Options and restrictions relating to access types in DMS

Not all access types are possible with all access methods/file types. The following table provides an overview of the relationship between access types and access methods/file types:

Access type	SAM access method	ISAM access method	UPAM access method
Sequential	Reading/writing for SAM files. Physical record sequence.Shared update processing is not possible.	Reading/writing for ISAM files. Record sequence determined by primary key.Shared update processing is possible for reading or adding to opened files.	Reading/writing for PAM, SAM and ISAM files. Physical block sequence. Shared update processing is possible only for PAM files opened for reading.
Random	Reading/writing for SAM files. When records of variable length are overwritten, the record to be written must be of the same length as the record to be overwritten. Shared update processing is not possible.	Not possible.	Reading/writing for PAM, SAM and ISAM files. Shared update processing is possible only for PAM files opened as “INPUT” or “INOUT”.
Keyed	Not possible.	Reading/writing for ISAM files. Shared update processing is possible for files opened as “INPUT” or “INOUT”. Only the first opening application may open “OUTIN”.	Not possible.

Table 1: Overview of the relationship between access types and access methods/file types inthe DMS file system