Modernization on IBM i doesn’t require abandoning RPG — it requires extending it. One of the most effective ways to do that is by building IBM i REST APIs.
In 2026, APIs are the foundation of integration. They allow IBM i applications to connect with web apps, mobile platforms, cloud services, and third-party tools — all without rewriting core business logic.
This guide walks through how RPG developers can start building REST APIs on IBM i.
What Are IBM i REST APIs?
A REST API (Representational State Transfer API) allows systems to communicate over HTTP using standard methods like:
- GET (retrieve data)
- POST (create data)
- PUT (update data)
- DELETE (remove data)
On IBM i, REST APIs expose existing RPG programs and data so they can be accessed by external applications.
Why RPG Developers Should Build APIs
Building APIs directly in RPG provides several advantages:
- No need to rewrite existing programs
- Faster development using familiar tools
- Lower infrastructure complexity
- Greater control over business logic
- Seamless integration with modern systems
This approach allows IBM i teams to modernize without disruption.
Step 1: Identify the Right Use Cases
Not every program needs to be an API. Start with high-value use cases such as:
- Customer data lookup
- Order entry and tracking
- Inventory availability
- Pricing and product information
Focus on processes that benefit from real-time access.
Step 2: Prepare Your RPG Programs
Before exposing a program as an API:
- Ensure it has clear input and output parameters
- Remove dependencies on interactive screens
- Structure it for modular reuse
- Validate and sanitize inputs
Well-structured RPG programs make API development smoother.
Step 3: Define API Endpoints
Each API should have a clear endpoint and purpose.
Examples:
/api/customers/api/orders/api/inventory
Use consistent naming conventions and keep endpoints intuitive.
Step 4: Handle JSON Data
Modern APIs use JSON for data exchange.
Your IBM i environment should:
- Convert RPG data structures to JSON
- Parse incoming JSON requests
- Validate data formats
This ensures compatibility with modern applications.
Step 5: Implement Security
Security is critical when exposing IBM i functionality.
Best practices include:
- Use HTTPS for all API communication
- Implement authentication (API keys or tokens)
- Restrict access based on roles
- Log and monitor API usage
Security should be built into your API from the start.
Step 6: Test Your APIs
Before deploying:
- Test endpoints with tools like Postman
- Validate responses and error handling
- Ensure performance under load
- Confirm security controls are working
Thorough testing prevents issues in production.
Step 7: Deploy and Monitor
Once your API is live:
- Monitor usage and performance
- Track errors and failures
- Optimize for speed and scalability
- Update endpoints as needed
APIs should evolve alongside your business needs.
Common Challenges (and How to Avoid Them)
Overcomplicating APIs
Keep endpoints simple and focused.
Ignoring Security
Always implement authentication and encryption.
Poor Documentation
Provide clear documentation for developers and users.
Lack of Monitoring
Track usage to ensure reliability and performance.
How IBM i REST APIs Support Modernization
REST APIs enable IBM i systems to:
- Power web and mobile applications
- Integrate with cloud platforms
- Support real-time data exchange
- Extend existing business logic
They are the bridge between traditional systems and modern technology.
Final Thoughts
For RPG developers, REST APIs are one of the most practical ways to modernize IBM i.
By starting small, focusing on high-value use cases, and following best practices, organizations can unlock new capabilities without disrupting existing systems.
Ready to start building IBM i REST APIs?
Kato Integrations provides tools and expertise to help you simplify API development and accelerate modernization.