Entry-sequenced file organization
The records in an entry-sequenced file are store up in the order they are written into the file. New records are for all time appended to the end of the file. When records are removed from an entry-sequenced file the space formerly allocated to those records is not automatically reclaimed or reused. The only means to reclaim this space is by using the sfs_ReorganizeFile function. Entry-sequenced files are often used when records in the file will be accessed in the order that they are written to the file. This type of file organization is frequently used for log files, audit trail files, or for any other files that keep time-sequenced records of events.
Entry-sequenced files are able to contain fixed-length or variable-length records. When an existing record is updated then the updated record must be no longer than the original record. Every record in an entry-sequenced file has an entry sequence number (ESN) which are corresponds to the order in which it was inserted into the file. The ESN isn't part of the record, and is therefore said to be implicit. Because the value of the primary index key for entry-sequenced files isn't part of the record, merely the name of the primary index must be specified when creating an entry-sequenced file in SFS. It isn't necessary to supply names of fields on which to base the index.
When records are read by an entry-sequenced file a sequential scan on the primary index (ESN) for the file returns records in the order in which they were inserted regardless of whether they have subsequently been updated. Records that have been deleted don't show up when an entry-sequenced file is scanned.