CNOS - Change Number of Sessions

Enhanced Definition

CNOS (Change Number of Sessions) is an APPC (Advanced Program-to-Program Communication) verb or function used to dynamically negotiate and modify the number of parallel sessions that can be established between two LU 6.2 partners. It allows applications to adjust session capacity on the fly without requiring a restart of the underlying communication infrastructure. CNOS (Change Number of Sessions) is an APPC/APPN verb that allows for the dynamic modification of the number of parallel LU 6.2 sessions between two logical units. It provides a mechanism to adjust session limits between partner applications without requiring a full system restart or deactivation/reactivation of the LUs involved.

Key Characteristics

- Dynamic Negotiation: Allows session limits to be changed *on the fly* without requiring system or application restarts, providing flexibility in resource management.
- APPC/LU 6.2 Specific: Exclusively applies to APPC (Advanced Program-to-Program Communication) using LU 6.2 (Logical Unit Type 6.2) protocols, which are fundamental for peer-to-peer communication on z/OS.
- Session Capacity Adjustment: Modifies the maximum number of concurrent conversations (sessions) that can exist between two specific Logical Units (LUs).
- Bidirectional Control: Either LU involved in the communication can initiate a CNOS request, and the negotiation process determines the mutually agreed-upon session limit.
- VTAM/z/OS Role: In a z/OS environment, VTAM (Virtual Telecommunications Access Method) manages the underlying LU 6.2 connections and processes CNOS requests, ensuring proper network resource allocation.
- Pre-allocation Negotiation: The negotiation occurs *before* individual sessions are allocated, setting the upper bound for subsequent ALLOCATE requests.

Use Cases

- Workload Balancing: Dynamically increasing session limits for critical APPC links (e.g., CICS-to-CICS or CICS-to-IMS) during peak transaction processing hours to handle higher volumes.
- Resource Conservation: Decreasing session limits during off-peak hours to free up VTAM control blocks, network buffers, and other system resources.
- Application Scaling: Adjusting the number of available sessions when new application instances are brought online or taken offline, requiring more or fewer concurrent connections.
- Troubleshooting and Maintenance: Temporarily reducing session counts to isolate issues related to resource contention or session management, or to prepare for planned outages.
- Capacity Planning: Using CNOS to test the impact of different session limits on application performance and system resource utilization without system reconfigurations.

Related Concepts

CNOS operates within the framework of APPC (Advanced Program-to-Program Communication) and LU 6.2 (Logical Unit Type 6.2), which define the peer-to-peer communication protocols for distributed transaction processing on z/OS. It relies on VTAM (Virtual Telecommunications Access Method) to manage the underlying network connections and session establishment. While CNOS changes the *potential* number of sessions, the actual allocation and deallocation of individual sessions are handled by APPC verbs like ALLOCATE and DEALLOCATE within application programs (e.g., COBOL programs in CICS or IMS). It complements static session definitions in VTAM application major nodes by providing dynamic flexibility.

Best Practices:

Monitor Session Usage: Regularly monitor actual session utilization using VTAM display commands (D NET,ID=lu_name,SCOPE=ALL) to determine optimal CNOS values and avoid over-allocation or under-allocation of resources.
Coordinate with Partners: Ensure that both communicating LU 6.2 partners are configured to handle CNOS requests and have compatible session limits defined to prevent negotiation failures.
Implement Graceful Changes: When reducing session counts, allow existing sessions to complete naturally before the new, lower limit takes effect to avoid disrupting active transactions.
Automate Adjustments: Consider automating CNOS requests based on workload metrics, time-of-day schedules, or system events to proactively optimize resource usage and performance.
Document Configurations: Clearly document the default and dynamically adjusted CNOS settings for critical APPC links to aid in troubleshooting, capacity planning, and compliance.