In this module, we will explore how to integrate RPG with modern technologies, focusing on APIs and JSON. Understanding these concepts is crucial for developing applications that can communicate with other systems and services over the web.

An API (Application Programming Interface) is a set of rules and protocols for building and interacting with software applications. APIs allow different software systems to communicate with each other.

• Endpoints: Specific URLs where API services are accessed.
• HTTP Methods: Common methods include GET, POST, PUT, DELETE.
• Request and Response: Data sent to and received from the API.

JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write and easy for machines to parse and generate.

• Objects: Enclosed in curly braces {} and contain key-value pairs.
• Arrays: Enclosed in square brackets [] and contain a list of values.
• Values: Can be strings, numbers, objects, arrays, true, false, or null.

To interact with APIs in RPG, you typically use HTTP functions to send requests and handle responses. Let's go through a practical example.

We'll use a simple public API to fetch user data.

1. Define the API Endpoint:

   Dcl-S apiUrl Varchar(256) Inz('https://jsonplaceholder.typicode.com/users/1');
   

2. Send HTTP GET Request:

   Dcl-S httpResponse Varchar(32767);
   Dcl-S httpStatus Int(10);
   
   // Use HTTP API to send GET request
   httpStatus = http_url_get(apiUrl: httpResponse);
   

3. Check Response Status:

   If httpStatus = 200;
      // Successful response
      Dsply httpResponse;
   Else;
      // Handle error
      Dsply 'Error fetching data';
   EndIf;
   

To parse JSON data in RPG, you can use the JSON_TABLE function or a JSON parsing library.

Assume the response is:

1. Define Data Structure for JSON:

   Dcl-Ds userData Qualified;
      id Int(10);
      name Varchar(50);
      username Varchar(50);
      email Varchar(50);
   End-Ds;
   

2. Parse JSON Response:

   // Assuming a JSON parsing procedure 'parseJson' is available
   parseJson(httpResponse: userData);
   

3. Access Parsed Data:

   Dsply userData.name; // Displays "Leanne Graham"
   

Write an RPG program that fetches a list of users from the API https://jsonplaceholder.typicode.com/users and displays the name and email of each user.

• Incorrect URL: Ensure the API endpoint URL is correct.
• HTTP Methods: Use the correct HTTP method (GET, POST, etc.) as required by the API.
• Error Handling: Always check the response status and handle errors appropriately.
• JSON Parsing: Ensure the JSON structure matches the data structure in your RPG program.

In this section, we learned about APIs and JSON, how to send HTTP requests from RPG, and how to parse JSON responses. These skills are essential for integrating RPG applications with modern web services and external systems. In the next module, we will explore XML handling, another important data interchange format.