Disconnect
In the mainframe context, a **disconnect** refers to the termination of an established logical or physical connection between two entities, such such as a user terminal and an application, an application and a database, or two network components. It involves releasing resources associated with the connection and ending the communication session.
Key Characteristics
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- Resource Release: Upon disconnection, system resources (e.g., memory, control blocks, file handles, database locks) held by the connection are typically released, making them available for other processes.
- Session Termination: A logical session, which represents an ongoing interaction, is ended. This can be between a 3270 terminal and a CICS region, a batch job and a DB2 subsystem, or an IMS client and an IMS control region.
- Initiation Methods: Disconnects can be initiated explicitly by a user (e.g., logging off), programmatically by an application (e.g.,
DISCONNECTSQL statement, CICSRETURNwithLOGOFF), or implicitly by system events (e.g., timeout, network failure, abnormal program termination). - Graceful vs. Abrupt: A graceful disconnect ensures all pending operations are completed, and resources are properly released. An abrupt disconnect (e.g., network cable pulled, system crash) can leave resources locked or in an inconsistent state, potentially requiring recovery actions.
- Impact on Transactions: For transactional systems like CICS or DB2, a disconnect often implies the end of a transaction or unit of work, potentially triggering commit or rollback operations to maintain data integrity.
Use Cases
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- CICS Terminal Session Termination: A 3270 terminal user logs off from a CICS application, causing the CICS session to terminate and the terminal to disconnect from the CICS region.
- DB2 Application Disconnection: A COBOL batch program or an online CICS transaction completes its database operations and explicitly issues a
DISCONNECT CURRENTorRELEASEstatement to sever its connection to the DB2 subsystem, releasing database resources. - IMS Transaction Completion: After an IMS transaction processes a message and sends a response, the logical terminal (LTERM) might be disconnected or returned to an available pool, ending the specific interaction.
- VTAM/TCP/IP Session End: A TN3270 client closes its emulator window, leading to the termination of the TCP/IP session and the underlying VTAM logical unit (LU) session with the mainframe application.
- MQ Queue Manager Disconnect: An application program finishes sending or receiving messages and disconnects from the IBM MQ queue manager, releasing the connection handle.
Related Concepts
Disconnect is intrinsically linked to sessions (e.g., CICS sessions, VTAM sessions, DB2 connections) and resource management. It is the counterpart to connect or establish session, marking the end of a communication lifecycle. In transactional systems, a disconnect often coincides with the completion of a unit of work or transaction, ensuring data integrity through commit/rollback mechanisms. It also relates to error handling and recovery, as unexpected disconnects can necessitate specific procedures to clean up orphaned resources or restore data consistency. Network protocols like VTAM and TCP/IP manage the underlying physical and logical connections that are subject to disconnection.