Http Client in COBOL

Our program will demonstrate how to create a simple HTTP client using COBOL. Here’s the full source code:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. HTTP-CLIENT.
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT RESPONSE-FILE ASSIGN TO "response.txt"
               ORGANIZATION IS LINE SEQUENTIAL.
       DATA DIVISION.
       FILE SECTION.
       FD RESPONSE-FILE.
       01 RESPONSE-LINE PIC X(80).
       WORKING-STORAGE SECTION.
       01 WS-URL PIC X(50) VALUE "https://example.com".
       01 WS-COMMAND PIC X(100).
       01 WS-STATUS PIC 9(3).
       01 WS-EOF PIC X VALUE 'N'.
       01 WS-COUNTER PIC 9 VALUE 1.
       
       PROCEDURE DIVISION.
       MAIN-PROCEDURE.
           PERFORM SEND-REQUEST.
           PERFORM READ-RESPONSE.
           STOP RUN.

       SEND-REQUEST.
           STRING "curl -o response.txt " WS-URL 
               DELIMITED BY SIZE INTO WS-COMMAND.
           CALL "SYSTEM" USING WS-COMMAND.

       READ-RESPONSE.
           OPEN INPUT RESPONSE-FILE.
           PERFORM UNTIL WS-EOF = 'Y' OR WS-COUNTER > 5
               READ RESPONSE-FILE
                   AT END
                       MOVE 'Y' TO WS-EOF
                   NOT AT END
                       DISPLAY RESPONSE-LINE
                       ADD 1 TO WS-COUNTER
               END-READ
           END-PERFORM.
           CLOSE RESPONSE-FILE.

This COBOL program demonstrates a basic HTTP client functionality. Here’s how it works:

1. We define a file to store the HTTP response.

2. In the WORKING-STORAGE SECTION, we declare variables for the URL, the system command to execute, and other control variables.

3. The MAIN-PROCEDURE calls two subroutines: SEND-REQUEST and READ-RESPONSE.

4. In SEND-REQUEST, we use the curl command to send an HTTP GET request to the specified URL and save the response to a file. This is done using the CALL "SYSTEM" statement, which allows us to execute system commands.

5. In READ-RESPONSE, we open the response file, read and display the first 5 lines (or until the end of the file), and then close the file.

To run this program:

1. Save the code in a file named http-client.cob.
2. Compile the program:

$ cobc -x http-client.cob

3. Run the compiled program:

$ ./http-client

This will send a GET request to https://example.com and display the first 5 lines of the response.

Note that this example uses the curl command, which must be available on your system. Also, error handling is minimal in this example and should be expanded for a production environment.

COBOL doesn’t have built-in HTTP client libraries like some modern languages, so we’re using system commands to achieve similar functionality. In a real-world scenario, you might use more sophisticated methods or third-party libraries to handle HTTP requests in COBOL.