APPC - Advanced Program-to-Program Communication

Enhanced Definition

Advanced Program-to-Program Communication (APPC) is a peer-to-peer communication protocol within IBM's Systems Network Architecture (SNA) that enables application programs on different systems to communicate directly and exchange data. It provides a robust, high-level interface for distributed transaction processing, allowing programs to act as both clients and servers.

Key Characteristics

- Peer-to-Peer Communication: Unlike traditional host-terminal communication, APPC allows any two programs to communicate directly as equals, without a hierarchical relationship.
- LU 6.2 Implementation: APPC is the primary implementation of SNA's Logical Unit Type 6.2 (LU 6.2), which is specifically designed for program-to-program communication.
- Conversation-Based: Communication occurs within a defined "conversation" between two programs, which can be synchronous (requiring immediate response) or asynchronous (allowing deferred processing).
- Distributed Transaction Processing: APPC is fundamental for coordinating transactions that span multiple systems, often managed by a Transaction Manager like CICS.
- Built on SNA: It leverages the underlying SNA network services provided by VTAM on z/OS for reliable data transfer and network management.
- Common Programming Interface: Provides a consistent API for application developers to build distributed applications, abstracting the complexities of network communication.

Use Cases

- CICS-to-CICS Communication: Enabling transactions to flow seamlessly between CICS regions on different z/OS systems or other platforms, supporting distributed transaction processing.
- Distributed Database Access: Allowing applications on one system to access and update data in a DB2 database located on another z/OS system, often coordinated via Distributed Data Facility (DDF).
- Batch Job Initiation: A program on one z/OS system initiating a batch job or another application on a remote z/OS system.
- Client-Server Interactions: Mainframe applications acting as servers for distributed clients (e.g., PC applications) or vice-versa, using APPC for the underlying communication.
- Data Transfer Between Applications: Secure and reliable exchange of application-specific data between disparate systems, often for integration purposes.

Related Concepts

APPC is a crucial component of SNA (Systems Network Architecture), specifically leveraging LU 6.2 for its peer-to-peer capabilities. On z/OS, VTAM (Virtual Telecommunications Access Method) manages the underlying network connections and resources that APPC conversations utilize. It is extensively used by CICS (Customer Information Control System) for distributed transaction processing and by DB2 for distributed database access, enabling these subsystems to communicate reliably across the network and participate in multi-system operations.

Best Practices:

Secure APPC Connections: Implement robust security using RACF or equivalent external security managers to define access controls for APPC Logical Units (LUs) and transaction programs (TPs).
Monitor APPC Activity: Regularly use VTAM display commands (e.g., D NET,APPC) and analyze SMF records to monitor conversation status, resource utilization, and identify potential performance bottlenecks.
Design for Recovery: Ensure application programs are designed to handle APPC conversation failures gracefully, implementing appropriate rollback or retry mechanisms for distributed transactions to maintain data integrity.
Optimize VTAM Configuration: Tune VTAM buffer sizes and other APPC-related parameters to match network characteristics and application workload for optimal performance.
Use Qualified LU Names: Employ fully qualified LU names (e.g., NETID.LUNAME) to ensure uniqueness and proper routing in complex multi-network environments, especially when dealing with NETVIEW or other network management tools.