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63-538: Channel Definition Format (CDF) was an XML file format formerly used in conjunction with Microsoft 's Active Channel , Active Desktop and Smart Offline Favorites technologies. The format was designed to "offer frequently updated collections of information, or channels, from any web server for automatic delivery to compatible receiver programs." Active Channel allowed users to subscribe to channels and have scheduled updates delivered to their desktop. Smart Offline Favorites, like channels, enabled users to view webpages from

126-549: A numeric character reference . Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as &#20013; or &#x4e2d; . Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as I &lt;3 J&#xF6;rg . &#0;

189-554: A feed's URI into the reader or by clicking on the browser's feed icon . The RSS reader checks the user's feeds regularly for new information and can automatically download it, if that function is enabled. The RSS formats were preceded by several attempts at web syndication that did not achieve widespread popularity. The basic idea of restructuring information about websites goes back to as early as 1995, when Ramanathan V. Guha and others in Apple 's Advanced Technology Group developed

252-408: A generic XML file. Although RSS formats have evolved from as early as March 1999, it was between 2005 and 2006 when RSS gained widespread use, and the (" [REDACTED] ") icon was decided upon by several major web browsers. RSS feed data is presented to users using software called a news aggregator and the passing of content is called web syndication . Users subscribe to feeds either by entering

315-448: A list of syntax rules provided in the specification. Some key points in the fairly lengthy list include: The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing. This policy, occasionally referred to as " draconian error handling", stands in notable contrast to

378-561: A major new version of the format, RSS 2.0, that redubbed its initials Really Simple Syndication. RSS 2.0 removed the type attribute added in the RSS 0.94 draft and added support for namespaces. To preserve backward compatibility with RSS 0.92, namespace support applies only to other content included within an RSS 2.0 feed, not the RSS 2.0 elements themselves. (Although other standards such as Atom attempt to correct this limitation, RSS feeds are not aggregated with other content often enough to shift

441-522: A mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser (and in some cases not even UTF-16, even though the standard mandates it to also be recognized). XML provides escape facilities for including characters that are problematic to include directly. For example: There are five predefined entities : All permitted Unicode characters may be represented with

504-555: A more compact non-XML syntax; the two syntaxes are isomorphic and James Clark 's conversion tool— Trang —can convert between them without loss of information. RELAX NG has a simpler definition and validation framework than XML Schema, making it easier to use and implement. It also has the ability to use datatype framework plug-ins ; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes. Schematron

567-566: A podcast, a user adds the RSS feed to their podcast client, and the client can then list available episodes and download or stream them for listening or viewing. To be included in a podcast directory the feed must for each episode provide a title, description, artwork, category, language, and explicit rating. There are some services that specifically indexes and is a search engine for podcasts. Some BitTorrent clients support RSS. RSS feeds which provide links to .torrent files allow users to subscribe and automatically download content as soon as it

630-522: A result, publishers began placing HTML markup into the titles and descriptions of items in their RSS feeds. This behavior has become expected of readers, to the point of becoming a de facto standard. Though there is still some inconsistency in how software handles this markup, particularly in titles. The RSS 2.0 specification was later updated to include examples of entity-encoded HTML; however, all prior plain text usages remain valid. As of January 2007 , tracking data from www.syndic8.com indicates that

693-506: A rich datatyping system and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them. xs:schema element that defines a schema: RELAX NG (Regular Language for XML Next Generation) was initially specified by OASIS and is now a standard (Part 2: Regular-grammar-based validation of ISO/IEC 19757 – DSDL ). RELAX NG schemas may be written in either an XML based syntax or

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756-471: A synopsis, but the RSS 1.* markup can be (and often is) used as an extension. There are also several common outside extension packages available, e.g. one from Microsoft for use in Internet Explorer 7. The most serious compatibility problem is with HTML markup. Userland's RSS reader—generally considered as the reference implementation—did not originally filter out HTML markup from feeds. As

819-421: A validity error must be able to report it, but may continue normal processing. A DTD is an example of a schema or grammar . Since the initial publication of XML 1.0, there has been substantial work in the area of schema languages for XML. Such schema languages typically constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and

882-527: A vocabulary to refer to the constructs within an XML document, but does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized. Existing APIs for XML processing tend to fall into these categories: Stream-oriented facilities require less memory and, for certain tasks based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs typically require

945-461: Is a lexical , event-driven API in which a document is read serially and its contents are reported as callbacks to various methods on a handler object of the user's design. SAX is fast and efficient to implement, but difficult to use for extracting information at random from the XML, since it tends to burden the application author with keeping track of what part of the document is being processed. It

1008-726: Is a language for making assertions about the presence or absence of patterns in an XML document. It typically uses XPath expressions. Schematron is now a standard (Part 3: Rule-based validation of ISO/IEC 19757 – DSDL ). DSDL (Document Schema Definition Languages) is a multi-part ISO/IEC standard (ISO/IEC 19757) that brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NG full and compact syntax, Schematron assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based routing of document fragments to different validators. DSDL schema languages do not have

1071-578: Is an XML industry data standard. XML is used extensively to underpin various publishing formats. One of the applications of XML is in the transfer of Operational meteorology (OPMET) information based on IWXXM standards. The material in this section is based on the XML Specification . This is not an exhaustive list of all the constructs that appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use. XML documents consist entirely of characters from

1134-498: Is better suited to situations in which certain types of information are always handled the same way, no matter where they occur in the document. Pull parsing treats the document as a series of items read in sequence using the iterator design pattern . This allows for writing of recursive descent parsers in which the structure of the code performing the parsing mirrors the structure of the XML being parsed, and intermediate parsed results can be used and accessed as local variables within

1197-442: Is not permitted because the null character is one of the control characters excluded from XML, even when using a numeric character reference. An alternative encoding mechanism such as Base64 is needed to represent such characters. Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. Comments begin with <!-- and end with --> . For compatibility with SGML ,

1260-816: Is published. Some services deliver RSS to an email inbox, sending updates from user's personal selection and schedules. Examples of such services include IFTTT , Zapier and others. Conversely, some services deliver email to RSS readers. Further services like e. g. Gmane allow to subscribe to feeds via NNTP . It may be noted that email clients such as Thunderbird supports RSS natively. Both RSS and Atom are widely supported and are compatible with all major consumer feed readers. RSS gained wider use because of early feed reader support. Technically, Atom has several advantages: less restrictive licensing, IANA -registered MIME type , XML namespace, URI support, RELAX NG support. The following table shows RSS elements alongside Atom elements where they are equivalent. Note:

1323-496: Is to extend the basic XML schema established for more robust syndication of content. This inherently allows for more diverse, yet standardized, transactions without modifying the core RSS specification. To accomplish this extension, a tightly controlled vocabulary (in the RSS world, "module"; in the XML world, "schema") is declared through an XML namespace to give names to concepts and relationships between those concepts. Some RSS 2.0 modules with established namespaces are: Although

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1386-511: The .NET Framework , and the DOM traversal API (NodeIterator and TreeWalker). RSS RSS ( RDF Site Summary or Really Simple Syndication ) is a web feed that allows users and applications to access updates to websites in a standardized , computer-readable format. Subscribing to RSS feeds can allow a user to keep track of many different websites in a single news aggregator , which constantly monitors sites for new content, removing

1449-509: The Internet . It is a textual data format with strong support via Unicode for different human languages . Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures , such as those used in web services . Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many application programming interfaces (APIs) to aid

1512-662: The Meta Content Framework . RDF Site Summary, the first version of RSS, was created by Dan Libby , Ramanathan V. Guha, and Eckart Walther at Netscape . It was released in March 1999 for use on the My.Netscape.Com portal. This version became known as RSS 0.9. In July 1999, Dan Libby of Netscape produced a new version, RSS 0.91, which simplified the format by removing RDF elements and incorporating elements from Dave Winer 's news syndication format. Libby also renamed

1575-756: The RSS Advisory Board with Brent Simmons and Jon Udell , a group whose purpose was to maintain and publish the specification and answer questions about the format. In September 2004, Stephen Horlander created the now ubiquitous RSS icon ( [REDACTED] ) for use in the Mozilla Firefox browser . In December 2005, the Microsoft Internet Explorer team and Microsoft Outlook team announced on their blogs that they were adopting Firefox's RSS icon. In February 2006, Opera Software followed suit. This effectively made

1638-458: The Unicode repertoire. Except for a small number of specifically excluded control characters , any character defined by Unicode may appear within the content of an XML document. XML includes facilities for identifying the encoding of the Unicode characters that make up the document, and for expressing characters that, for one reason or another, cannot be used directly. Unicode code points in

1701-457: The asterisk character (*) indicates that an element must be provided (Atom elements "author" and "link" are only required under certain conditions). Several major sites such as Facebook and Twitter previously offered RSS feeds but have reduced or removed support. Additionally, widely used readers such as Shiira , FeedDemon, and particularly Google Reader , have all been discontinued as of 2013, citing declining popularity in RSS. RSS support

1764-410: The infoset augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these. A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term "XML" is used to refer to XML together with one or more of these other technologies that have come to be seen as part of

1827-429: The XML core. Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude , XLink , and XPointer . The design goals of XML include, "It shall be easy to write programs which process XML documents." Despite this, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset specification provides

1890-555: The XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes. DTD technology is still used in many applications because of its ubiquity. A newer schema language, described by the W3C as the successor of DTDs, is XML Schema , often referred to by the initialism for XML Schema instances, XSD (XML Schema Definition). XSDs are far more powerful than DTDs in describing XML languages. They use

1953-434: The allowable parent/child relationships. The oldest schema language for XML is the document type definition (DTD), inherited from SGML. DTDs have the following benefits: DTDs have the following limitations: Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining entities , which are arbitrary fragments of text or markup that

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2016-621: The base language for communication protocols such as SOAP and XMPP . It is one of the message exchange formats used in the Asynchronous JavaScript and XML (AJAX) programming technique. Many industry data standards, such as Health Level 7 , OpenTravel Alliance , FpML , MISMO , and National Information Exchange Model are based on XML and the rich features of the XML schema specification. In publishing, Darwin Information Typing Architecture

2079-401: The behavior of programs that process HTML , which are designed to produce a reasonable result even in the presence of severe markup errors. XML's policy in this area has been criticized as a violation of Postel's law ("Be conservative in what you send; be liberal in what you accept"). The XML specification defines a valid XML document as a well-formed XML document which also conforms to

2142-547: The board revised their version of the specification to confirm that namespaces may extend core elements with namespace attributes, as Microsoft has done in Internet Explorer 7. According to their view, a difference of interpretation left publishers unsure of whether this was permitted or forbidden. RSS is XML -formatted plain text. The RSS format itself is relatively easy to read both by automated processes and by humans alike. An example feed could have contents such as

2205-535: The cache. Submitted to the World Wide Web Consortium (W3C) in March 1997 for consideration as a web standard , CDF marked Microsoft's attempt to capitalize on the push technology trend led by PointCast . The most notable implementation of CDF was Microsoft's Active Desktop , an optional feature introduced with the Internet Explorer 4.0 browser in September 1997. Smart Offline Favorites

2268-423: The case of C1 characters, this restriction is a backwards incompatibility; it was introduced to allow common encoding errors to be detected. The code point U+0000 (Null) is the only character that is not permitted in any XML 1.1 document. The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover

2331-492: The company, also removing documentation and tools that supported the format. Two parties emerged to fill the void, with neither Netscape's help nor approval: The RSS-DEV Working Group and Dave Winer, whose UserLand Software had published some of the first publishing tools outside Netscape that could read and write RSS. Winer published a modified version of the RSS 0.91 specification on the UserLand website, covering how it

2394-429: The data structure and contain metadata . What is within the tags is data, encoded in the way the XML standard specifies. An additional XML schema (XSD) defines the necessary metadata for interpreting and validating XML. (This is also referred to as the canonical schema.) An XML document that adheres to basic XML rules is "well-formed"; one that adheres to its schema is "valid." IETF RFC 7303 (which supersedes

2457-442: The direct use of almost any Unicode character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the less-than sign, "<"). The following is a well-formed XML document including Chinese , Armenian and Cyrillic characters: The XML specification defines an XML document as a well-formed text, meaning that it satisfies

2520-523: The entire repertoire; well-known ones include UTF-8 (which the XML standard recommends using, without a BOM ) and UTF-16 . There are many other text encodings that predate Unicode, such as ASCII and various ISO/IEC 8859 ; their character repertoires are in every case subsets of the Unicode character set. XML allows the use of any of the Unicode-defined encodings and any other encodings whose characters also appear in Unicode. XML also provides

2583-498: The following ranges are valid in XML 1.0 documents: XML 1.1 extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F. At the same time, however, it restricts the use of C0 and C1 control characters other than U+0009 (Horizontal Tab), U+000A (Line Feed), U+000D (Carriage Return), and U+0085 (Next Line) by requiring them to be written in escaped form (for example U+0001 must be written as &#x01; or its equivalent). In

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2646-703: The following versions: The RSS 2.* branch (initially UserLand, now Harvard) includes the following versions: Later versions in each branch are backward-compatible with earlier versions (aside from non-conformant RDF syntax in 0.90), and both versions include properly documented extension mechanisms using XML Namespaces, either directly (in the 2.* branch) or through RDF (in the 1.* branch). Most syndication software supports both branches. "The Myth of RSS Compatibility", an article written in 2004 by RSS critic and Atom advocate Mark Pilgrim , discusses RSS version compatibility issues in more detail. The extension mechanisms make it possible for each branch to copy innovations in

2709-431: The following: When retrieved, RSS reading software could use the XML structure to present a neat display to the end users. There are various news aggregator software for desktop and mobile devices, but RSS can also be built-in inside web browsers or email clients like Mozilla Thunderbird . There are several different versions of RSS, falling into two major branches (RDF and 2.*). The RDF (or RSS 1.*) branch includes

2772-485: The format from RDF to RSS Rich Site Summary and outlined further development of the format in a "futures document". This would be Netscape's last participation in RSS development for eight years. As RSS was being embraced by web publishers who wanted their feeds to be used on My.Netscape.Com and other early RSS portals, Netscape dropped RSS support from My.Netscape.Com in April 2001 during new owner AOL 's restructuring of

2835-708: The functions performing the parsing, or passed down (as function parameters) into lower-level functions, or returned (as function return values) to higher-level functions. Examples of pull parsers include Data::Edit::Xml in Perl , StAX in the Java programming language, XMLPullParser in Smalltalk , XMLReader in PHP , ElementTree.iterparse in Python , SmartXML in Red , System.Xml.XmlReader in

2898-560: The name RDF Site Summary from RSS 0.9, reintroduced support for RDF and added XML namespaces support, adopting elements from standard metadata vocabularies such as Dublin Core . In December 2000, Winer released RSS 0.92 a minor set of changes aside from the introduction of the enclosure element, which permitted audio files to be carried in RSS feeds and helped spark podcasting . He also released drafts of RSS 0.93 and RSS 0.94 that were subsequently withdrawn. In September 2002, Winer released

2961-548: The need for the user to manually check them. News aggregators (or "RSS readers") can be built into a browser , installed on a desktop computer , or installed on a mobile device . Websites usually use RSS feeds to publish frequently updated information, such as blog entries, news headlines, episodes of audio and video series, or for distributing podcasts . An RSS document (called "feed", "web feed", or "channel") includes full or summarized text, and metadata , like publishing date and author's name. RSS formats are specified using

3024-474: The number of items in an RSS channel is theoretically unlimited, some news aggregators do not support RSS files larger than 150KB. For example, applications that rely on the Common Feed List of Windows might handle such files as if they were corrupt, and not open them. Interoperability can be maximized by keeping the file size under this limit. Podcasts are distributed using RSS. To listen to

3087-550: The older RFC 3023 ), provides rules for the construction of media types for use in XML message. It defines three media types: application/xml ( text/xml is an alias), application/xml-external-parsed-entity ( text/xml-external-parsed-entity is an alias) and application/xml-dtd . They are used for transmitting raw XML files without exposing their internal semantics . RFC 7303 further recommends that XML-based languages be given media types ending in +xml , for example, image/svg+xml for SVG . Further guidelines for

3150-475: The orange square with white radio waves the industry standard for RSS and Atom feeds, replacing the large variety of icons and text that had been used previously to identify syndication data. In January 2006, Rogers Cadenhead relaunched the RSS Advisory Board without Dave Winer's participation, with a stated desire to continue the development of the RSS format and resolve ambiguities. In June 2007,

3213-412: The other. For example, the RSS 2.* branch was the first to support enclosures , making it the current leading choice for podcasting, and as of 2005 is the format supported for that use by iTunes and other podcasting software; however, an enclosure extension is now available for the RSS 1.* branch, mod_enclosure. Likewise, the RSS 2.* core specification does not support providing full-text in addition to

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3276-645: The popularity from RSS to other formats having full namespace support.) Because neither Winer nor the RSS-DEV Working Group had Netscape's involvement, they could not make an official claim on the RSS name or format. This has fueled ongoing controversy in the syndication development community as to which entity was the proper publisher of RSS. One product of that contentious debate was the creation of an alternative syndication format, Atom, that began in June 2003. The Atom syndication format, whose creation

3339-449: The processing of XML data. The main purpose of XML is serialization , i.e. storing, transmitting, and reconstructing arbitrary data. For two disparate systems to exchange information, they need to agree upon a file format. XML standardizes this process. It is therefore analogous to a lingua franca for representing information. As a markup language , XML labels, categorizes, and structurally organizes information. XML tags represent

3402-487: The rules of a Document Type Definition (DTD). In addition to being well formed, an XML document may be valid . This means that it contains a reference to a Document Type Definition (DTD), and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies. XML processors are classified as validating or non-validating depending on whether or not they check XML documents for validity. A processor that discovers

3465-469: The string "--" (double-hyphen) is not allowed inside comments; this means comments cannot be nested. The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding. An example of a valid comment: <!--no need to escape <code> & such in comments--> XML 1.0 (Fifth Edition) and XML 1.1 support

3528-431: The three main versions of RSS in current use are 0.91, 1.0, and 2.0, constituting 13%, 17%, and 67% of worldwide RSS usage, respectively. These figures, however, do not include usage of the rival web feed format Atom. As of August 2008 , the syndic8.com website is indexing 546,069 total feeds, of which 86,496 (16%) were some dialect of Atom and 438,102 were some dialect of RSS. The primary objective of all RSS modules

3591-530: The use of XML in a networked context appear in RFC 3470 , also known as IETF BCP 70, a document covering many aspects of designing and deploying an XML-based language. XML has come into common use for the interchange of data over the Internet. Hundreds of document formats using XML syntax have been developed, including RSS , Atom , Office Open XML , OpenDocument , SVG , COLLADA , and XHTML . XML also provides

3654-472: The use of much more memory, but are often found more convenient for use by programmers; some include declarative retrieval of document components via the use of XPath expressions. XSLT is designed for declarative description of XML document transformations, and has been widely implemented both in server-side packages and Web browsers. XQuery overlaps XSLT in its functionality, but is designed more for searching of large XML databases . Simple API for XML (SAX)

3717-426: The vendor support of XML Schemas yet, and are to some extent a grassroots reaction of industrial publishers to the lack of utility of XML Schemas for publishing . Some schema languages not only describe the structure of a particular XML format but also offer limited facilities to influence processing of individual XML files that conform to this format. DTDs and XSDs both have this ability; they can for instance provide

3780-533: Was being used in his company's products, and claimed copyright to the document. A few months later, UserLand filed a U.S. trademark registration for RSS, but failed to respond to a USPTO trademark examiner's request and the request was rejected in December 2001. The RSS-DEV Working Group, a project whose members included Aaron Swartz , Guha and representatives of O'Reilly Media and Moreover , produced RSS 1.0 in December 2000. This new version, which reclaimed

3843-455: Was in part motivated by a desire to get a clean start free of the issues surrounding RSS, has been adopted as IETF Proposed Standard RFC   4287 . In July 2003, Winer and UserLand Software assigned the copyright of the RSS 2.0 specification to Harvard's Berkman Klein Center for Internet & Society , where he had just begun a term as a visiting fellow. At the same time, Winer launched

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3906-493: Was introduced in Internet Explorer 5.0 . CDF prefigured aspects of the RSS file format introduced by Netscape in March 1999, and of web syndication at large. Unlike RSS, CDF was never widely adopted and its use remained very limited. As a consequence, Microsoft removed CDF support from Internet Explorer 7 in 2006. A generic CDF file: XML The design goals of XML emphasize simplicity, generality, and usability across

3969-802: Was removed in OS X Mountain Lion 's versions of Mail and Safari , although the features were partially restored in Safari 8. Mozilla removed RSS support from Mozilla Firefox version 64.0, joining Google Chrome and Microsoft Edge which do not include RSS support, thus leaving Internet Explorer as the last major browser to include RSS support by default. Since the late 2010s there has been an uptick in RSS interest again. In 2018, Wired published an article named "It's Time for an RSS Revival", citing that RSS gives more control over content compared to algorithms and trackers from social media sites. At that time, Feedly

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