Call Level Interface (CLI) Programming Method

Enhanced Definition

The Call Level Interface (CLI) is a standard programming interface that allows applications to access and manipulate data in a relational database (such as DB2 for z/OS) using a set of function calls, rather than embedding SQL statements directly into the application's source code. It provides a standardized, vendor-neutral API for database interaction, primarily facilitating dynamic SQL execution.

Key Characteristics

- Standardized API: Adheres to industry standards (e.g., X/Open SQL CAE, ODBC), providing a consistent interface across different database systems, though implementations can vary.
- Function-Based: Database operations are performed by calling specific functions (e.g., SQLAllocHandle, SQLExecDirect, SQLFetch) from a CLI library.
- Dynamic SQL: Primarily designed for executing SQL statements constructed at runtime, offering flexibility for ad-hoc queries and dynamic data access.
- Runtime Binding: SQL statements are prepared and executed at runtime, contrasting with embedded SQL which typically requires a precompilation step.
- Language Independence: While often associated with C/C++ applications, CLI can be used by various programming languages that can call external functions.
- Driver-Dependent: Relies on a specific database driver (e.g., the DB2 CLI driver for z/OS) to translate generic CLI calls into database-specific operations.

Use Cases

- Dynamic Query Tools: Developing applications that allow users to construct and execute arbitrary SQL queries against DB2 for z/OS without recompilation.
- Middleware and Gateways: Building database middleware or gateways that provide a common access layer to DB2 for z/OS from distributed client applications.
- C/C++ Applications on z/OS: Creating C or C++ programs that require flexible and dynamic interaction with DB2 databases on the mainframe.
- ODBC/JDBC Driver Implementations: CLI serves as the underlying mechanism for how ODBC and JDBC drivers connect to and interact with DB2 for z/OS from client applications on other platforms.
- Generic Database Utilities: Developing utilities that need to connect to and query various databases, including DB2 for z/OS, using a common programming model.

Related Concepts

CLI is a powerful alternative to Embedded SQL (e.g., EXEC SQL in COBOL or PL/I), which requires a precompiler to convert SQL statements into host language calls. While embedded SQL is often preferred for static, performance-critical transactions, CLI offers greater flexibility for dynamic SQL. It is fundamental to how DB2 for z/OS is accessed programmatically, especially for applications requiring runtime SQL construction. Furthermore, CLI forms the bedrock for higher-level database connectivity standards like ODBC (Open Database Connectivity) and JDBC (Java Database Connectivity), enabling distributed applications to connect to DB2 for z/OS.

Best Practices:

Robust Error Handling: Always check the return code of every CLI function call and use SQLGetDiagRec or SQLGetDiagField to retrieve detailed error and warning information for proper diagnostics.
Resource Management: Ensure that all allocated handles (environment SQLHENV, connection SQLHDBC, statement SQLHSTMT, descriptor SQLHDESC) are properly freed using SQLFreeHandle to prevent resource leaks.
Parameter Markers: Utilize parameter markers (?) in dynamic SQL statements to bind input values. This prevents SQL injection vulnerabilities and allows the database to reuse prepared statements, improving performance.
Transaction Control: Explicitly manage transactions using SQLSetConnectAttr to control auto-commit behavior and SQLCommit or SQLRollback to ensure data integrity.
Statement Caching: For frequently executed dynamic SQL statements, consider using statement caching mechanisms provided by the CLI driver or implementing your own to reduce the overhead of preparing statements repeatedly.