VSAM data sets: KSDS, ESDS, RRDS

Key-sequenced data set (KSDS)

A key-sequenced data set has each of its records identified by a key. (The key of each record is a field in a predefined position within the record.) Each key must be unique in the data set.

When the data set is initially loaded with data, or when new records are added, the logical order of the records depends on the collating sequence of the key field. This also fixes the order in which you retrieve records when you browse through the data set.

To find the physical location of a record in a KSDS, VSAM creates and maintains an index. This relates the key of each record to the record's relative location in the data set. When you add or delete records, this index is updated accordingly.

With releases of DFSMS/MVS 1.4 and later, a data set can be greater than 4 GB in size if it is defined as extended format and extended addressability in the storage class. CICS supports, in both RL and non-RLS mode, KSDS data sets that are defined with these extended attributes.

Entry-sequenced data set (ESDS)

An entry-sequenced data set is one in which each record is identified by its relative byte address (RBA).

Records are held in an ESDS in the order in which they were first loaded into the data set. New records added to an ESDS always go after the last record in the data set. You cannot delete records or alter their lengths. After a record has been stored in an ESDS, its RBA remains constant. When browsing, records are retrieved in the order in which they were added to the data set.

A standard RBA is an unsigned 32 bit number. The use of a 32 bit RBA means that a standard ESDS cannot contain more than 4 GB of data. However, there is a different ESDS that supports 64 bit extended relative byte addresses (XRBAs) and which is therefore not subject to the 4 GB limit. This type of ESDS is called an extended format, extended addressing ESDS data set. For brevity, we will refer to it as an extended addressing ESDS, or as an extended ESDS. From CICS TS for z/OS®, Version 3.2 onwards, CICS supports 64 bit XRBAs and extended ESDS data sets.

Relative record data set (RRDS)

A relative record data set has records that are identified by their relative record number (RRN). The first record in the data set is RRN 1, the second is RRN 2, and so on.

Records in an RRDS can be fixed or variable length records, and the way in which VSAM handles the data depends on whether the data set is a fixed or variable RRDS. A fixed RRDS has fixed-length slots predefined to VSAM, into which records are stored. The length of a record on a fixed RRDS is always equal to the size of the slot. VSAM locates records in a fixed RRDS by multiplying the slot size by the RRN (which you supply on the file control request), to calculate the byte offset from the start of the data set.

A variable RRDS, can accept records of any length up to the maximum for the data set. In a variable RRDS VSAM locates the records with an index.

A fixed RRDS generally offers better performance. A variable RRDS offers greater function.

With releases of DFSMS/MVS 1.5 and later, a data set can be greater than 4 GB in size if it is defined as extended format and extended addressability in the storage class. CICS supports access to extended RRDS or VRRDS data sets if you use an RRN that can be specified in a four byte RRN field to access the records that reside beyond the 4 GB boundary.

Empty data sets

An empty data set is a data set that has not yet had any records written to it. VSAM imposes several restrictions on an empty data set that is opened in non-RLS access mode. However, CICS hides all these restrictions from you, allowing you to use an empty data set in the same way as a data set that contains data, regardless of the access mode.

VSAM alternate indexes

Sometimes you want to access the same set of records in different ways. For example, you can have records in a personnel data set that have as their key an employee number. No matter how many Smiths you have, each of them has a unique employee number. Think of this as the primary key.

If you were producing a telephone directory from the data set, you would want to list people by name rather than by employee number. You can identify records in a data set with a secondary (alternate) key instead of the primary key described previously. So the primary key is the employee number, and the employee name is the alternate key. Alternate keys are just like the primary key in a KSDS—fields of fixed length and fixed position within the record. You can have any number of alternate keys per base file and, unlike the primary or base key, alternate keys need not be unique.

To continue the personnel example, the employee's department code might be defined as a further alternate key.

VSAM allows KSDS and ESDS (but not RRDS or extended ESDS) data sets to have alternate keys. When the data set is created, one secondary or alternate index is built for each alternate key in the record and is related to the primary or base key. To access records using an alternate key, you must define a further VSAM object, an alternate index path. The path then behaves as if it were a KSDS in which records are accessed using the alternate key.

When you update a record by way of a path, the corresponding alternate index is updated to reflect the change. However, if you update the record directly by way of the base, or by a different path, the alternate index is only updated if it has been defined to VSAM (when created) to belong to the upgrade set of the base data set. For most applications, you probably want your alternate index to be in the upgrade set.

A CICS application program disregards whether the file it is accessing is a path or the base. In a running CICS system, access to a single base data set can be made by way of the base and by any of the paths defined to it, if each access route is defined to CICS.

It is also possible for a CICS application program to access a file that has been directly defined as an alternate index rather than a path. This results in index data being returned to the application program rather than file data. This operation is not supported for files opened in record-level sharing (RLS) mode.