Checking the stored file

An original explanatory article about digital preservation.

A file can remain in a folder for years while the question of whether its contents have changed goes unanswered. Digital preservation therefore includes checks on the stored data, rather than relying only on a reassuring filename.

A checksum is a value calculated from a file. Preservation systems can retain a reference value and calculate the checksum again later. Comparing the results provides a way of detecting changes during transfer or storage. Repeating the check makes this an ongoing activity rather than a single inspection on arrival.

A difference requires attention, but the checksum does not point to the exact part of the file which changed. It supplies a signal for investigation. Keeping a known good additional copy can make recovery possible when unwanted alteration is found; the checksum itself is not a replacement copy.

Deliberate change also needs to be recognised. Converting material into another file format changes the data being checked. A preservation record should not confuse that planned action with an unexplained failure. A new reference checksum can support later checks of the converted file.

The value of the process is its repeated connection between stored material and a recorded baseline. A digital collection needs evidence about its condition, as well as somewhere to put it.

Explain why checksum checks are important in the preservation process described in the article.