The Virtual Storage Access Method (VSAM)

VSAM Data Access Types

In sequential access, a series of records is retrieved in sequence. Sequence has a different meaning for each of the three VSAM data set organizations:

In an ESDS, sequential access means that a record is retrieved by its entry sequence.
In a KSDS, sequential access means that a record is retrieved by its key sequence.
In an RRDS, sequential access means that a record is retrieved by its relative-record sequence.

In all three cases, a record is located by its position relative to the last record accessed.

Direct Access

Introduction to Direct Access

With direct access, data storage or retrieval depends only on the location of the record and not on a reference to records previously accessed. Each record is stored or retrieved directly, according to its logical address (its key or its relative-record number, or RRN), or its address relative to the beginning of the data set (relative-byte address, or RBA). Thus, there are two direct access modes: keyed by key or relative-record number, and addressed by relative-byte address.

Keyed Direct Access

In keyed direct access, there are two methods in which records are retrieved or stored:

an index that relates the record's key to its relative location in the data set.
a relative-record number (RRN) that identifies the record that is wanted. The RRN is relative to the first record in the data set.

SAS supports keyed access to logical records in both KSDS and RRDS data sets. Keyed access to data records in KSDS data sets is by key; in RRDS data sets, keyed access is by the relative-record number.

Addressed Direct Access

In addressed direct access, the entire data set is treated as a continuous stream of bytes. A record is retrieved and stored directly by its address relative to the beginning of the data set (relative-byte address, or RBA), which is dependent on the record's location relative to records previously accessed. SAS supports addressed access to logical records in ESDS and KSDS data sets. It also supports addressed access (read-only) to control intervals in all three data set types.

Keyed Direct Access with an Alternate Index

An alternate key index, commonly called an alternate index (AIX), provides another way to access a VSAM data set. The advantage of an alternate index is that you effectively reorganize the data set instead of keeping separate copies organized in different ways for different applications. Suppose you have a KSDS with the employee number as the prime key. By building alternate indexes using employee names and department numbers, you can access the same data set in three ways: by employee name, by employee number, or by department number. The alternate key does not have to be unique; that is, there can be more than one record with the same alternate key.

The following figure illustrates an alternate index with nonunique keys over a KSDS. The base cluster records are sequenced by employee number, which is the prime key. The alternate index records are sequenced by department number, which is the alternate key. Each alternate index data record points to the prime key (employee number) in the base cluster. Note that because the alternate keys are nonunique, there can be multiple base records with the same department number.

KSDS with Alternate Index (Nonunique Alternate Keys)

[KSDS with Alternate Index (Nonunique Alternate Keys)]

Alternate indexes can be built over a KSDS or an ESDS. You define and build an alternate index using the IBM utility program Access Method Services (AMS). The data set over which an alternate index is built is called the base cluster. The alternate key can be any field having a fixed length and a fixed position within each record. The alternate index itself is a KSDS. The data component of an alternate index contains the alternate key, followed by a pointer to the appropriate record or records in the base cluster. In a KSDS, the pointer is the prime key; in an ESDS, the pointer is the RBA of the base record or records.

A path logically relates a base cluster and one of its alternate indexes. You define and name a path to access the base cluster records through a specific alternate index with AMS. See Using Alternate Indexes for VSAM Data Sets and IBM documentation for more information on defining alternate indexes.

Skip Sequential Access

A combination of both direct and sequential access can be used in a two-step process called skip sequential access. The process uses keyed direct access to find a starting point. After the initial record is obtained, additional records are retrieved sequentially. Skip sequential processing can be used with a KSDS, RRDS, and, if it has an alternate index, an ESDS.

Skip sequential processing is useful for the following reasons:

Skip sequential processing can improve performance and reduce overhead, because a simple sequential retrieval is faster than direct retrieval.
It enables you to find records when you know the key, RBA, or RRN of the first record you want, but do not know the key, RBA, or RRN of the subsequent records.
It enables you to process the data set sequentially, starting at a record other than the first.

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