Enterprise service bus is the most promising new middleware approach. ESB generally refers to integration software that supports simple, expedited, loosely coupled, standards-based, service-oriented integration. It also refers to a segment of the middleware market that converges the best features of message-oriented middleware, integration brokers and Web services.The ESB market is heating up but possibly also melting down. Recently we've seen a leading ESB vendor - Sonic Software (an operating unit of Progress Software) - merge with service-oriented architecture (SOA) governance vendor Actional. ESB environments need the policy-driven management provided by robust IT governance tools.However, Sonic-Actional won't be able to compete for long against the SOA platform vendors - especially BEA, IBM, Microsoft and Oracle - that are adding ESB and IT governance functionality to their suites at a rapid clip. When the ESB market matures by the end of this decade, pure-play ESB vendors will find their value proposition usurped by platform vendors that have embedded ESB functionality into their offerings.Many SOA platform vendors are embedding ESB functionality more deeply into their environments. They do so in order to address a broader range of integration scenarios, support their own integration software products and position their platforms as alternatives to third-party integration software. What, for example, is Microsoft's Windows Communication Foundation (WCF) if not an attempt to push ESB functionality more deeply into Windows?In the next two to three years, the ESB wave may give some platform vendors an advantage over their direct competitors. When Microsoft delivers commercial WCF and Windows Workflow Foundation (WWF) functionality in Vista and Longhorn, the company will be able to position its server and client platforms as ESB-enabled out of the box. Microsoft also has committed to running WCF on pre-Longhorn Windows platforms - Windows XP and Windows Server 2003 - which will further strengthen its ESB and SOA story.Over the next several years, platform vendors that fail to address ESB functionality in their road maps will marginalize themselves out of the SOA market. Minor platform vendors won't be able to survive in a market that eventually will be dominated by SOA platforms. At the very least, all platform vendors will need to implement WS Reliable Messaging in their architectures in order to enable reliable, guaranteed, once-only delivery of Simple Object Access Protocol (SOAP) messages over Web services environments.By the end of this decade, ESB functionality will be common-denominator functionality implemented on all platforms, leveraging the industry's common denominator interoperability stack: the WS-* stack. As ESB functionality becomes ubiquitous in application platforms, pure-play ESB middleware vendors will find the going tough. Today's ESB middleware market segment will fade away, absorbed into the SOA platforms that will dominate all distributed environments. In order for these SOA platform vendors to distinguish their commoditized ESB features, they'll have to keep evolving up the functionality stack, adding Web services management (WSM), dynamic content-based routing, distributed transactions and other advanced features.As regards WSM functionality, there's little of that in today's ESB market - that's why the merger of Sonic (the ESB pioneer) and Actional (one of the WSM pioneers) is so significant. ESB vendors will layer WSM functionality on their product architectures in the coming years. It's likely that other WSM pioneers, such as AmberPoint, will find ESB suitors. Another likely development is for network appliance vendors - such as Cast Iron Systems, Cisco Systems, F5 Networks and Solace Systems - to reposition their products as high-performance ESB message processing nodes.