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舊 2004-03-03, 11:22 AM   #1
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預設 什麼是RSS

RSS(Rich Site Summary或者RDF Site Summary)是一種用於網站內容集成的技術。這種最初源自瀏覽器「新聞頻道」的技術,現在卻在企業門戶(portal)、企業應用集成(EAI)等方面得到了更加寬廣的用武之地。

————————————————

What is RSS?
By Mark Pilgrim

RSS is a format for syndicating news and the content of news-like sites, including major news sites like Wired, news-oriented community sites like Slashdot, and personal weblogs. But it's not just for news. Pretty much anything that can be broken down into discrete items can be syndicated via RSS: the "recent changes" page of a wiki, a changelog of CVS checkins, even the revision history of a book. Once information about each item is in RSS format, an RSS-aware program can check the feed for changes and react to the changes in an appropriate way.

RSS-aware programs called news aggregators are popular in the weblogging community. Many weblogs make content available in RSS. A news aggregator can help you keep up with all your favorite weblogs by checking their RSS feeds and displaying new items from each of them.

A brief history
But coders beware. The name "RSS" is an umbrella term for a format that spans several different versions of at least two different (but parallel) formats. The original RSS, version 0.90, was designed by Netscape as a format for building portals of headlines to mainstream news sites. It was deemed overly complex for its goals; a simpler version, 0.91, was proposed and subsequently dropped when Netscape lost interest in the portal-making business. But 0.91 was picked up by another vendor, UserLand Software, which intended to use it as the basis of its weblogging products and other web-based writing software.

In the meantime, a third, non-commercial group split off and designed a new format based on what they perceived as the original guiding principles of RSS 0.90 (before it got simplified into 0.91). This format, which is based on RDF, is called RSS 1.0. But UserLand was not involved in designing this new format, and, as an advocate of simplifying 0.90, it was not happy when RSS 1.0 was announced. Instead of accepting RSS 1.0, UserLand continued to evolve the 0.9x branch, through versions 0.92, 0.93, 0.94, and finally 2.0.

What a mess.

So which one do I use?
That's 7 -- count 'em, 7! -- different formats, all called "RSS". As a coder of RSS-aware programs, you'll need to be liberal enough to handle all the variations. But as a content producer who wants to make your content available via syndication, which format should you choose?

RSS versions and recommendations Version Owner Pros Status Recommendation
0.90 Netscape Obsoleted by 1.0 Don't use
0.91 UserLand Drop dead simple Officially obsoleted by 2.0, but still quite popular Use for basic syndication. Easy migration path to 2.0 if you need more flexibility
0.92, 0.93, 0.94 UserLand Allows richer metadata than 0.91 Obsoleted by 2.0 Use 2.0 instead
1.0 RSS-DEV Working Group RDF-based, extensibility via modules, not controlled by a single vendor Stable core, active module development Use for RDF-based applications or if you need advanced RDF-specific modules
2.0 UserLand Extensibility via modules, easy migration path from 0.9x branch Stable core, active module development Use for general-purpose, metadata-rich syndication


What does RSS look like?
Imagine you want to write a program that reads RSS feeds, so that you can publish headlines on your site, build your own portal or homegrown news aggregator, or whatever. What does an RSS feed look like? That depends on which version of RSS you're talking about. Here's a sample RSS 0.91 feed (adapted from XML.com's RSS feed):

<rss version="0.91">
<channel>
<title>XML.com</title>
<link>http://www.xml.com/</link>
<description>XML.com features a rich mix of information and services for the XML community.</description>
<language>en-us</language>
<item>
<title>Normalizing XML, Part 2</title>
<link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
<description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
</item>
<item>
<title>The .NET Schema Object Model</title>
<link>http://www.xml.com/pub/a/2002/12/04/som.html</link>
<description>Priya Lakshminarayanan describes in detail the use of the .NET Schema Object Model for programmatic manipulation of W3C XML Schemas.</description>
</item>
<item>
<title>SVG's Past and Promising Future</title>
<link>http://www.xml.com/pub/a/2002/12/04/svg.html</link>
<description>In this month's SVG column, Antoine Quint looks back at SVG's journey through 2002 and looks forward to 2003.</description>
</item>
</channel>
</rss>

Simple, right? A feed comprises a channel, which has a title, link, description, and (optional) language, followed by a series of items, each of which have a title, link, and description.

Now look at the RSS 1.0 version of the same information:

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns="http://purl.org/rss/1.0/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
>
<channel rdf:about="http://www.xml.com/cs/xml/query/q/19">
<title>XML.com</title>
<link>http://www.xml.com/</link>
<description>XML.com features a rich mix of information and services for the XML community.</description>
<language>en-us</language>
<items>
<rdf:Seq>
<rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/normalizing.html"/>
<rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/som.html"/>
<rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/svg.html"/>
</rdf:Seq>
</items>
</channel>
<item rdf:about="http://www.xml.com/pub/a/2002/12/04/normalizing.html">
<title>Normalizing XML, Part 2</title>
<link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
<description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
<dc:creator>Will Provost</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
<item rdf:about="http://www.xml.com/pub/a/2002/12/04/som.html">
<title>The .NET Schema Object Model</title>
<link>http://www.xml.com/pub/a/2002/12/04/som.html</link>
<description>Priya Lakshminarayanan describes in detail the use of the .NET Schema Object Model for programmatic manipulation of W3C XML Schemas.</description>
<dc:creator>Priya Lakshminarayanan</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
<item rdf:about="http://www.xml.com/pub/a/2002/12/04/svg.html">
<title>SVG's Past and Promising Future</title>
<link>http://www.xml.com/pub/a/2002/12/04/svg.html</link>
<description>In this month's SVG column, Antoine Quint looks back at SVG's journey through 2002 and looks forward to 2003.</description>
<dc:creator>Antoine Quint</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
</rdf:RDF>

Quite a bit more verbose. People familiar with RDF will recognize this as an XML serialization of an RDF document; the rest of the world will at least recognize that we're syndicating essentially the same information. In fact, we're including a bit more information: item-level authors and publishing dates, which RSS 0.91 does not support.






by Mark Pilgrim

Despite being RDF/XML, RSS 1.0 is structurally similar to previous versions of RSS -- similar enough that we can simply treat it as XML and write a single function to extract information out of either an RSS 0.91 or RSS 1.0 feed. However, there are some significant differences that our code will need to be aware of:

The root element is rdf:RDF instead of rss. We'll either need to handle both explicitly or just ignore the name of the root element altogether and blindly look for useful information inside it.

RSS 1.0 uses namespaces extensively. The RSS 1.0 namespace is http://purl.org/rss/1.0/, and it's defined as the default namespace. The feed also uses http://www.w3.org/1999/02/22-rdf-syntax-ns# for the RDF-specific elements (which we'll simply be ignoring for our purposes) and http://purl.org/dc/elements/1.1/ (Dublin Core) for the additional metadata of article authors and publishing dates.

We can go in one of two ways here: if we don't have a namespace-aware XML parser, we can blindly assume that the feed uses the standard prefixes and default namespace and look for item elements and dc:creator elements within them. This will actually work in a large number of real-world cases; most RSS feeds use the default namespace and the same prefixes for common modules like Dublin Core. This is a horrible hack, though. There's no guarantee that a feed won't use a different prefix for a namespace (which would be perfectly valid XML and RDF). If or when it does, we'll miss it.

If we have a namespace-aware XML parser at our disposal, we can construct a more elegant solution that handles both RSS 0.91 and 1.0 feeds. We can look for items in no namespace; if that fails, we can look for items in the RSS 1.0 namespace. (Not shown, but RSS 0.90 feeds also use a namespace, but not the same one as RSS 1.0. So what we really need is a list of namespaces to search.)

Less obvious but still important, the item elements are outside the channel element. (In RSS 0.91, the item elements were inside the channel. In RSS 0.90, they were outside; in RSS 2.0, they're inside. Whee.) So we can't be picky about where we look for items.

Finally, you'll notice there is an extra items element within the channel. It's only useful to RDF parsers, and we're going to ignore it and assume that the order of the items within the RSS feed is given by their order of the item elements.

But what about RSS 2.0? Luckily, once we've written code to handle RSS 0.91 and 1.0, RSS 2.0 is a piece of cake. Here's the RSS 2.0 version of the same feed:

<rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/">
<channel>
<title>XML.com</title>
<link>http://www.xml.com/</link>
<description>XML.com features a rich mix of information and services for the XML community.</description>
<language>en-us</language>
<item>
<title>Normalizing XML, Part 2</title>
<link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
<description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
<dc:creator>Will Provost</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
<item>
<title>The .NET Schema Object Model</title>
<link>http://www.xml.com/pub/a/2002/12/04/som.html</link>
<description>Priya Lakshminarayanan describes in detail the use of the .NET Schema Object Model for programmatic manipulation of W3C XML Schemas.</description>
<dc:creator>Priya Lakshminarayanan</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
<item>
<title>SVG's Past and Promising Future</title>
<link>http://www.xml.com/pub/a/2002/12/04/svg.html</link>
<description>In this month's SVG column, Antoine Quint looks back at SVG's journey through 2002 and looks forward to 2003.</description>
<dc:creator>Antoine Quint</dc:creator>
<dc:date>2002-12-04</dc:date>
</item>
</channel>
</rss>

As this example shows, RSS 2.0 uses namespaces like RSS 1.0, but it's not RDF. Like RSS 0.91, there is no default namespace and items are back inside the channel. If our code is liberal enough to handle the differences between RSS 0.91 and 1.0, RSS 2.0 should not present any additional wrinkles.

How can I read RSS?
Now let's get down to actually reading these sample RSS feeds from Python. The first thing we'll need to do is download some RSS feeds. This is simple in Python; most distributions come with both a URL retrieval library and an XML parser. (Note to Mac OS X 10.2 users: your copy of Python does not come with an XML parser; you will need to install PyXML first.)

from xml.dom import minidom
import urllib

def load(rssURL):
return minidom.parse(urllib.urlopen(rssURL))

This takes the URL of an RSS feed and returns a parsed representation of the DOM, as native Python objects.

The next bit is the tricky part. To compensate for the differences in RSS formats, we'll need a function that searches for specific elements in any number of namespaces. Python's XML library includes a getElementsByTagNameNS which takes a namespace and a tag name, so we'll use that to make our code general enough to handle RSS 0.9x/2.0 (which has no default namespace), RSS 1.0 and even RSS 0.90. This function will find all elements with a given name, anywhere within a node. That's a good thing; it means that we can search for item elements within the root node and always find them, whether they are inside or outside the channel element.

DEFAULT_NAMESPACES = \
(None, # RSS 0.91, 0.92, 0.93, 0.94, 2.0
'http://purl.org/rss/1.0/', # RSS 1.0
'http://my.netscape.com/rdf/simple/0.9/' # RSS 0.90
)

def getElementsByTagName(node, tagName, possibleNamespaces=DEFAULT_NAMESPACES):
for namespace in possibleNamespaces:
children = node.getElementsByTagNameNS(namespace, tagName)
if len(children): return children
return []

Finally, we need two utility functions to make our lives easier. First, our getElementsByTagName function will return a list of elements, but most of the time we know there's only going to be one. An item only has one title, one link, one description, and so on. We'll define a first function that returns the first element of a given name (again, searching across several different namespaces). Second, Python's XML libraries are great at parsing an XML document into nodes, but not that helpful at putting the data back together again. We'll define a textOf function that returns the entire text of a particular XML element.

def first(node, tagName, possibleNamespaces=DEFAULT_NAMESPACES):
children = getElementsByTagName(node, tagName, possibleNamespaces)
return len(children) and children[0] or None

def textOf(node):
return node and "".join([child.data for child in node.childNodes]) or ""

That's it. The actual parsing is easy. We'll take a URL on the command line, download it, parse it, get the list of items, and then get some useful information from each item:

DUBLIN_CORE = ('http://purl.org/dc/elements/1.1/',)

if __name__ == '__main__':
import sys
rssDocument = load(sys.argv[1])
for item in getElementsByTagName(rssDocument, 'item'):
print 'title:', textOf(first(item, 'title'))
print 'link:', textOf(first(item, 'link'))
print 'description:', textOf(first(item, 'description'))
print 'date:', textOf(first(item, 'date', DUBLIN_CORE))
print 'author:', textOf(first(item, 'creator', DUBLIN_CORE))
print

Running it with our sample RSS 0.91 feed prints only title, link, and description (since the feed didn't include any other information on dates or authors):

$ python rss1.py http://www.xml.com/2002/12/18/examples/rss091.xml.txt
title: Normalizing XML, Part 2
link: http://www.xml.com/pub/a/2002/12/04/normalizing.html
description: In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.
date:
author:

title: The .NET Schema Object Model
link: http://www.xml.com/pub/a/2002/12/04/som.html
description: Priya Lakshminarayanan describes in detail the use of the .NET Schema Object Model for programmatic manipulation of W3C XML Schemas.
date:
author:

title: SVG's Past and Promising Future
link: http://www.xml.com/pub/a/2002/12/04/svg.html
description: In this month's SVG column, Antoine Quint looks back at SVG's journey through 2002 and looks forward to 2003.
date:
author:

For both the sample RSS 1.0 feed and sample RSS 2.0 feed, we also get dates and authors for each item. We reuse our custom getElementsByTagName function, but pass in the Dublin Core namespace and appropriate tag name. We could reuse this same function to extract information from any of the basic RSS modules. (There are a few advanced modules specific to RSS 1.0 that would require a full RDF parser, but they are not widely deployed in public RSS feeds.)

Here's the output against our sample RSS 1.0 feed:

$ python rss1.py http://www.xml.com/2002/12/18/examples/rss10.xml.txt
title: Normalizing XML, Part 2
link: http://www.xml.com/pub/a/2002/12/04/normalizing.html
description: In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.
date: 2002-12-04
author: Will Provost

title: The .NET Schema Object Model
link: http://www.xml.com/pub/a/2002/12/04/som.html
description: Priya Lakshminarayanan describes in detail the use of the .NET Schema Object Model for programmatic manipulation of W3C XML Schemas.
date: 2002-12-04
author: Priya Lakshminarayanan

title: SVG's Past and Promising Future
link: http://www.xml.com/pub/a/2002/12/04/svg.html
description: In this month's SVG column, Antoine Quint looks back at SVG's journey through 2002 and looks forward to 2003.
date: 2002-12-04
author: Antoine Quint

Running against our sample RSS 2.0 feed produces the same results.

This technique will handle about 90% of the RSS feeds out there; the rest are ill-formed in a variety of interesting ways, mostly caused by non-XML-aware publishing tools building feeds out of templates and not respecting basic XML well-formedness rules. Next month we'll tackle the thorny problem of how to handle RSS feeds that are almost, but not quite, well-formed XML.

Related resources
Sample RSS feeds: RSS 0.91, RSS 1.0, RSS 2.0.
rss1.py
Specifications: RSS 0.90, RSS 0.91, RSS 1.0, RSS 2.0.
Syndic8.com, a directory of 10,000 publicly available RSS feeds.
News Readers in the Open Directory, a variety of client-side and server-side programs for reading RSS feeds.
psac 目前離線  
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舊 2004-03-03, 11:23 AM   #2 (permalink)
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預設

什麼是RSS?

文章來源http://blog.sunmast.com/
討論與Blog相關的技術,不可不談的就是RSS,這個縮寫在英文中可以有幾個源頭,並被不同的技術團體做不同的解釋。既可以是「Rich Site Summary」,或「RDF Site Summary」,也可以是「Really Simple Syndication」。為什麼有這麼多含義呢?這還要從RSS的一段今天也沒有理清的關係說起。

今天肯定有人還記得IE 4剛剛推出來的時候有一個有趣的功能,那就是新聞頻道。這個新聞頻道的功能與Netscape推出的新聞頻道是很相似的(當時Netscape還是市場上領先的瀏覽器)。為此Netscape 定義了一套描述新聞頻道的語言,這就是RSS,只不過Netscape自當時起每況愈下,所以最終也沒有發佈一個正式的RSS規範(只發佈了一個0.9版本)。而微軟也在當時推出了支持自己IE的CDF(Channel Definition Format)數據規格,與RSS非常接近。微軟試圖用新聞頻道的功能把「推」(Push)技術變成一個應用主流,並與Netscape抗衡。不過出乎預測的是,「推」技術自始至終沒有找到合適的商業模型,而且伴隨著其他各類網絡特性的出現,也日益無法顯現自身的優勢。新聞頻道在瀏覽器中的地位最終日暮西山,最後也在IE的後續版本中消失了。

新聞頻道的確進入了低谷,但是RSS並沒有被業界人士所拋棄。過去兩年,Blog從一個專業群體開始,逐步成為了網絡上最熱門的新話題。而RSS成為了描述Blog主題和更新信息的最基本方法。於是RSS這項技術被著名Blogger/Geek戴夫·溫那(Dave Winner)的公司UserLand所接手,繼續開發新的版本,以適應新的網絡應用需要。新的網絡應用就是Blog,因為戴夫·溫那的努力,RSS升級到了0.91版,然後達到了0.92版,隨後在各種Blog工具中得到了應用,並被眾多的專業新聞站點所支持。在廣泛的應用過程中,眾多的專業人士認識到需要組織起來,把RSS發展成為一個通用的規範,並進一步標準化。一個聯合小組根據W3C新一代的語義網技術RDF對RSS進行了重新定義,發佈了RSS 1.0,並把RSS定義為「RDF Site Summary」。這項工作並沒有與戴夫·溫那進行有效的溝通,而戴夫則堅持在自己設想的方向上進一步開發RSS的後續版本,也並不承認RSS 1.0的有效性。RSS由此開始分化形成了RSS 0.9x/2.0和RSS 1.0兩個陣營,也由此引起了在專業人群中的廣泛爭論。

因為爭論的存在,一直到今天,RSS 1.0還沒有成為標準化組織的真正標準。而戴夫·溫那卻在2002年9月獨自把RSS升級到了2.0版本,其中的定義完全是全新的模式,並沒有任何RSS 1.0的影子。這引發了網絡上進一步爭議,究竟讓一個越來越普及的數據格式成為一個開放的標準,還是被一家公司所定義和控制,成為了爭議的焦點。戴夫·溫那並沒有為自己辯解,他的觀點是RSS還需要進一步發展,需要專業人士更明確的定義,不過恐怕這種輕描淡寫不能消除人們對RSS「被一家商業公司獨佔」的擔心。

前面的鋪墊對用戶來說也許沒有什麼太大的意義,可能更多人關心如何在自己的Blog增加RSS輸出,這樣可以讓很多新聞聚合工具(例如CNBlog剛剛推薦的NewzCrawler)很容易找到你並自動獲得你在Blog中的更新內容。

它是什麼:站點用來和其他站點之間共享內容的簡易方式(也叫聚合內容)。 RSS使用XML作為彼此共享內容的標準方式。

它代表什麼:Really Simple Syndication (或RDF Site Summary,RDF站點摘要)

例如:一些免費的軟件能夠讓你閱讀那些RSS使能的站點,比如 NewsIsFree 和 Amphetadesk。

它有什麼用處:讓別人容易的發現你已經更新了你的站點,讓人們很容易的追蹤他們閱讀的所有weblogs。
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