CGI - Common Gateway Interface

Enhanced Definition

CGI (Common Gateway Interface) is a standard protocol that defines how an information server, such as a web server running on z/OS, communicates with external executable programs to generate dynamic content. It allows web servers to offload the task of processing user requests and generating responses to external applications, enabling interactive web experiences from mainframe data and logic.

Key Characteristics

- Standard Protocol: Provides a well-defined interface for web servers to execute external programs and exchange data via environment variables and standard input/output streams.
- Language Agnostic: CGI programs on z/OS can be written in various languages, including REXX, COBOL, C/C++, Perl, and shell scripts, leveraging existing mainframe programming skills.
- Process-Based Execution: For each incoming web request, the web server typically forks a new process to execute the CGI program, which can lead to significant overhead and resource consumption under heavy load.
- Environment Variables: Web server passes request-specific information (e.g., HTTP headers, query strings, server variables) to the CGI program through environment variables.
- Standard I/O: The CGI program receives POST data via standard input (stdin) and sends its HTTP response (including headers and body) to the web server via standard output (stdout).

Use Cases

- Dynamic Web Page Generation: Retrieving real-time data from mainframe databases (DB2, IMS) or files (VSAM) and formatting it into HTML for display in a web browser.
- Web Form Processing: Handling user input submitted through HTML forms, validating data, updating mainframe records, and providing immediate feedback.
- Legacy Application Integration: Exposing existing COBOL or Assembler business logic to web users by wrapping it in a CGI program, allowing web access without rewriting core applications.
- Generating Reports On-Demand: Creating customized reports based on user criteria and serving them as HTML, PDF, or CSV files directly from z/OS.

Related Concepts

CGI programs are typically executed by the IBM HTTP Server for z/OS, which acts as the front-end web server. These programs run within the z/OS UNIX System Services (USS) environment, utilizing its process management and file system capabilities. While CGI can invoke traditional COBOL or REXX programs, it often serves as a bridge to access mainframe data sources like DB2, IMS, or VSAM. For more scalable and performant dynamic web applications, WebSphere Application Server for z/OS with Java servlets or z/OS Connect EE are often preferred modern alternatives to traditional CGI.

Best Practices:

Minimize Resource Usage: Due to the process overhead, design CGI programs to be as efficient as possible, minimizing execution time and resource consumption.
Input Validation and Security: Rigorously validate all user input to prevent common web vulnerabilities like cross-site scripting (XSS), SQL injection, and buffer overflows.
Error Handling and Logging: Implement robust error handling and comprehensive logging within CGI programs to aid in debugging and monitoring, writing logs to z/OS UNIX files or SYSOUT.
Consider Alternatives: For new development or high-volume applications, evaluate more modern and efficient alternatives like FastCGI, Java servlets running in WebSphere Application Server for z/OS, or z/OS Connect EE.
Secure File Permissions: Ensure that CGI scripts and any data files they access have appropriate z/OS