Overview of shared data tables
The CICS® shared data table facility is an extension of the CICS file management services. Using shared data tables, all files that are defined as data tables can potentially be shared using cross-memory services. No changes are required to the file definitions for existing data tables.
The concept of shared data tables exploits the fact that it is more efficient:
- To use MVS cross-memory services instead of CICS function shipping to share a file of data between two or more CICS regions in the same MVS image.
- To access data from memory instead of from DASD.
- To access a file of data from memory using services integrated within CICS file management instead of using VSAM services and a local shared resource (LSR) pool.
The two versions of data tables are:
- Shared data tables
- Coupling facility data tables (CFDTs) (see CICS coupling facility data tables for details)
Benefits of shared data tables
The use of cross-memory services is one of the major benefits of shared data tables. This improves the performance of applications that currently use function shipping and makes file sharing feasible for applications that cannot accept the performance overhead of function shipping.
The other major enhancement is that nearly all read requests are supported for use with data
tables. This enhancement extends the use of data tables to applications that include:
- Browse requests
- Read requests that use an imprecise key
Besides the use of cross-memory services, shared data tables offer benefits for performance and security:
- Very large reductions in path length can be achieved for remote accesses because function shipping is avoided for most read and browse requests.
- When cross-memory services are used, the requests are processed by the AOR, thus freeing the FOR to process other requests. This increases multiprocessor exploitation.
- Increased security of data is provided because the record information in shared data tables is stored outside the CICS region and is not included in CICS system dumps (either formatted or unformatted).
- For CICS-maintained data tables, all forms of non-update, keyed access (including browse requests and imprecise-key read requests) are processed by reference to the data table.
- For user-maintained data tables, all forms of non-update, keyed access (including browse requests and imprecise-key read requests) are supported.
- Any number of files referring to the same source data set that are open at the same time can retrieve data from the one CICS-maintained data table.
- An enhancement to the XDTRD user exit allows you to skip over a range of records while loading the data table.