One of the defining characteristics of Unix is modularity - the operating system shell provides a set of small and flexible utilities (such as grep, sed and awk) that can be \u201cglued\u201d together to perform more complex tasks.Most open-source applications are also composed of modular components that connect through well-defined interfaces. The Linux kernel, the Apache Web server and the Gnome Desktop are all examples of highly modular design.A modular approach is well suited to the open-source model of software development because it allows a highly distributed group of volunteers to independently improve each component while maintaining the overall functionality of the application. For end users, the modular design of open-source applications means that they can be customized - cut down to the essentials, simplifying maintenance and improving security by removing unused modules.Web services, which are lightweight application interfaces based on XML and other open protocols, allow application components to be \u201cexposed\u201d as fine-grained machine-readable interfaces. Web services can be used to create libraries of modular software components that can be \u201cbundled\u201d as new applications or to tightly integrate applications.The open-source software development model has a lot to gain from Web services. Application modules can interface \u201cinternally\u201d within the application and \u201cexternally\u201d with other applications using the same protocols. This will allow open-source developers to further accelerate the software development cycle, adding features and re-using components while reducing the number of bugs introduced by re-engineering for integration. Also, Web services-enabled applications can benefit from the open-source model, which emphasizes open standards and distributed innovation.For data center managers Web services and open source are two mutually reinforcing trends to watch. In building data centers based on service-oriented architectures (SOA), IT executives should pay attention to companies that combine open source and Web services in their strategic plans and software portfolio. Leading the pack are IBM and Novell.IBM has been an early pioneer in both areas. IBM\u2019s on-demand strategy is based on SOAs, with the WebSphere, DB2, Lotus and Tivoli software platforms designed with Web services at their core. Furthermore, IBM distinguished itself as one of the first large vendors to appreciate the potential of Linux and the open-source model by reportedly investing $1 billion in Linux-related activities. As Web services and open source start reinforcing each other, IBM stands to gain from both trends.Novell has become a powerful player in the open-source world. It has acquired SuSE Linux, an enterprise-level Linux distribution; and Ximian, a contributor to the Gnome Linux desktop and the developer of Mono, which allows Linux to run .Net applications. Meanwhile, Novell\u2019s Web services platform, Extend 5, won the LinuxWorld award for best server application. Novell\u2019s strategy is clearly moving into the nexus of open source and Web services.In building a next-generation data center, IT executives can combine the best elements of open-source software and SOAs based on Web services to deliver flexible, open and powerful application services to the enterprise. It\u2019s a match made in heaven.