Interface

Enhanced Definition

In the mainframe context, an **interface** serves as a defined point of interaction or communication between distinct software components, hardware devices, or a user and a system. It specifies the methods, protocols, and data formats required for interaction, enabling different parts of a complex system to connect and exchange information in a structured manner.

Key Characteristics

- Standardized Communication: Interfaces define a contract for interaction, ensuring that components can communicate reliably by adhering to agreed-upon rules, data structures, and invocation methods.
- Abstraction Layer: They often hide the internal complexity of a component, presenting a simplified view to the outside world. For example, a program uses a CALL to a CICS service without needing to know the internal CICS implementation details.
- Defined Protocols and APIs: Interfaces are typically implemented through Application Programming Interfaces (APIs), system calls (SVCs), macros, or specific communication protocols (e.g., SNA, TCP/IP sockets).
- Hardware and Software Manifestations: Interfaces exist at various levels, from hardware interfaces (e.g., I/O channels, device controllers) to software interfaces (e.g., program-to-program, program-to-OS, user-to-application).
- Input/Output Specification: An interface explicitly defines the expected inputs (parameters, data structures) and the anticipated outputs (return codes, results, updated data) for any interaction.

Use Cases

- Application-to-Application Communication: A COBOL batch program might use a CALL statement to invoke a subprogram, or a CICS transaction program might communicate with a DB2 database using SQL statements, where SQL acts as the interface.
- Program-to-System Services: Application programs interact with the z/OS operating system through SVC (Supervisor Call) instructions or system macros (e.g., GETMAIN for storage allocation, OPEN for file access), which are direct interfaces to OS services.
- User-to-System Interaction: Users interact with mainframe systems via interfaces like TSO/ISPF for command-line and panel-driven access, or 3270 terminal emulators for CICS and IMS transactions.
- Hardware Device Control: Channel programs provide a low-level interface for the CPU to communicate with and control I/O devices (e.g., disk drives, tape drives, printers) through channel command words (CCWs).
- Network Communication: VTAM (Virtual Telecommunications Access Method) provides an interface for applications to communicate over SNA networks, while TCP/IP sockets provide an interface for network communication using TCP/IP protocols.