CICS/400 uses the OS/400 Data Management facilities to control access to physical files for CICS/400 applications.
CICS/400 emulates the VSAM support required by CICS file control using the OS/400 file structure. CICS/400 uses the following file types:
– Key-sequenced (KSDS)
A KSDS has each of its records identified by a key; that is, a field at a predefined position in the file. To find the physical location of a record, emulated VSAM creates and maintains an index. This index is updated whenever you add or delete records. The use of indexes is supported through OS/400 logical files. – Entry-sequenced (ESDS) An ESDS has each of its records identified by its displacement from the beginning of the file that is by its relative byte address (RBA). After a record has been added to a file, its RBA stays constant.New records are added to the end of the file. You cannot delete records, nor alter their lengths.
– Relative record (RRDS)
An RRDS has fixed-length slots in which records can be stored. Each slot has a unique relative record number (RRN). Each record is therefore identified by its RRN. Records can be inserted or deleted without affecting the position of other data records. File functions available are reading, writing, updating, and deleting, plus sequential reading, that is, browsing in CICS/400 terms, both forwards and backwards through the file.
A relational database manager is provided as an integral part of OS/400. It can be accessed through OS/400 file operations or through the IBM DB2/400 Query Manager and SQL Development Kit. SQL statements may be embedded in application program source code. The CICS/400 precompiler automatically invokes the SQL precompiler when EXEC SQL statements are detected within the source code.
Under CICS/400, distributed relational database support is not available. There is no DL/I support on the OS/400. To access an IMS or DL/I database on a mainframe CICS system, you use either the distributed transaction processing function, or the distributed program link function.
Temporary storage is a mechanism provided by CICS/400 for storing data that must be available to more than one transaction.Data in temporary storage tends to be short-lived, with the emphasis on ease of storage and retrieval. You need not allocate temporary storage until it is required and you keep it only for as long as necessary.
Data is written to temporary storage in the form of a queue of items. These items can be read by any number of application programs in any order, and may be updated. The queue is created when the first item is written to it, and is held in an emulated VSAM file. You have the option of creating the queue in main storage or in auxiliary temporary storage.
Transient data is another storage queue mechanism provided by CICS/400. Data created by one transaction can be held in a transient data queue for use by the same transaction or another. Transient data queues might be used for capturing input data from several workstations and applications. When the number of records in the queue reaches a predefined limit, the transaction that processes those records can be started automatically to update a database or create hardcopy output on a printer, for example. This is known as automatic transaction initiation (ATI).
Data is stored sequentially, and therefore transient data is only suitable for those application programs that need to process data sequentially. Unlike temporary storage, there is no random retrieval or updating of records in transient data queues. Also, records can be read only once. Transient data queues are known as destinations.
The CICS/400 printer spooling facility provides support for writing data to OS/400 print spools,thus allowing an application to be customized to the user’s specific environment for printed output.This facility may be used for output to Intelligent Printer Data Stream™ (IPDS) and other advanced function printers. As an alternative to printing, the OS/400 printer spool facility supports output to diskette.
CICS/400 commands may be used within application programs to support spooling, and OS/400 facilities are used to create the spool files.
OS/400 journal management facility, if used, automatically records all data changes to files. OS/400 can be used for reading and processing journal files; there is no equivalent function within CICS/400. OS/400 also provides the system journaling log. OS/400 journals and journal receivers are an integral part of OS/400 commitment control, which is used by CICS/400 for recovery and sync point management.
CICS/400 supports user journaling only through CICS/400 commands.This facility provides automatic journaling for records read from and written to application files. User journal files can be accessed using OS/400 facilities and non-CICS programs. Journals can be archived either automatically or under operator control.
A syncpoint is a point in an application program when changes to resources are committed. The CICS/400 syncpoint control facility performs commit and backout processing. Support includes the provision of commands for inclusion in the application program to mark explicit syncpoints. Implicit syncpoints occur at normal and abnormal task termination. When a syncpoint occurs, either explicitly or implicitly, the CICS/400 syncpoint control facility issues the corresponding OS/400 commit or rollback operation.
When CICS systems are linked by protected conversations, syncpoint control can extend to the resources on all participating systems. The CICS systems will cooperate to ensure that resources are consistently committed or rolled back, even in the event of most system or conversation failures.
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