Fast Infoset specifies a binary encoding for the XML Information Set. An XML infoset [http://www.w3.org/TR/xml-infoset/] (such as a DOM [http://java.sun.com/j2se/1.4.2/docs/api/org/w3c/dom/package-summary.html] tree, StAX [http://jcp.org/en/jsr/detailid=173] events or SAX [http://java.sun.com/j2se/1.4.2/docs/api/org/xml/sax/package-summary.html] events in programmatic representations) may be serialized to an XML 1.x document or, as specified by Fast Infoset, may be serialized to a fast infoset document. Fast infoset documents are generally smaller in size and faster to parse and serialize than equivalent XML documents.

Analogies taken too far can lead to all sorts of misconceptions but one can think of Fast Infoset as gzip for XML. Like the gzip encoding, and unlike a schema-based encodings, a Fast Infoset document can be read only knowing that it was encoded using Fast Infoset. A major difference is that Fast Infoset optimizes the properties of size and performance, while gzip optimizes only the property of size.

The Fast Infoset specification is a standard from the ITU-T [http://www.itu.int/ITU-T/studygroups/com17/index.asp] and ISO [http://www.iso.org/iso/en/stdsdevelopment/tc/tclist/TechnicalCommitteeDetailPage.TechnicalCommitteeDetailCOMMID=34]. The standard has two different formal names, one from each standard organization: ITU-T Rec. X.891 | ISO/IEC 24824-1 (Fast Infoset), but it is commonly refered to by the term Fast Infoset. ITU-T approved the standard on May 14, 2005, while it is at the stage of a Final Committee Draft under the ISO process.

A common misconception is that Fast Infoset requires ASN.1 support. This is not true: although the formal description uses ASN.1 formalisms, ASN.1 support is not required by implementations. A document based on Annex D of ITU-T Rec. X.891 | ISO/IEC 24824-1 has been published [http://asn1.elibel.tm.fr/xml/example-of-Fast-Infoset-for-UBL.htm] that presents examples of encoding XML infosets as fast infoset documents.

The FI project is an open source (using .

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