DCCTL environment
DCCTL is an IMS Transaction Manager subsystem that has no database components. A DCCTL environment is similar to the DB⁄DC environment. The primary difference is that a DCCTL control region owns no databases and does not service DL⁄I database calls.
DCCTL subsystems do not support the IMS catalog.
DCCTL, in conjunction with the IMS External Subsystem Attach Facility (ESAF) or the Db2 for z/OS® Recoverable Resource Manager Services Attach Facility (RRMS), provides a Transaction Manager facility to external subsystems (for example, Db2 for z/OS). In a DCCTL environment, transaction processing and terminal management is identical to transaction processing and terminal management in a DB⁄DC environment. DCCTL contains the programming support necessary for:
- Master terminal support
- Terminal network support
- Data communication
- Message handling
- Transaction processing
- Application program execution
- IMS command execution
DCCTL also supports online change, Message Format Service (MFS), Multiple Systems Coupling (MSC), and Database Recovery Control (DBRC).
All IMS commands are supported in a DCCTL environment except database commands and database-related keywords.
DBRC is required, and is used to maintain system log information for restart. DBRC in a DCCTL environment maintains logs only for transactions. External database subsystems must maintain their own database logs.
DCCTL consists of three address space types:
- Control region
- DBRC
- Dependent regions (up to 999)
Dependent regions and DBRC are subordinate to the control region.
The DCCTL control region contains three structural components:
- A data communication manager, which controls terminal states and input⁄output message traffic. It also contains security controls that prevent unauthorized access to DC resources.
- A message manager, which is the read⁄write and I⁄O interface between terminal input from the data communication manager and the scheduling services of the Transaction Manager.
- A Transaction Manager, which manages MPPs, BMPs, and IFPs, schedules application programs in those dependent regions, and owns and responds to the application programming interface (API).
Each manager (data communication manager, Transaction Manager, and message manager) controls the use of its resources and the recoverability of its resources during a system failure. Like DB⁄DC dependent regions, MPP, BMP, IFP, JMP, and JBP dependent regions are used by the Transaction Manager to schedule application programs.
The following figure represents a DCCTL environment that is attached to an external subsystem.
DCCTL coordinates the sync point recovery process with the connected external subsystems. DCCTL ensures that database updates and terminal messages are committed when an application program reaches a sync point.
Two methods exist for connecting a DCCTL control region to another subsystem. You can have DCCTL use the control region EXEC parameter, SSM, to select the PROCLIB member. Or, you can use the /START SUBSYSTEM SSM command, which allows DCCTL to connect to other subsystems even though you did not request this option when you started IMS.
You can also specify the dependent region EXEC parameter, SSM, for dependent regions. The control region SSM member definition allows the dependent region to select one or more external subsystem connections. The SSM member can contain no definitions (null members) to prevent a connection to an external subsystem.
After you use the /START SUBSYSTEM SSM command, you must stop active dependent regions and then restart them if they require an external subsystem connection.
After IMS has established a connection between the dependent region and the external subsystem, a thread is created between the connected regions. The thread is used for subsequent application program calls, committing the data, or in failure situations, backing out the data.
The application programs managed by DCCTL are identical to those managed by the DC manager and TM manager in the DB⁄DC environment.
Related reading:
- For more information about the ESAF and RRSAF interfaces, see Accessing external systems from within IMS.
- For more information on using the ⁄START SUBSYSTEM SSM command, see IMS Version 15.4 Commands, Volume 2: IMS Commands N-V.