Compare Xi / OPC .NET3.0 to other OPC Specifications

The Express Interface was developed by a group of companies to provide a .NET-based migration path from COM-based systems to Microsoft .NET and Windows Communication Foundation (WCF). The OPC Foundation adopted Xi as part of the OPC Foundation umbrella because many OPC community requested a web services based standard with the same functionality as the COM based OPC specifications.

The following table compares the design goals and main features of the Xi and OPC specifications.

The DA/HDA/AE specifications base on DCOM communication. DCOM was the state of the art Microsoft communication platform when the OPC specifications were created. Microsoft moved from DCOM to web services but DCOM is still supported in all Windows operating systems. DCOM is efficient for local server access but it's difficult to configure for remote server access and problematic for communication through firewalls.
The OPC specification define DCOM interfaces with methods and data structures.

The XML DA specification was created in 2003 as a web service SOAP messages specification with the functionality of OPC DA (Data Access). The XML DA specification hasn't been widely accepted, mostly because of performance and security concerns.
Since the publication of Xi there is interestingly an increase in the use of XML DA. Developers appear to realize that XML DA can be implemented on WCF instead of .NET2 web services and take advantage of the wide range of communication and security options offered by WCF. However, security is not enforced and performance optimized as in Xi.
The development of XML DA clients is real simple. The communication is state-less, a client can e.g. use nothing but the Read method.
More information can be found at http://advosol.com/c-2-opc-xml-webservices.aspx.

The UA specification addressed the XML DA performance and security deficiencies with a UA specific communication stack that is suitable for implementation on Windows and non-Windows platforms. UA has the combined functionality of DA (Data Access), HDA (Historical Data Access), A&E (Alarms and Events) and a complex network data structure that allows UA servers to model a wide range of applications.
The specification defines the messages that can be exchanged between client and server.

The Xi specification is based on the .NET WCF (Windows Communication Foundation) for web services based communication and is designed to provide a .NET interface that is easy to wrap to and from OPC DA (Data Access), OPC HDA (Historical Data), OPC A&E (Alarms & Events).
WCF implements multiple web services standards to provide the performance and security required for different applications. The communication can be configured for different protocols (http, tcp, pipes) and different security levels for authentication and encryption.
The specification defines WCF contracts. These are .NET interface definitions that are implemented in the server. WCF provides the tools to create client proxy classes and a WSDL document describing the interface as a web service.

Xi / UA Structure Comparison

Xi bases on WCF and uses the communication options and security levels provided by WCF. UA is designed for multi-platform use and cannot depend on features of a particular operating system. UA needs to implement a communication stack down to the TCP/IP or HTTP level, that can be expected to be available on all platforms. The Xi Client library consists of the WCF proxy layer and optionally additional layers. The client application interface is not standardized, allowing vendors to provide implementation tailored for specific needs and preferences. The UA client library interfaces the UA communication stack and provides a common interface for the different modes of communication. The client application interface is not standardized. The Xi Server Library implements the WCF contracts. A server base layer implementation is available to the public together with the specification. The UA server library is provided by the OPC Foundation to member companies in form of the UA SDK.

Xi on non-Windows platforms

The Xi specification is focused on the Windows platform and based on WCF, the Windows Communication Foundation.
WCF is a superset of the web services specifications and the basicHttp WCF communication binding complies with standard web services.
Xi clients or servers can be implemented on any platform that supports standard web services, but the communication is restricted to HTTP/SOAP.
Client libraries and server toolkits are available for the Windows platform but may not be for a particular non-Windows platform. In this case the client or server has to be developed on the web services interface. This requires a significantly higher development effort.

Xi / UA Data Structure comparison

The Xi specification is designed for the combined functionality of the classic OPC DA, HDA, A&E specifications and uses the same structure as the classic OPC servers. Objects are organized in a hierarchical structure and Xi offers browsing with starting points anywhere in the hierarchy and with strong filter capabilities. DA, HDA and A&E instances can be linked to the same object.	UA servers organize the data objects in a network structure. Each object can reference other objects to define relations. Complex object structures can be modeled. Views can be defined with a subset to simplify the structure for particular clients.