Enterprises are demanding integration tools that enable more seamless interoperability across diverse integration-ware paradigms: old, current, and emerging. The industry has coined a new three-letter acronym - enterprise service bus (ESB) - that speaks to the dream of standards-based integration of legacy middleware with the new world of Web services.

ESB denotes an emerging segment of the middleware market. ESB middleware vendors include Cape Clear Software, Fiorano Software, IBM, IONA, Sonic Software, Systinet, TIBCO Software, and webMethods. ESB products bring together the legacy world of vendor-proprietary, message-oriented middleware (MOM) protocols with the growing "WS-*" stack of vendor-independent Web services standards. Fundamentally, ESB is "MOM++."

ESB products support reliable, guaranteed messaging, which has traditionally been the core MOM functionality. They leverage Web services standards and interface with established reliable-messaging MOM protocols such as IBM WebSphere MQ, TIBCO Rendezvous, and Sonic Software's SonicMQ. Common features of ESB products include the ability to bridge heterogeneous MOMs, wrap MOM protocols with Web Services Description Language (WSDL) interfaces, and tunnel Simple Object Access Protocol (SOAP) traffic over MOM transports. In addition, most ESB products support direct, peer-to-peer interactions among distributed applications, in addition to or in lieu of hub-and-spoke interactions through intermediaries such as integration brokers.

The evolution of MOMs won't be complete until the Web services stack includes ubiquitous standards for reliable messaging, event notification, and pub/sub. The Web services stack still lacks finalized, profiled, widely adopted standards in all of these areas, although it has strong contenders in Web Services Reliable Messaging (WS-RM), WS-Eventing, and WS-Notification (though it appears increasingly likely that the industry will produce a convergence draft that incorporates the overlapping functionality of WS-Eventing and WS-Notification). Some ESB vendors - most notably, Systinet - have already implemented these specifications in their products but most still have the specifications on their futures roadmaps.

ESB vendors are seriously committed to implementing Web services reliability standards - such as WS-RM - as they emerge. One fortunate consequence of this trend will be continuing reductions in vendor opportunities to lock customers in through proprietary specifications. Already, vendor-proprietary MOM application programming interfaces (APIs) have taken a backseat to the common Java API - Java Message Service (JMS) - and all ESB vendors are supporting further abstraction through the ability to wrap proprietary MOMs as Web services via WSDL and SOAP.

By the end of this decade, Web services-based MOM functionality will become the dominant ESB approach for reliable messaging. We call this approach "MOM abstraction": the ability to use Web services standards and specifications to provide MOM-grade reliability services in lieu of, or to bridge between, pre-Web services MOM environments. Increasingly, Web services will have native reliability features through WS-RM and other SOAP extensions. As Web services-based MOM abstraction takes hold in the market, enterprises will gradually de-emphasize traditional MOMs, slowly pushing them out of the integration picture.

However, until WS-RM and other Web services reliability protocols are widely adopted, enterprises will continue to rely on traditional MOMs and other established middleware approaches to provide the end-to-end reliable messaging that the Web services stack (in its current incomplete state) can't yet support. We expect that, over the next 1-2 years, consensus, profiled Web services standards will emerge for reliable messaging, event notification, pub/sub, and other robust functionality. It will take 2-3 more years before these are implemented widely, profiled by the Web Services Interoperability Organization (WS-I), and tested for interoperability across multivendor environments.

Once WS-RM, WS-Notification, and other key Web services reliability standards are universally implemented, the need for vendor-proprietary ESB protocol stacks on nodes will start to wither away. Vendors such as IBM, Microsoft, and BEA embed ESB functionality in their application servers. Increasingly, enterprises will access ESB functionality within every Web services application platform that they buy, thereby taking advantage of those platforms' native support for peer-to-peer, Web services-based reliable messaging. As that trend intensifies, ESB pure-play vendors such as Sonic, TIBCO, and Fiorano will find themselves under increasing pressure to distinguish themselves in the crowded middleware market.