A CICS® file is a representation of a data set on DASD. If you specify that the file is to use data table services, CICS copies the contents of the data set into an MVS™ data space when the file is opened and uses that copy whenever possible.
Because of the way that the data table services access the records, they can be used only with a VSAM key-sequenced data set (KSDS). The KSDS is called the source data set. The copy in memory is called the data table. The process of copying the records is called loading the data table.
When the file is read by a CICS application, the record is normally retrieved from the data table. When the file is updated by a CICS application, the effect depends on the type of data table that you have defined for the file.
CICS data table services support two types of data table:
A CICS-maintained data table is a data table whose records are automatically reflected in the source data set; when you update the file, CICS changes both the source data set and the data table.
A CICS-maintained data table is easy to implement--you need to know little about the data table services, you do not need to change your existing application programs, and full recovery support of the file is retained. CICS-maintained data tables are discussed in more detail in CICS-maintained data tables. A data set being accessed in Record Level Sharing (RLS)mode cannot be used as the source for a CICS-maintained data table. The source data set must be accessed in non-RLS mode.
A user-maintained data table is a data table whose records are not automatically reflected in the source data set; when you update the file, CICS changes only the data table.
A user-maintained data table lets you optimize the benefits of using a data table by allowing you to eliminate activity on the source data set, for update requests as well as read requests.
A small number of file operations are not supported for user-maintained data tables. Thus, you might need to make minor changes to existing application programs. Also, recovery of the file is supported after a transaction failure, but not a system failure. User-maintained data tables are discussed in more detail in User-maintained data tables.
A base VSAM KSDS accessed in either non-RLS or RLS mode can be used as the source data set for a user-maintained data table. You might want to make an RLS-mode data set the source of a user-maintained data table if you have other file definitions that access the data set and the data set is updated by other CICS regions.