Logical Records and Visible Records

As mentioned before, at an abstract level, DLIS file consists of multiple logical records (LRs). They can be viewed as abstract units, containing a specific type of data and/or metadata.

On the other hand, in physical sense, a DLIS file is divided into visible records (VRs). They are byte structures of pre-defined format, consisting of a 4-byte header (which includes a visible record start mark and record length) and a body (which can be filled with any bytes carrying data and/or metadata, coming from the logical records).

Visible records have a limited length, which is often lower than that of logical records. In this case, the contents of a logical record can be split among several visible records’ bodies. The logical record segments (parts of the split logical record) contain additional header information indicating e.g. whether the given segment is the first and/or the last one of the original logical record. (In case a logical record fits entirely into a single visible record, its body is also wrapped in a logical record segment structure, with indication that the given segment is both the first and the last part of the original logical record.)

The maximum length of a VR is defined in the file’s Storage Unit Label. According to the standard, the minimum length is not explicitly defined, but because the minimum length of a LR segment is 16 bytes (including 4 LR segment header bytes), the resulting minimum length of a VR is 20 bytes.