Green Screen Modernization
Replacing the 5250 green screen interface is usually the first modernization step organizations take, because it is visible, relatively low-risk, and does not require touching the underlying RPG or COBOL logic. Screen wrapping, rehosting, and native web front ends are the three main approaches.
The green screen is the most visible part of an AS400 system to end users, and it is usually the first thing a modernization initiative targets, both because the payoff is immediately visible to non-technical stakeholders and because it can often be done without rewriting the application logic behind it.
Why Start With the Interface
Interface modernization is popular as a first step for a practical reason: it is possible to change how a system looks without changing what it does. The 5250 protocol separates screen presentation from the underlying program logic, which means a well-designed modernization approach can replace the visual layer while leaving decades of tested RPG or COBOL business logic completely untouched.
Screen Wrapping and Rehosting
Screen wrapping (sometimes called screen scraping or rehosting) intercepts the 5250 data stream and re-renders it as a web page or GUI, without modifying the underlying program. This is the fastest and lowest-risk approach, since the existing application continues to run exactly as before; only the presentation layer changes. The tradeoff is that a wrapped screen is still fundamentally constrained by the original screen's field layout and navigation flow, which limits how far the interface can genuinely improve without deeper work.
Native Web Front Ends
A more involved approach builds a genuinely new web or mobile front end that calls into existing RPG or COBOL programs through defined interfaces, rather than simply re-rendering the old screen. This requires more upfront development than screen wrapping, but produces an interface unconstrained by the original 5250 layout, and it is often paired with API integration work so the same underlying logic can serve both the new interface and other systems.
Choosing an Approach
Screen wrapping is typically the right choice for internal, back-office applications where the primary goal is a faster, less intimidating interface for existing staff. Native front-end development is typically the right choice for customer-facing or high-visibility applications where the interface itself is a competitive factor. Many organizations use both: wrapping for internal tools, native development for anything customers or partners interact with directly.