j. Composability Composability is defined for XMSF as the ability to select and combine components in various combinations to create new functionality which satisfies specific user requirements across a variety Automated,tool-based support is a composability gdi n and implementation, and during runtime of application domains. This applies both during des Run-time composition of new components and existing components is a long-running area of research that finally appears to be ready for widespread practical application. Both backwards compatibility(for legacy applications)and forwards compatibility (with as-yet unknown applications)can be enabled through composable software. A Defense Modeling and Simulation Office ( DMsO) Workshop on Software Components held July 2002 explored these topics in some detail, with further work to follow [DMSO 2002] It is interesting to consider that the platform-independent techniques used by Web Services can significantly reduce the number of software components which need to be directly composable Exposing object-method functionality via XML-based remote procedure calls(e.g. XML- RPC, SOAP)can provide lightweight client-side access to heavyweight server-side capabilities XMSF Workshop Symposium Report, October 2002 page 17
XMSF Workshop & Symposium Report, October 2002 page 17 j. Composability Composability is defined for XMSF as the ability to select and combine components in various combinations to create new functionality which satisfies specific user requirements across a variety of application domains. This applies both during design and implementation, and during runtime. Automated, tool-based support is a composability goal. Run-time composition of new components and existing components is a long-running area of research that finally appears to be ready for widespread practical application. Both backwards compatibility (for legacy applications) and forwards compatibility (with as-yet unknown applications) can be enabled through composable software. A Defense Modeling and Simulation Office (DMSO) Workshop on Software Components held July 2002 explored these topics in some detail, with further work to follow [DMSO 2002]. It is interesting to consider that the platform-independent techniques used by Web Services can significantly reduce the number of software components which need to be directly composable. Exposing object-method functionality via XML-based remote procedure calls (e.g. XML- RPC, SOAP) can provide lightweight client-side access to heavyweight server-side capabilities
3.3 Web services overview Web Services has been an active area of work for several years. While there is no fixed definition or locked-down architecture, certain capabilities appear to be common. A summary table follows which presents a possible XMSF Stack for Web Services, adapted from [Cerami 2002 Table 3-1. Multiple layers of Functionality, Composed to Provide Accessible web services. Repositories Administrative Locations for providing approved Exemplars: DoD XML Registry, XML. Gov (or ad hoc)Web Services with http://diides.ncrdisamil/xmlreg/user/index.cfm ntegrated registry services http:/xml.gov/efforts.htmI Services discovery UDDL, LDAP Centralized access via repositories Universal Description, Discovery and Integration, is made accessible to web-based Lightweight Directory Access Protocol applications via service publish and Oasis:http://www.uddi.org search capabilities IeTf:http://www.ietforg/rfc/rfc2251.txt Services Description WSDL Describe detailed methods and Web Services Description Language parameter signatures of each service W3c:http://www.w3.org/2002/ws XML Messaging XMI-RPC. SOAP. XMLP Express messages in common XML Remote procedure Calls. SOAP. XML Protocol formats for simple encoding and http://www.xmlrpc.orghttp://www.w3.org/2000/xp/group Service Transport Http, Smtp, Ftp, Beep Transporting messages between applications. Typically requires Hypertext Transfer Protocol, Simple Mail Transfer Protocol reliable(i.e. guaranteed) deliver File Transfer Protocol, Blocks Extensible Exchange Protocol Rob Glidden's presentation on the Seven Successful Habits of Web Services sparked several interesting discussions and is summarized below. Further details appear in his Symposium slideset Enablement of developer community. Supporting developers drives overall success Services are not equal to applications. Rethinking and restructuring the architecture Incrementalism: results are greater than effort. True and measurable benefits occur Federation: accept political uncertainties and interests. Many sizes fit all, not one size Assembly combining. Assembling applications becomes combining web services Virtualization distillation. Exposing functionality requires distilling key capabilities System stability: equilibrium or dynamic change. Adaptable systems a An important further resource is the W3C Workshop on Web Services, held 11-12 April 2001 in SanjOseCaliforniaOnlineathttp://www.w3org/2001/01/wsws XMSF Workshop Symposium Report, October 2002 page 18
XMSF Workshop & Symposium Report, October 2002 page 18 3.3 Web Services Overview Web Services has been an active area of work for several years. While there is no fixed definition or locked-down architecture, certain capabilities appear to be common. A summary table follows which presents a possible XMSF Stack for Web Services, adapted from [Cerami 2002]. Table 3-1. Multiple Layers of Functionality, Composed to Provide Accessible Web Services. Repositories Locations for providing approved (or ad hoc) Web Services with integrated registry services. Administrative Exemplars: DoD XML Registry, XML.Gov http://diides.ncr.disa.mil/xmlreg/user/index.cfm http://xml.gov/efforts.htm Services Discovery Centralized access via repositories is made accessible to web-based applications via service publish and search capabilities UDDI, LDAP Universal Description, Discovery and Integration, Lightweight Directory Access Protocol OASIS: http://www.uddi.org IETF: http://www.ietf.org/rfc/rfc2251.txt Services Description Describe detailed methods and parameter signatures of each service WSDL Web Services Description Language W3C: http://www.w3.org/2002/ws XML Messaging Express messages in common XML formats for simple encoding and decoding XML-RPC, SOAP, XMLP Remote Procedure Calls, SOAP, XML Protocol http://www.xmlrpc.org , http://www.w3.org/2000/xp/Group Service Transport Transporting messages between applications. Typically requires reliable (i.e. guaranteed) delivery. HTTP, SMTP, FTP, BEEP Hypertext Transfer Protocol, Simple Mail Transfer Protocol, File Transfer Protocol, Blocks Extensible Exchange Protocol Rob Glidden’s presentation on the Seven Successful Habits of Web Services sparked several interesting discussions and is summarized below. Further details appear in his Symposium slideset. ß Enablement of developer community. Supporting developers drives overall success. ß Services are not equal to applications. Rethinking and restructuring the architecture. ß Incrementalism: results are greater than effort. True and measurable benefits occur. ß Federation: accept political uncertainties and interests. Many sizes fit all, not one size. ß Assembly ‡ combining. Assembling applications becomes combining web services. ß Virtualization ‡ distillation. Exposing functionality requires distilling key capabilities. ß System stability: equilibrium or dynamic change. Adaptable systems are most robust. An important further resource is the W3C Workshop on Web Services, held 11-12 April 2001 in San Jose California. Online at http://www.w3.org/2001/01/WSWS