Rational Open Access: RPG Edition

Rational® Open Access: RPG Edition provides a way for RPG programmers to use the simple and well-understood RPG I/O model to access resources and devices that are not directly supported by RPG.

Open Access opens up RPG's file I/O capabilities, allowing anyone to write innovative I/O handlers to access other devices and resources such as:
  • Browsers
  • Mobile devices
  • Cloud computing resources
  • Web services
  • External databases
  • XML files
  • Spreadsheets
  • And more
An Open-Access application has three parts:
  1. An RPG program that uses standard RPG coding to define an Open Access file and use I/O operations against the file.
  2. A handler procedure or program that is called by Open Access to handle the I/O operations for the file.
  3. The resource or device that the handler is using or communicating with.

Open Access is the linkage between parts 1 and 2.

Open-Access handlers

Open Access does not provide the handlers.

Anyone can write the handlers that extend RPG IV's I/O capabilities to new resources and devices.
  • Software tool vendors
  • Business partners
  • Services organizations
  • Users

The provider of the handler can choose the RPG device type whose I/O operations best fit the functions provided by the handler. For example, a user-interface application could map to a WORKSTN file, an Excel document could map to a PRINTER file, and a web service could map to a keyed DISK file.

Two ways to approach Open Access

  1. The handler is written after the application is written.
    For example, an existing application that uses 5250 display files is modified to use Open Access for the WORKSTN files.
    • The RPG program is modified by adding the HANDLER keyword to the WORKSTN files
    • The handler must handle all the operations and requirements of the existing RPG program.
    • This type of handler will often be provided by an outside expert such as a software tool vendor or business partner.
  2. The handler is written before the application is written.
    For example, the RPG programmer wants to use a web service that returns information for a specific set of criteria.
    • The handler provider creates a keyed database file matching the web service with a field for each piece of information returned by the web service, and a key field for each criterion needed by the web service. This file will not hold any data; it will only be used for defining externally-described files and data structures in the RPG program and the handler.
    • The handler provider can tell the RPG programmer what I/O operations that the handler will support. For example, it might only support OPEN, CHAIN, CLOSE.
    • The RPG programmer codes the RPG program using the file as an externally described keyed DISK file, with the HANDLER keyword to identify the Open-Access handler.
    • The handler uses externally-described data structures defined from the same file.
    • This type of handler might be written by the same RPG programmer who uses the Open-Access file, or it might be provided by an outside expert.

How Open Access works

When an RPG program performs an I/O operation for a system file, a system data management function is called to handle the operation.

When an RPG program performs an I/O operation for an Open Access file, the Open-Access handler is called. The handler receives a data structure parameter with subfields that enable the handler to perform the correct I/O operation, and provide information back to the RPG program.

If the file is externally-described, it must be available to the RPG compiler at compile-time. Open Access does not require the file to be present at run-time, but an individual handler may require the file to be present.