Drain

Enhanced Definition

In the mainframe context, "draining" refers to the controlled process of stopping new work from being accepted by a system, subsystem, or resource, while allowing all currently active or in-flight work to complete normally. This ensures an orderly shutdown, resource release, or transition without data loss or transaction integrity issues. In mainframe computing, particularly within z/OS, CICS, IMS, and other subsystems, "draining" refers to the process of allowing all currently active work (e.g., transactions, messages, jobs) to complete processing while preventing any new work from starting. It's an orderly cessation of activity for a specific resource or component, typically in preparation for maintenance, shutdown, or reconfiguration.

Key Characteristics

- Controlled Cessation: It's a deliberate, managed process to cease operations, not an abrupt termination.
- No New Work: The system or resource stops accepting new requests, transactions, or tasks once the drain process begins.
- Completion of Existing Work: All work that was active or queued *before* the drain commenced is allowed to finish processing.
- Resource Release: Once all active work completes, the resource (e.g., a CICS region, an IMS control region, a DB2 subsystem, a queue) becomes quiescent and can be safely taken offline, shut down, or modified.
- Command-Driven: Draining is typically initiated via specific operator commands or system utilities (e.g., CICS CEMT PERFORM SHUTDOWN, IMS /PGR).

Use Cases

- CICS Region Shutdown: Before shutting down a CICS region for maintenance or upgrades, it's drained to allow active transactions to complete, preventing data integrity issues and ensuring a clean shutdown.
- IMS Control Region Termination: An IMS control region is drained to ensure all dependent regions and active transactions finish before the control region is brought down, maintaining database consistency.
- DB2 Subsystem Quiesce: A DB2 subsystem or specific database is often "quiesced" (a form of draining) before applying maintenance, taking a backup, or performing recovery, ensuring no new updates occur while existing ones complete.
- Queue Management: Draining a message queue (e.g., in MQSeries) means processing all existing messages before the queue is cleared, redefined, or taken offline, preventing message loss.
- Application Server Shutdown: In environments with multiple z/OS application servers, one might be drained to remove it from a workload distribution group, allowing its active tasks to complete before it's taken offline for maintenance.

Related Concepts

Draining is fundamental to system availability and data integrity during planned outages. It contrasts sharply with an abend (abnormal end) or a forced termination, which immediately stops all activity and can lead to data inconsistencies or recovery challenges. It is a critical step in maintenance procedures, disaster recovery planning, and resource management, ensuring that critical subsystems like CICS, IMS, and DB2 can be brought down and up gracefully.

Best Practices:

Always Drain Before Shutdown: For critical production systems, always initiate a drain process before a full shutdown to preserve data integrity and ensure an orderly transition.
Monitor Drain Progress: Actively monitor the number of active transactions or tasks during a drain (e.g., using CEMT INQ TASK in CICS) to ensure it's progressing as expected and to identify any hung processes.
Communicate Outages: Inform users and dependent systems about planned drains and associated outages to manage expectations and minimize business impact.
Automate Where Possible: Incorporate drain commands into automated shutdown scripts or procedures to ensure consistency, reduce human error, and streamline maintenance windows.
Understand Drain Options: Be aware of different drain options (e.g., CICS SHUTDOWN, IMMEDIATE, DUMP) and their implications for transaction completion, recovery, and the overall shutdown duration.