CID - Connection Identifier

Enhanced Definition

In the context of IBM z/OS, a Connection Identifier (CID) is a unique alphanumeric name used primarily within CICS (Customer Information Control System) to identify a specific logical connection. It facilitates communication between CICS regions (Multi-Region Operation - MRO) or between a CICS region and an external client or batch job (External CICS Interface - EXCI). A CID, or Connection Identifier, is a unique token used primarily within IBM z/OS environments, especially CICS, to identify a specific connection or thread between an application region (like CICS) and an external resource manager (like DB2 or IBM MQ). It allows the resource manager to track and manage the resources associated with that particular connection.

Key Characteristics

- Uniqueness: Each CID must be unique within the CICS region where it is defined, representing a distinct communication path.
- Purpose: Primarily used to identify MRO (Multi-Region Operation) or APPC (Advanced Program-to-Program Communication) links between CICS regions, or EXCI connections from non-CICS address spaces.
- Definition: CIDs are typically defined in CICS resource definitions, specifically CONNECTION and SESSION resources, which specify the network attributes and session parameters.
- Usage: Referenced by CICS system components and application programs (via SYSID) to route transactions, data, and program links to remote systems.
- Format: Usually an 8-character alphanumeric name, adhering to standard CICS naming conventions.
- Scope: Can represent a connection to another CICS region, a VTAM Logical Unit (LU), or a specific EXCI client.

Use Cases

- Multi-Region Operation (MRO): A CICS application in one region needs to invoke a program or access a file in another CICS region. The CID identifies the logical link to the target region for Distributed Program Link (DPL) or Function Shipping.
- External CICS Interface (EXCI): A z/OS batch job, a z/OS UNIX System Services application, or another non-CICS address space needs to initiate a CICS transaction or link to a CICS program. The CID identifies the connection to the target CICS region.
- Distributed Transaction Processing (DTP): Two CICS regions or a CICS region and an APPC partner engage in a synchronized, two-phase commit transaction. The CID identifies the communication path for these APPC conversations.
- CICSplex Workload Management: In a CICSplex, CIDs are fundamental for defining the communication paths that allow CICSPlex SM to route work requests to the most appropriate target CICS region.

Related Concepts

A CID is a crucial component of CICS intercommunication. It works in conjunction with SYSID (System Identifier), a 4-character name used by application programs to refer to a remote system, which in turn maps to a CONNECTION definition identified by a CID. CIDs are underpinned by VTAM (Virtual Telecommunications Access Method) LU (Logical Unit) definitions for network connectivity and enable advanced features like MRO, APPC, and EXCI for distributed processing across the z/OS enterprise.

Best Practices:

Descriptive Naming: Assign meaningful CIDs (e.g., CICSPROD, EXCIBATCH) to clearly indicate the purpose or target of the connection, aiding in management and troubleshooting.
Consistent Definitions: Ensure that CID definitions and their associated CONNECTION and SESSION resources are consistently configured across all interconnected CICS regions to avoid communication failures.
Resource Monitoring: Regularly monitor the status and usage of CIDs and their underlying CONNECTION resources to identify potential bottlenecks or inactive links.
Security Implementation: Apply appropriate CICS security (e.g., RACF profiles for CICSCONNECTION resources) to control which users or address spaces can utilize specific CIDs, thereby securing inter-region communication.
Error Handling: Design CICS applications to include robust error handling for DPL or EXCI calls, specifically addressing SYSIDERR or LENGERR conditions that might arise from issues with the identified CID.