Free dcom software for linux

PyOPC is a Python library implementing the OPC XML Data Access Protocol Version 1.01 (OPC XML-DA 1.01).

PyOPC supports the easy creation of XMLDA-compliant clients and servers without having to deal with the SOAP protocol and server issues.

The Open Linking and Embedding for Process Control (OPC) consortium released several open standards, which address interfaces for vertical integration in industrial automation.

Historically, OPC used the Distributed Component Object Model (DCOM) for the underlying communication technology. DCOM has the disadvantage of being platform specific: it is only available for Microsoft Windows based systems. Other platforms, such as Linux, can therefore not retrieve fieldbus data from DCOM based servers.

Another disadvantage of DCOM is that it can not easily bypass firewalls, hence access will often be limited to certain segments of a corporate network.
In the last years, a new technology, called SOAP Web services, emerged. [LIV02] defines a Web Service as: ``a method or function that is available for other applications to access over the Internet.. Web services enable Remote Procedure Calls (RPC) and have the following key features:

High level of interoperability:

Web services technologies are all based on strictly defined open standards1.

High networking abilities:

As an underlying communication protocol, Web services utilize Internet protocols such as the Hyper Text Transfer Protocol (HTTP) or the Simple Mail Transfer Protocol (SMTP). These protocols have high networking abilities and may moreover penetrate firewalls.

Protocol legible by humans:

The Simple Object Access Protocol (SOAP)2 is based on the Extended Markup Language (XML), which is legible to humans. This way, testing and debugging of Web services is far easier than with binary protocols.

Documentation:

Another underlying technology of SOAP is the ``Web Service Definition Language (WSDL) which may be used to define the service, especially by constraining the format of the SOAP protocol. WSDL utilizes the XML Schema language for defining these SOAP messages3. These WSDL documents can be utilized by frameworks to generate stubs that provide a base for accessing a Web service.

Validation:

WSDL in combination with a validating XML parser enable the validation of SOAP messages. This way, custom code will never receive syntactically or semantically erroneous data, which should improve the stability of the service.

SOAP Web services are seen as a successor to several alternative technologies such as DCOM and are already broadly accepted by the industry. More information about the SOAP protocol can be found in [SEE02] and [LIV02].

The OPC consortium reacted on this technological evolution by adopting SOAP Web services for their standards. One recent addition of OPC is the "XML Data Access Version 1.0" (XML-DA 1.0) standard. This standard deals with access of underlying fieldbus technologies and covers the following aspects:

Information model:

The specification provides a simple information model, based on ``OPC Items which represent a piece of information, similar to fieldbus data points. These items can be arranged hierarchically.

Data types:

OPC XML-DA adopts several XML-Schema based data types, such as integer, float, date/time specific types. Moreover it defines arrays which are based on these basic types.

Operations:

The standard specifies 8 operations such as reading/writing and browsing which can be used to access the underlying fieldbus.

Subscription:

The specification further introduces a mechanism to retrieve only changed items, called ``Subscription. Clients may thus subscribe to items and use a dedicated polling operation to retrieve changed data.

This project attempts to provide a working implementation of a few multimedia-related utilities for x86 Linux. At the time of its creation in May 2000 its primary goal was creation of video capture and recompression applications that would work with most popular AVI file format and newest data compression methods ( Indeo Video and variations of MPEG-4 for image compression and MPEG Layer-3/Windows Media Audio for sound ).

The core idea of the project was in using Win32 dynamic-link libraries in Linux environment. However, it has become most popular because of its side product - an AVI movie player that could play DivX ;-) movies in Linux with reasonable performance and stability. Since then most of work was done in this direction.

Now it supports a wide range of codecs ( compressors/decompressors ) such as DivX ;-), Indeo Video, I263, and others, able to show subtitles and perform video output using YUV overlays when necessary support from hardware and operating system is available.

It is also able to play most files in ASF format, and current development CVS code is capable of streaming ASF media over HTTP. The project is not intended to provide universal media framework for Linux, nor to do much more than what it currently does. Moreover, its mostly a proof-of-concept work. It was the first project that introduced the idea of using Windows DLLs and very limited ( ~50 Kbytes of code ) subset of Win32 API for audio/video (de)compression in *nix environments, the idea which is already reused in several other - more general - software projects.

It is the first project that extends that idea to the usage of DirectShow audio/video decoders for the same purpose ( by emulating DirectShow/DCOM environment from decoder point of view ). It is the only project that includes a player for files in Advanced Streaming Format. All these features are essential for the complete modern multimedia environment, and hopefully will become available in near future in such perspective architectures as Arts ( KDE ) or GStreamer ( Gnome ).

Aside from reusing the ideas, the project is used as is in a few media players for Linux, such as XMMS, XTheater or LAMP. The code of this project is distributed under General Public License version 2. Basically it means that you may do anything you want with this code, but if you want to redistribute it or any its derivatives, you have to do it under GPL and you have to make the source code available. For more details visit the site of Free Software Foundation.

Legal issues covering Win32 DLLs which accompany source code are a bit more complicated. These DLLs are freely available in the Internet ( exact URLs to most of them are available on this site ). For those DLLs which come with the license, their copyright owners allow using them at no cost if you do not disassemble, reverse-engineer them, etc. In some cases ( Indeo Video ) they explicitly allow to include these files into other projects under mentioned restrictions. Many DLLs are available without having to accept any license agreement at all ( DivX ;-), all DirectShow codecs ), what obviously means that any kind of activity with them is acceptable.

There is no warranty about the quality of this project. It is written mostly by one former university student with background in the area of Applied Physics in his spare time. I cannot even guarantee that it compiles properly on your system, because I dont have resources to test it on all existing distributions of Linux and flavors of Unix. I try to resolve the issues that Im informed about.