An Entry Sequenced Data Set (ESDS) is a type of data set used by the VSAM computer data storage system.
A Key Sequenced Data Set (KSDS) is a type of data set used by the IBM VSAM computer data storage system.
In computer science, random access (more precisely and more generally called direct access) is the ability to access any item of data from a population of addressable elements roughly as easily and efficiently as any other, no matter how many elements may be in the set.
In computer science, sequential access means that a group of elements (such as data in a memory array or a disk file or on magnetic tape data storage) is accessed in a predetermined, ordered sequence.
Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS.