Sidecar file

Sidecar files, also known as buddy files or connected files, are a method of storing data (often metadata) which is not supported by the source file format, in external files.

For each source file one or more sidecar files can be created. This is in contrast to "metadata databases" where one database contains metadata for several source files.

In most cases the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension. The problem with this system is that most operating systems and file managers have no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.

Examples

Various examples of sidecar files are:

* XMP sidecar files. For file formats that have no internal support for XMP data, the data is stored in separate .xmp files with the same base file name. Many photo cataloging applications have support for this file format.
* Windows connected files. When an HTML document is written to a local disk, all images, stylesheets and other connected resources are stored inside a subfolder that has the base name of the source HTML file with "_files" added. Windows Explorer will move or delete all files when either the source HTML file or the folder is moved or deleted. Applications that support this format are mainly Windows Explorer, Internet Explorer and Microsoft Word.
* AVI + THM. Many digital photocameras will store a .thm (thumbnail) file alongside recorded movies, with the same base filename as the movie file. These thumbnail files are plain JPEG files with another extension. This system allows for quickly displaying a still preview of the movie, and storing camera data which is not supported by the AVI file format.
* JPEG + WAV. Some digital photocameras allow for voice/audio annotations with photos. These are then stored as WAV audio files alongside the JPEG photo file, with the same base filename.
* Mac OS resource fork. The Mac OS operating system has internal support for metadata/resources that are not stored inside the file data. Because the support is build into the operating system, these resource forks will not show up as separate files and all applications support it. However, when files with a resource fork are copied over to a non-Mac OS disk format, such as an ISO-format CD-ROM or a MS-DOS compatible disk, this resource fork will be stored as a separate file alongside the main source file.

A variation of this are copies of the source file which contain largely the same information, but in a different format or from a previous version:

* Backup and versioning files; many applications create backup files alongside the main file, often with a .bak extension.
* .exf backup files. Since many JPEG editing software used to destroy Exif metadata stored in digital photos, some photo cataloging applications allow to extract the Exif data and store that in an .exf file, so that the metadata can later be re-inserted into the JPEG file.
* Raw + JPEG. Many digital photocameras allow to store both uncompressed raw data and a JPEG image file when shooting in "raw mode". This allows for faster previewing the photo, and support by applications that do not support the (often undocumented) raw format.

References

* [http://windowsitpro.com/article/articleid/75159/jsi-tip-3841-what-is-the-windows-2000-connected-file-feature.html Windows IT Pro - What is the Windows 2000 Connected File feature?]