このチュートリアルはRSSと呼ばれるウェブ上で利用されるフォーマットの特徴や利点を解説し、更にその技術的な概要も手短に説明します。また、RSSと同じようなフォーマットであるAtomについての情報も含まれます。読者はXMLやその他ウェブ上で利用される技術について良く知っていることと仮定します。このチュートリアルは徹底的なものではないので、より多くの情報については更なる情報セクションを参照してください.
ニュース・ヘッドラインや検索結果、更新履歴、雇用情報など、日々アクセスするウェブ上の情報について考えてみてください。それらの大量なコンテンツはリスト、おそらくHTMLのli要素ではないですが、リスト的な傾向を持つ情報と考えることが出来ます。
多くの人々はたくさんのこれらリストを追いかける必要がありますが、ひとたび手に余るほどの量になったら追いかけることは難しくなります。それぞれのページを訪問し、読み込み、どのようなページであったかを思い出した上で、リストの最後にチェックした時の場所を探さなければならないからです。
RSSはXMLベースのフォーマットなので、他の情報やメタデータと共にハイパーリンク、閲覧者がそのリンクを辿るかどうか決められるように、のリストを配信することが出来ます。
また、コンピューターで情報を取得し理解することができるので、全ての興味のあるリストを追いかけ、個人々々に合わせることが可能です。つまり、これはむしろHTMLのように直接人々に見せられるものではなく、人に代わってコンピューターが利用するためのフォーマットです。
これを可能にするためには、ウェブサイトは他のファイルやリソースと同じように利用できるフィード、またはチャンネルを作成しなければならないでしょう。一度フィードを用意したのならば、コンピューターはリストの最新の項目を得るために定期的にそれを取得することが出来るようになります。大抵の場合、人々はそれをアグリゲーター、単一のインターフェイスで管理と表示を行うことの出来るプログラム、で行うでしょう。
フィードはリスト的な傾向を持つ情報、例えばリンクと共に内容物そのものの配信(ウェブログのような)、にも使うことが出来ます。しかし、このチュートリアルではリンクの配信にRSSを利用することに焦点を置きます。
フィードはリンクによって特定されるアイテムまたはエントリのリストから成ります。それぞれのアイテムは、同じようにリンクと関連付けられた様々なメタデータを持つことが出来ます。
最も基本的なエントリのメタデータは、リンクのタイトルとその概要です。例えば、ニュース・ヘッドラインの配信の場合、これらはニュースの見出しと最初の段落または要約になるでしょう。例えば、シンプルなエントリは以下のようになります。
<item> <title>Earth Invaded</title> <link>http://news.example.com/2004/12/17/invasion</link> <description>The earth was attacked by an invasion fleet from halfway across the galaxy; luckily, a fatal miscalculation of scale resulted in the entire armada being eaten by a small dog.</description> </item>
加えて、フィードそのものもそれに関連付けられるメタデータを持つことが出来るので、タイトル(例: "Bob’s news headlines")や概要、その他の発行者や著作権情報などを付記することが出来ます。
完全なフィードがどのようなものかは、"フォーマットのバージョンとモジュール"を参照してください。
アグリゲーターが一般的なフィードを利用するもので、それらには様々なタイプのものがあります。ウェブ・アグリゲーター(ポータルなどと呼ばれることもあります)はフィードをウェブ・ページに表示してくれます。代表的なものはMy Yahoo!でしょう。アグリゲーターはE-メール・クライアントやユーザーのデスクトップに統合された形、または独立した専用のソフトウェアとして提供されているかもしれません。
アグリゲーターは、複数の関連したフィードをまとめて表示する機能や既に閲覧したエントリを隠す機能、フィードやエントリをカテゴリ分けする機能などいろいろな機能を提供します。
フィードの他の利用としては、フィードは機械的に扱え、検索エンジンはサイトのどの部分が重要であるかどうかやどの部分が単なるナビゲーションや広告であるかどうかを把握する必要が無いため、検索エンジンやその他ソフトウェアでサイトのトラッキングに利用することが挙げられます。人々が彼らのウェブ・サイトでフィードを再利用すること、必要に応じてあなたのコンテンツを表示するといったことを許可することもできます。
Your viewers will thank you, and there will be more of them, because it allows them to see your site without going out of their way to visit.
While this seems bad at first glance, it actually improves your site’s visibility; by making it easier for your users to keep up with your site - allowing them to see it the way they want to - it’s more likely that they’ll know when something that interests them is available on your site.
For example, imagine that your company announces a new product or feature every month or two. Without a feed, your viewers have to remember to come to your site and see if they find anything new - if they have time. If you provide a feed for them, they can point their aggregator or other software at it, and it will give them a link and a description of developments at your site almost as soon as they happen.
News is similar; because there are so many sources of news on the Web, most of your viewers won’t come to your site every day. By providing a feed, you are in front of them constantly, improving the chances that they’ll click through to an article that catches their eye.
いいえ！(あなたが望むなら)あなたは依然として自分のコンテンツに対する著作権を保有します。
フィードで配信する情報を、完全な内容か煽りだけかといったように制御することもできます。Your content can still be protected by your current access control mechanisms; only the links and metadata are distributed. You can also protect the RSS feed itself with SSL encryption and HTTP username/password authentication too, if you’d like.
In many ways, syndication is similar to the subscription newsletters that many sites offer to keep viewers up-to-date. The big difference is that they don’t have to supply an e-mail address, lowering the barrier of privacy concerns, while still giving you a direct channel to your viewers. Also, they get to see the content in the manner that’s most convenient to them, which means that you get more eyes looking at your content.
Any list-oriented information on your site that your viewers might be interested in tracking or reusing is a good candidate for a feed. This can encompass news headlines and press releases, job listings, conference calendars and rankings (like ‘top 10’ lists).
For example;
While it’s a good start to have a “master feed” for your site that lists recent news and events, don’t stop there. Generally, each area of your site that features a changing list of information should have a corresponding feed; this allows viewers to precisely target their interests.
For example, if your news site has pages for World news, national news, local news, business, sports, etc., there should be a feed for each of these sections.
If your site offers a personalized view of data (e.g., people can choose categories of information that will show up on their home page), offer this as a feed, so that the viewers’ Web pages match the content of their feeds.
A great example of this is the variety of feeds that Netflix provides; not only can you keep track of new releases, but also personalised reccommendations and even a listing of the movies in your queue.
Another good example is Apple’s iTunes Music Store RSS feed generator; you can customize it based on your preferences, and the views it allows match those provided in the Music Store itself.
Finally, remember that feeds are just as - if not more - useful on an Intranet as they are on the Internet. Syndication can be a powerful tool for sharing and integrating information inside a company.
There are a number of ways to generate a feed from your content. First of all, explore your content management system - it might already have an option to generate an RSS feed.
If that option isn’t available, you have a number of choices;
For more information about all of these options, see “Feed Tools” and “More Information”.
An important step after publishing a feed is letting your viewers know that it exists; there are a lot of feeds available on the Web now, but it’s hard to find them, making it difficult for viewers to utilize them.
Pages that have an associated RSS feed should clearly indicate this to viewers by using a link containing like ‘RSS feed’. For example,
<a type="application/rss+xml" href="feed.rss">RSS feed for this page</a>
where ‘feed.rss’ is the URL for the feed. the ‘type’ attribute tells browsers that this is a link to an RSS feed in a way that they understand.
Additionally, some programs look for a link in the <head> section of your HTML. To support this, include a <link> tag;
<head> <title>My Page</title> <link rel="alternate" type="application/rss+xml" href="feed.rss" title="RSS feed for My Page"> </head>
These links should be placed on the Web page that is most similar to the feed content; this enables people to find them as they browse.
Note that Atom feeds should use application/atom+xml rather than application/rss+xml in both styles of use.
Finally, there are a number of guides and registries for RSS feeds that people can search and browse through, much like the Yahoo directory for Web sites; it’s a good idea to register your feed; see More Information.
There are a number of different versions of the RSS format in use today, but the main choices are RSS 1.0 and RSS 2.0. Each version has its benefits and drawbacks; RSS 2.0 is known for its simplicity, while RSS 1.0 is more extensible and fully specified. Both formats are XML-based and have the same basic structure.
There’s one more choice; Atom is an effort in the IETF (an Internet standards body) to come up with a well-documented, standard syndication format. Although it has a different name, it has the same basic functions as RSS, and many people use the term “RSS” to refer to RSS or Atom syndication.
This section presents a quick overview of each; for more information, see their specifications and supporting materials.
RSS 2.0 is championed by UserLand’s Dave Winer. In this version, RSS stands for “Really Simple Syndication,” and simplicity is its focus.
This branch of RSS is based on RSS 0.91, which was first documented at Netscape and later refined by Userland.
Included in 2.0.1 - the latest stable version of this branch - are channel metadata like link, title, description; image, which allows you to specify a thumbnail image to display with the feed); webMaster and managingEditor, to identify who’s responsible for the feed, and lastBuildDate, which shows when the feed was last updated.
Items have the standard link, title and description metadata, as well as other, more experimental facilities like enclosure, which allows attachments to be automatically downloaded (don’t expect these features to be supported by all aggregators, however). Finally, items can have a guid element that identifies the item uniquely; this allows some advanced functionality in some aggregators.
Here’s an example of a minimal RSS 2.0 feed:
<?xml version="1.0"?>
<rss version="2.0">
<channel>
<title>Example Channel</title>
<link>http://example.com/</link>
<description>My example channel</description>
<item>
<title>News for September the Second</title>
<link>http://example.com/2002/09/01</link>
<description>other things happened today</description>
</item>
<item>
<title>News for September the First</title>
<link>http://example.com/2002/09/02</link>
</item>
</channel>
</rss>
In the RSS 2.0 roadmap, Winer states that this branch is, for all practical purposes, frozen, except for clarifications to the specification.
However, exensions to the format are allowed in separate modules, using XML Namespaces to avoid conflicts in their names. For example, if you had an ISBN module to track books, it might look like this;
<item xmlns:book="http://namespace.example.com/book/1.0"> <title>Excession</link> <link>http://www.amazon.com/exec/obidos/tg/detail/-/0553575376</link> <book:isbn>0553575376</book:isbn> </item>
Generally, though, you should look for available RSS Modules, rather than defining your own, unless you’re sure that what you need doesn’t exist.
RSS 1.0 stands for “RDF Site Summary.” This flavor of RSS incorporates RDF, a Web standard for metadata. Because RSS 1.0 uses RDF, any RDF processor can understand RSS without knowing anything about it in particular. This allows syndicated feeds to easily become part of the Semantic Web.
RSS 1.0 also uses XML Namespaces to allow extensions, in a manner similar to RSS 2.0.
RSS 1.0 feeds look very similar to RSS 2.0 feeds, with a few key differences;
RSS 1.0 is developed and maintained by an ad hoc group of interested people; see their Web site for more information about RSS 1.0 and RSS Modules. See below for an example of an RSS 1.0 feed.
The most well-known example of an RSS 1.0 Module is the Dublin Core Module. The Dublin Core is a set of metadata developed by librarians and information scientists that standardizes a set of common metadata that is useful for describing documents, among other things. The Dublin Core Module uses these metadata to attach information to both feeds (in the channel metadata) and to individual items.
This module includes useful elements like dc:date, for associating dates with items, dc:subject, which can be useful for categorizing items or feeds, and dc:rights, for dictating the intellectual property rights associated with an item or a feed.
Here’s an example of a minimal RSS 1.0 feed that uses the Dublin Core Module:
<?xml version="1.0"?>
<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://example.com/news.rss">
<title>Example Channel</title>
<link>http://example.com/</link>
<description>My example channel</description>
<items>
<rdf:Seq>
<rdf:li resource="http://example.com/2002/09/01/"/>
<rdf:li resource="http://example.com/2002/09/02/"/>
</rdf:Seq>
</items>
</channel>
<item rdf:about="http://example.com/2002/09/01/">
<title>News for September the First</title>
<link>http://example.com/2002/09/01/</link>
<description>other things happened today</description>
<dc:date>2002-09-01</dc:date>
</item>
<item rdf:about="http://example.com/2002/09/02/">
<title>News for September the Second</title>
<link>http://example.com/2002/09/02/</link>
<dc:date>2002-09-02</dc:date>
</item>
</rdf:RDF>
As you can see, RSS 1.0 is a bit more verbose than 2.0, mostly because it needs to be compatible with other versions of RSS while containing the markup that RDF processors need.
Both RSS 1.0 and 2.0 are informal specifications; that is, they aren’t published by a well-known standards body or industry consortium, but instead by a small group of people.
Some people are concerned by this, because such specifications can be changed at the whim of the people who control it. Standards bodies bring stability, by limiting change and having well-established procedures for introducing it. To introduce such stability to syndication, a group of people established an IETF Working Group to standardise a format called Atom.
Atom is functionally similar to both branches of RSS, and is also an XML-based format.
For example;
<?xml version="1.0" encoding="utf-8"?> <feed xmlns="http://www.w3.org/2005/Atom"> <title>Example Feed</title> <link href="http://example.org/"/> <updated>2003-12-13T18:30:02Z</updated> <author> <name>John Doe</name> </author> <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id> <entry> <title>Atom-Powered Robots Run Amok</title> <link href="http://example.org/2003/12/13/atom03"/> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2003-12-13T18:30:02Z</updated> <summary>Some text.</summary> </entry> </feed>
As you can see, Atom has a feed element that contains both the feed-level metadata as well as the entrys (analogous to RSS’ items), and entry can contain similar metadata, such as title, link, id (instead of RSS 1.0’s rdf:about or RSS 2.0’s guid), and a short textual summary (instead of RSS’ description).
Generally, Atom isn’t as widely supported as RSS 1.0 or 2.0 right now, because it’s relatively new. However, it should catch up quickly, because of the broad base of vendors supporting the standardisation effort.
One of the most confusing and unfortunate problems in syndication is the large number of formats in use. In addition to those listed above, there are many other formats (e.g., RSS 0.9, 0.91, 0.92) that are commonly encountered on the Web.
For better or worse, the decision isn’t as critical as you might think. Most aggregators and other software use syndication libraries which abstract out the particular format that a feed is in, so that they can consume any popular syndication feed.
As a result, which format to choose is a matter of personal taste. RSS 1.0 is very extensible, and useful if you want to integrate it into Semantic Web systems. RSS 2.0 is very simple and easy to author by hand. Atom is now an IETF Standard, bringing stability and a natural community to support its use.
RSS and Atom are easy to work with, but like any new format, you may encounter some problems in using them. This section attempts to address the most common issues that arise when generating a feed.
This is an incomplete list of tools for creating feeds and checking them to make sure that you’ve done so correctly. Note that there are many more libraries that help parsing feeds; these haven’t been included here because this tutorial focuses on the Webmaster, not consumers of feeds.
This document is Copyright © 2002-2005 Mark Nottingham <mnot@pobox.com>. This work is licensed under a Creative Commons License.
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Version 0.91 — September 7, 2005