Call Level Interface

CLI

Enhanced Definition

In the context of mainframe databases like DB2 for z/OS, a Call Level Interface (CLI) is a standardized application programming interface (API) that allows applications to access database services using function calls, rather than embedded SQL statements. It provides a language-independent way for programs to connect to a database, execute SQL statements, and retrieve results dynamically at runtime.

Key Characteristics

- Dynamic SQL Execution: Unlike embedded SQL which is precompiled, CLI allows applications to construct and execute SQL statements dynamically during program execution.
- Language Independence: CLI APIs are designed to be callable from various programming languages, including C/C++, COBOL, and REXX, promoting application portability.
- Standardization: DB2 CLI is largely compliant with the Open Database Connectivity (ODBC) standard, making it easier for applications to connect to different relational databases.
- Function-Based API: Database operations (connection, statement preparation, execution, fetching results) are performed by calling specific functions provided by the CLI driver.
- Runtime Binding: SQL statements are bound to the database at runtime, offering flexibility in application design.

Use Cases

- Developing Client/Server Applications: Building applications (e.g., C/C++ or Java via JDBC/ODBC bridges) that run on distributed platforms and connect to DB2 for z/OS.
- Dynamic Query Tools: Creating tools or utilities that allow users to enter arbitrary SQL queries against a DB2 database without recompiling the application.
- Data Warehousing and Reporting: Developing applications that extract, transform, and load (ETL) data into or from DB2, often requiring dynamic SQL for flexible data manipulation.
- Middleware and Gateway Development: Implementing database access layers or gateways that translate requests from various clients into DB2 CLI calls.
- COBOL Applications with Dynamic SQL: Modern COBOL applications can use DB2 CLI to execute SQL statements that are not known until runtime, providing greater flexibility than static embedded SQL.

Related Concepts

DB2 CLI is a specific implementation of a database API, providing an alternative to Embedded SQL for accessing DB2 for z/OS. It is closely related to ODBC (Open Database Connectivity) and JDBC (Java Database Connectivity), with DB2 CLI often serving as the underlying driver for ODBC connections to DB2. It enables client/server architectures by allowing distributed applications to communicate with the mainframe DB2 subsystem.

Best Practices:

Resource Management: Explicitly allocate and free CLI handles (environment, connection, statement) to prevent resource leaks and ensure efficient database access.
Error Handling: Implement robust error checking after every CLI function call, using SQLGetDiagRec or SQLGetDiagField to retrieve detailed diagnostic information.
Prepared Statements: For SQL statements executed multiple times with different parameter values, use prepared statements (`SQLPrepare