of specifications and APIs that not only simplify the creation of enterprise-grade applications but also promote architectural best practices. Two key architectural concepts that have proven invaluable in the development of such complex modern systems are Domain-Driven Design (DDD) and layered architecture.