Extensible Metadata Platform (XMP) Specification - Adobe Partners

3
XMP Storage Model
External Storage of Metadata
External Storage of Metadata
It is recommended that XMP metadata be embedded in the file that the metadata describes.
There are cases where this is not appropriate or possible, such as database storage models,
extremes of file size, or due to format and access issues. Small content intended to be
frequently transmitted over the Internet might not tolerate the overhead of embedded
metadata. Archival systems for video and audio might not have any means to represent the
metadata. Some high-end digital cameras have a proprietary, non-extensible file format for
“raw” image data and typically store EXIF metadata as a separate file.
If metadata is stored separately from content, there is a risk that the metadata can be lost. The
question arises of how to associate the metadata with the file containing the content.
Applications should:
● Write the external file as a complete well-formed XML document, including the leading
XML declaration.
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The file extension should be .xmp. For Mac OS, optionally set the file’s type to 'TEXT'.
If a MIME type is needed, use application/rdf+xml.
● Write external metadata as though it were embedded and then had the XMP Packets
extracted and catenated by a postprocessor.
● If possible, place the values of the xmpMM:DocumentID, xmpMM:InstanceID, or other
appropriate properties within the file the XMP describes, so that format-aware applications
can make sure they have the right metadata.
For applications that need to find external XMP files, look in the same directory for a file with
the same name as the main document but with an .xmp extension. (This is called a sidecar
XMP file.)
36 September 2005 XMP Specification