ICC - Inter-Component Communication

Enhanced Definition

Inter-Component Communication (ICC) refers to the mechanisms and techniques used by different software components, programs, tasks, or address spaces within a z/OS system, or across multiple z/OS systems in a sysplex, to exchange data, signals, and control information. Its primary purpose is to enable cooperation and data sharing among disparate parts of an application or system, facilitating complex distributed processing.

Key Characteristics

- Scope: ICC can occur within a single address space (e.g., between different modules), across different address spaces on the same Logical Partition (LPAR), or across multiple LPARs within an IBM z/OS sysplex.
- Mechanisms: Various z/OS services facilitate ICC, including Cross-Memory Services (e.g., PC and ATSET macros), Message Queues, Shared Common Storage Area (CSA/ECSA), Global Resource Serialization (GRS), and the Cross-System Coupling Facility (XCF) for sysplex-wide communication.
- Synchronization: Communication can be synchronous (requiring a response before proceeding) or asynchronous (sending a message and continuing processing without waiting for an immediate reply).
- Performance: The performance characteristics of ICC vary significantly based on the chosen mechanism, with cross-memory services offering very high speed for local communication, while XCF involves more overhead for cross-system exchanges.
- Complexity: Implementing robust ICC often requires careful design to handle data integrity, serialization, error conditions, and recovery, especially in multi-threaded or multi-system environments.

Use Cases

- Application Integration: Different modules or programs within a complex application needing to share runtime data, status updates, or trigger specific actions in other components.
- Resource Sharing: Multiple address spaces or tasks needing to access and update a common data area or shared resource, such as a control block or a temporary data buffer.
- Workload Distribution: Distributing processing tasks or requests across multiple server address spaces or regions (e.g., CICS Multi-Region Operation - MRO) to balance load and improve throughput.
- System Management and Monitoring: Management agents communicating with managed subsystems or applications to collect performance metrics, issue commands, or receive alerts.
- Database Interaction: A COBOL application in one address space invoking a DB2 stored procedure that, in turn, might call another program or access data managed by a different component.

Related Concepts

ICC is fundamental to the operation of many core z/OS subsystems. It heavily relies on Address Spaces as the primary isolation boundary, with ICC mechanisms bridging these boundaries. For communication across multiple LPARs, the Cross-System Coupling Facility (XCF) is a foundational technology, providing high-speed, reliable communication and serialization services within a Sysplex. Subsystems like CICS extensively use ICC for Inter-Region Communication (IRC) and Multi-Region Operation (MRO), allowing transactions to flow between different CICS regions, and DB2 uses it for internal coordination and for applications to interact with its services. Serialization mechanisms like ENQ/DEQ and GRS are crucial alongside ICC to ensure data integrity when shared resources are accessed concurrently.

Best Practices:

Choose the Right Mechanism: Select the most appropriate ICC method based on the communication scope (intra-address space, cross -memory, cross-system), data volume, frequency, and performance requirements. For example, use cross -memory for high-speed local data exchange and XCF for sysplex-wide events.
Implement Robust Error Handling: Design for potential communication failures, timeouts, and component unavailability. Include retry logic, fallback mechanisms, and clear error reporting to ensure application resilience.
Minimize Overhead: Avoid excessive or chatty communication, especially across LPARs. Batch data transfers where possible and optimize message sizes to reduce CPU and I/O overhead.
Ensure Data Integrity: Always employ proper serialization techniques (e.g., ENQ/DEQ macros, latches, locks) when sharing resources or data through ICC to prevent race conditions and data corruption.
Monitor and Tune: Regularly monitor the performance and resource consumption of ICC pathways. Use z/OS performance tools (e.g., RMF, SMF) to identify bottlenecks and optimize communication patterns.