语义表达是不是一定要用RDF？
昨天参加了一场博士论文答辩，内容有关语义Web应用，论文架构很庞大，从基本概念、标准规范到元数据和本体的构建，到特定领域应用的实现和查询效果的比较等，感觉该童鞋很不容易。
答辩期间问了两个问题：
1、对于“语义”是如何理解的？机器如何理解语义？是不是Web加了元数据之后就是语义Web了？
2、如何对语义进行编码和查询？为什么没有试验用SAPRQLSPARQL进行查询？
第一个问题是因为论文中罗列了很多语义Web的定义而没有对语义进行定义，更没有说明语义是如何能被机器“理解”，保证机器理解的机制到底是什么？整篇文章给人的感觉好像是对于Web进行了元数据（语义）标注之后就是语义网了。
第二个问题是针对论文认为语义标注必须首先用XML进行结构化，RDF以及KOS转换的本体（OWL)起什么作用都没有明确说明，有点知其然不知其所以然的味道，后面更是没有提到语义Web查询语言SPARQL的独特作用，而是用SQL实现对语义数据的查询。
该 童鞋对于这两个问题的回答还是基本到位的，第一个问题她说到了语义就是所表达内容的含义，需要通过形式化编码才能被机器理解；第二个问题说需要用基于 XML的语义描述语言进行编码，因为在实验系统实现的过程中没有直接支持RDF的数据库系统，所以无法用SPARQL进行语义查询，等等。
这里想补充一些我的认识，有些认识可能比较绝对，对与不对大家可以批评指正。
1、 对于语义Web而言，RDF是基本的编码方式，是不是用了RDF可以作为具不具有明确外在的语义编码的判定标准。就是说，用了RDF，才能说是有语义的， 就像用了ASCII等字符编码标准才能进行文本编码一样。元数据可以不用RDF来表示，但是RDF是专门为了表达元数据而发明的语言（或者说框架或结 构），因为元数据就是“关于数据（主体）的数据（客体）”，主客体通过某种方式（谓词）相联系。这个问题在计算机界一直有争论，但是我这里对RDF的定义 是三元组方式，不一定是XML表达的三元组，也可以是其它方式（如N3等)表达的三元组，或者通过数据库方式能够输出三元组。本体OWL和SKOS都是基 于RDF的，因此它们肯定是表达语义的，而用XML自定义的任何表达方式，可以认为是系统内部局部的语义表达方式，到Web上就不具有可交换性了。因此它 虽然是结构化的，但不是表达全局语义的。
2、RDF是表达机器语义的必要条件，但并不充分。在语义Web中，必须结合URI机制，才能赋予任何一 个表达（资源）的全局语义，当然这个全局也仅仅是对于开放的Web来说的，这也就是URI能够解析的范围。任何一个局域网，无论其规模再大，都可能屏蔽这 种机器语义，而具有其独特的、更丰富的语义。因此，元数据和本体可以适用于比语义Web更广泛的领域，但到了Web上，这些内部语义如果要进行分享、重 用、交换，都有互操作问题。现有的技术架构、模型方案等，都是为了规范和减少这些互操作问题而提出。
3、采用了语义技术，语义Web就可能借助于SKOS或OWL等编码表达的概念体系，进行基于概念的检索，并可能进行知识挖掘和简单推理。SPARQL提供了强大的知识查询能力。
从理解XML到理解RDF，似乎在“思考范式”上要有一个转型。许多搞计算机的人都无法理解，对语义Web认识一直有一个障碍，就是“管它语义不语义，机器懂什么语义？系统只要能满足用户的需求，什么技术不能表达语义？”希望上述解释能够回答这个问题。
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Tags: OWL, RDF, 元数据, 语义技术Related posts
就建立“主题描述模型”与雨师的对话
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关于OWL-S的服务描述- -
服务发现是否假设请求者和提供者使用同一本体？肯定不是。否则 OWL-S 的使用会大大受限，甚至失去意义了，因为其目的本来就是为了寻找合适的本体，其前提假设就有问题。（但是是否假设使用同一本体编码语言呢？也不应该是，但是不一定）
会对本体中介进行一些规范吗？就像 WSMO 对面向对象的中间件一样？虽然可以这样做，但是至今还没有这方面的研究计划和进展。 OWL-S 是采用 OWL 语言的一种服务描述语言，并不规定是否一定有中间件或者服务实现某些功能。
是否仅包含输入描述和输出描述？答案也是否定的。 OWL-S 纲要是用于”广告”语义 Web 服务的，描述的内容包括服务描述、产品描述、输入、输出、前提条件、效果、存取条件、服务质量、安全参数等等，凡是与服务有关的参数均可以用 OWL-S 进行描述。
来自于Katia Sycara” < katia+@cs.cmu.edu >的一帖关于owl-s服务描述问题的澄清，感到有必要存档一下。
There were a number of issues raised in this discussion:
1. Does OWL-S discovery assumes that requesters and provides use a unique ontology?
The answer is NO. OWL-S does not assume the use of a single onrology. It is difficult, however, to see what you mean by “one single ontology”. If you mean “one single OWL file”, then of course trivially OWL-S does not assume a single ontology since you can import as many OWL files as you desire in an OWL-S description, and use any of the concepts defined in those files to describe the OWL-S profile or any other OWL-S component. During the discovery process the Profile of the requested service may refer to a concept, say a:A (the concept A defined in “file” a), and an advertisement may refer to concept b:B that belong to a different ontology (different owl files), and yet b:B may be defined as a subclass of a:A. In this case, matching engines would still be able to match exploiting the logical relation between A and B. At CMU, we have shown different kinds of matches (e.g. exact, plug-in etc) in our matching algorithm (see e.g. [3]).
Another way in which the use of different ontologies can be handled in OWL-S is through mapping rules that could be expressed in SWRL. In this way, to the extent that the similarity between A and B can be made explicit, then the mapping can be exploited. Of course there are issues of where these mappings live, how it is discovered where they live, since of course in the process of service discovery one does not know a priori what the ontological needs of one request would be vis a vis the current advertisement knowledge base. Even if one assumes a unique knowledge base containing such mappings, another set of issues is of course, how this knowledge base gets searched efficiently. 我的论文的一部分就是要解决这个问题：采用登记服务自动完成映射工作，但是基于怎样的请求？机制仍然成问题。
The issue of ontological mapping is an old and well known one that has predated Semantic Web Services. Work on how to express mappings to achieve semantic interoperability efficiently (even assuming the mapping rules are known) has been going on since the late 80's (perhaps even earlier).
The general problem of arbitrary ontology mapping is an open research problem. The real scientific work is in trying to attack the technical issues that I outlined (and others that are there but I did not refer to). After we solve these scientific problems (ie. How to derive the mappings, and how to use them), we can worry about what to call the algorithms.
Since ONTOLOGY MEDIATION is an open research issue, OWL-S is agnostic about the actual ontology mediation process used.( 这方面应该有研究论文，也可以参考一下，属于论文中创新性的内容 ) To the extent that the mediation process is a service, rather than a set of rules, it can be represented in OWL-S and discovered.
2. Should OWL-S do something about ontological mediation like WSMO is doing with the OO mediators?
Up to now, there is no clear operational definition of what a WSMO mediator is, neither is there a clear specification of an ontology or language for describing mediators, or an algorithm for ontological mediation.
To the extent that WSMO mediators are services, rather than sets of rules, they can be represented in OWL-S by specifying what is their profile, process model and grounding (for a detailed discussion on this point see [2]). Furthermore, the discovery mechanism may then become similar to a composition procedure where you combine discovery of the appropriate mediator with the discovery of the appropriate service.
Note that if you take this viewpoint, the sentence “OWL-S has no mediators” is non-sensical: it is analogous to sentences like “Java has no Operating System” or other such sentences. OWL-S is a language (it uses OWL semantics) that allows you to describe Web services, it does not tell you what infrastructure Web services need, nor does it stipulate the existence of mediators or of a discovery registry or any other component. If you think you need a mediator, the role of OWL-S is to provide you the tools to describe a mediator if you decide to implement it as a Web service. If you look at [2] there is a discussion on how to do that.
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关于OWL-S应用的一些问题- -
关于 OWL-S 应用的一些问题（摘自 W3C 语义万维网讨论组 public-sws-ig@w3.org Evan K. Wallace 的一个贴子）：
Eric Miller 在最近的一次会议上提到，许多软件公司对 OWL-S 的应用似乎比当初 RDF-S 和 OWL 来得迟缓，究其原因，大概是因为 OWL-S 目前还是一个 W3C submission 而不是推荐标准，正在讨论之中，变动还会比较大。另一方面好的工具比较少，参考文档和参考案例不多，也影响了应用。
实际上与 OWL-S 处于同一水平层次上的同类技术规范很多，例如 XPDL, BPML, BPE4WS, ebBPSS, BPRI, WMF, 以及 UML2 Action Semantics 等等。 其它更为形式化的如 PSL 和 SWSL 。 OWL-S 似乎并没有像 OWL 一样在同类语言中鹤立鸡群（特别作为概念建模语言方面）。 OWL-S 似乎没有吸收足够的同类语言的成果。
Popularity: 25% [?]
Tags: ontology, OWL, 语义技术