IAC - Inter Application Communication

Enhanced Definition

In the z/OS environment, Inter Application Communication (IAC) refers to the mechanisms and protocols that enable different applications or components, often residing in separate address spaces or even on different systems, to exchange data and invoke services. It is fundamental for building complex, distributed mainframe applications and integrating them with other platforms.

Key Characteristics

- Diverse Mechanisms: z/OS offers various IAC mechanisms, including APPC (Advanced Program-to-Program Communication), CPI-C (Common Programming Interface for Communications), IBM MQ, CICS MRO (Multi-Region Operation) and ISC (InterSystem Communication), and Cross-Memory Services.
- Synchronous and Asynchronous: IAC can be synchronous (requiring an immediate response, like a direct call) or asynchronous (message-based, where the sender doesn't wait for an immediate reply), depending on the chosen technology.
- Data Exchange: Facilitates the exchange of structured or unstructured data, often requiring data conversion or serialization/deserialization between different application contexts.
- Location Transparency: Many IAC mechanisms aim to provide a degree of location transparency, allowing applications to communicate without explicit knowledge of the partner application's physical location.
- Reliability and Security: Built-in features for reliable message delivery, transaction integrity, and robust security (e.g., RACF integration, AT-TLS for encryption) are critical considerations for mainframe IAC.

Use Cases

- CICS Transaction Routing: A CICS region routing a transaction to another CICS region (either local via MRO or remote via ISC over SNA or IPIC over TCP/IP) to access specific resources or services.
- Batch-to-Online Integration: A COBOL batch program submitting a request to an online CICS or IMS transaction using IBM MQ or APPC to update a real-time database or trigger an event.
- Distributed Data Access: An application on z/OS invoking a DB2 stored procedure that, in turn, communicates with an application on another platform (e.g., Linux, Windows) to retrieve or update data.
- Enterprise Service Bus (ESB) on z/OS: Using IBM MQ as the backbone for an ESB architecture, allowing various mainframe applications (CICS, IMS, Batch) to publish and subscribe to messages, integrating with external systems.
- Cross-Memory Communication: High-performance communication between applications in different address spaces on the *same* z/OS system, often used by system components or specialized applications for shared data access.

Related Concepts

IAC is foundational to distributed computing on z/OS. It heavily relies on underlying communication infrastructures like VTAM (for APPC, CPI-C, SNA) and TCP/IP (for MQ, CICS IPIC, Sockets). CICS MRO and ISC are specific implementations of IAC for CICS environments, while IBM MQ provides a robust, asynchronous messaging layer that decouples applications. `Cross