Web services for manufacturing
Last month, I recommended using loosely coupled and asynchronous interfaces for systems that must be robust in the face of failures, especially the temporary loss of communications or temporary unavailability of systems. Fortunately, there is a new IT technology that meets these needs. Web services are a new class of APIs (application program interface) standards that are rapidly becoming the p...
Last month, I recommended using loosely coupled and asynchronous interfaces for systems that must be robust in the face of failures, especially the temporary loss of communications or temporary unavailability of systems. Fortunately, there is a new IT technology that meets these needs. Web services are a new class of APIs (application program interface) standards that are rapidly becoming the preferred interfaces among IT systems. In manufacturing they complement OPC interfaces used for high-speed and synchronous communications. Web services provide a good alternative when high speed is not required or when asynchronous applications are used. Usually, a new IT technology takes years to become mature enough for manufacturing applications. Control engineers are, in general, reluctant to apply new technologies in critical systems if they came from other fields. Web services are different and are already suitable for use in manufacturing applications.
An interface standard based on the HTTP protocol used in the Internet, Web services take the concept of a request-response transaction for a Web page and apply it to application-to-application transactions. The most common implementations of Web services use XML (eXtensible Markup Language) to send and receive data because XML provides operating system and application-independent data representations. Web services also use SOAP (simple object application protocol) to send XML requests and receive XML responses. SOAP provides a standard format for messages, essentially defining the envelope that carries XML messages.
SOAP provides a standard format for sender and receiver information, which is the same as a delivery address and return address on a paper envelope. SOAP and XML are then combined with two other standards, WDSL and UDDI (universal description, discovery, and integration), for a full Web service implementation. WSDL (Web services description language) is the format for an XML document that describes a Web service. Each WSDL document contains an English definition of the service and all of the input and output data formats required to use the service. The final element of Web services is the UDDI standard. UDDI is used to define XML-based registries for Web services. These registries can be considered the electronic equivalent of the Yellow Pages. UDDI standard defines how an application publishes, finds, and binds to a Web service. UDDI uses WDSL as the means to document the Web service and uses SOAP as the way to access the UDDI directory.
Web services also are often combined with message-oriented middleware (MOM). MOM provides a buffering mechanism for SOAP messages, ensuring all messages are delivered, even if a temporary loss of communications occurs or if the receiving application is temporarily unavailable. Buffering also ensures that a message is received only once, even if it needs to be held until the receiving application is ready.
This may seem like a lot of technology for simple integration problems, but two things make it easy. First, Web service standards are already built into most software development tools. The tools will automatically generate code to send and receive XML files, generate SOAP interfaces, create WSDL documents, and publish the service to UDDI registries. Commercial off-the-shelf software is also available for message middleware, such as Microsoft's BizTalk service.
Web services also simplify the solution because manufacturing applications are usually simpler than pure business applications. Business applications also need security for messages and transactional control across multiple interactions, and much of the ongoing work in Web services is the development of standards in these areas. Manufacturing application security isn't as complicated an issue because most services involve single message exchanges, and rollback of transactions is not necessary or possible. This means that Web services are now a good technology for control system integration. They provide vendor-neutral and network-neutral methods for integrating systems and should be on your short list for integration technologies.
Dennis Brandl, firstname.lastname@example.org , is the president of BR&L Consulting, a consulting firm focusing on manufacturing IT solutions, based in Cary, NC.
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