Practical RPG: Encapsulating Your Data with Extension Files, Part 3

  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

Replace file specifications with your own field definitions.

In my previous articles on this topic, I introduced extension files as a way to future-proof a legacy database and showed how to encapsulate those files in SQL access to allow growth. But I left out one important detail: When the compiler sees a file specification, it defines all the fields in that file. If you remove the F-spec, you have to define those fields yourself. And that's even trickier if you already have an extension file.

Defining Fields Without F-Specs

Let's set the stage. In the previous article, I suggested a file name CUSMAS, with an extension file named CUSEXT. CUSEXT was used to hold an extension attribute named CXCNTY, which held the county value, and I suggested how to get the value of CXCNTY for a customer number stored in the variable iCust:

exec sql set :wCNTY = (select CXCNTY from CUSEXT where CXCUST = :iCust);

What I didn't show was how to define these variables. The customer number work field iCust is pretty easy. You've probably done this many times in your programs. We'll just define it like the customer number in the customer master (not the customer extension!).

dcl-s iCust like(CMCUST);                        

That works because the customer master is a legacy application file and I'm accessing it via traditional native I/O. So the fields all exist in the program, and I can easily create a LIKE field as shown. But remember, we don't have a file specification for the extension file; that's the whole point of encapsulating it in SQL! So how do we define fields for a file that isn't defined in the program?

I think the easiest way is to use a data structure. But we need to think about how we're going to do that. Let's start with the simplest case:

// Template variables (for SQL)                            

dcl-ds *n extname('CUSEXT') end-ds;

dcl-s wCNTY like(CXCNTY);                        

In this design, you end up with storage for each of the variables in the CUSEXT file, named the same as the fields in the file. That seems simple enough, but it has a hidden danger: Unless I examine the SQL statement associated with the file, I might assume that there are values in those fields, and in turn I might use them even though I haven't populated them. Even worse, I might be tempted to do use SELECT * in my SQL statement, which would then tie my program to the file structure the same as if I had a file specification (F-spec). Any time the extension file changed, I would have to recompile my program, and that's exactly the opposite of what I want!

So instead, I prefer to use templates. Unfortunately, this works only in ILE; we'll see the more primitive way we have to do this in RPG/400 a little later in the article. But assuming you’re using ILE, you can do something that's very clean:

// Template variables (for SQL)                            

dcl-ds T_CUSEXT extname('CUSEXT') template qualified end-ds;

dcl-s wCNTY like(T_CUSEXT.CXCNTY);                        

This defines all the fields in the extension file, but as part of a template and with qualified names. Remember that this file had two attributes (besides the key): CXCNTY and CXTXLK. With this template technique, those attributes are defined, but they are defined as qualified fields T_CUSEXT.CXCNTY and T_CUSEXT.CXTXLK. More than that, since they are defined as part of a template, even if I tried to use them I'd get a compiler error because they have no storage assigned! So as long as my SQL statement fills in all of the unqualified work fields, I have successfully encapsulated my extension file.

What If You Already Have an Extension File?

If you've already got one or more extension files, chances are that you access them with record-level access using good old file specifications. In order to implement truly encapsulated access, we'll have to modernize the access to those existing files. I actually originally developed the template technique to allow me to modernize existing programs that used direct CHAINs to the extension file. To accomplish this, I use a slightly different technique when defining my "work" fields:

// Template variables (for SQL)                            

dcl-ds T_CUSEXT extname('CUSEXT') template qualified end-ds;

dcl-s CXCNTY like(T_CUSEXT.CXCNTY);                        

Note that I create an unqualified field with the same name as the qualified field! I use a template and redefine the field as shown. That way, any statement in the program that used to access the field CXCNTY can still do so. Rather than having to go through the entire program and change the name of the field in every line that uses it, I just have to populate the unqualified field. The SQL changes slightly to match:

exec sql set :CXCNTY = (select CXCNTY from CUSEXT where CXCUST = :iCust);

It's actually a little more self-documenting to me; I set the field named CXCNTY in the program to the value from the column CXCNTY in the table. Now I just have to make sure I populate every field I define this way. For example, if I needed to add CXTXLK, I go through a couple of changes:

// Template variables (for SQL)                            

dcl-ds T_CUSEXT extname('CUSEXT') template qualified end-ds;

dcl-s CXCNTY like(T_CUSEXT.CXCNTY);                        

dcl-s CXTXLK like(T_CUSEXT.CXTXLK);                        

exec sql set (:CXCNTY, :CXTXLK) =

(select CXCNTY, CXTXLK from CUSEXT where CXCUST = :iCust);

I've grown pretty fond of this technique. The only time it can get a little cluttered is when you have many fields. In that case, you end up with a whole set of fields in the SET clause and then a matching set of fields in the SELECT clause. I played around with creating a data structure to define fields, but after a few attempts I decided I didn't like it, and instead I'll stick with this technique. If I need additional fields, I add their definitions under the template and then add them to the two places in the SQL statement.

SQL Access in RPG/400

As I said, your options for RPG/400 are more limited. You can simply create an externally described data structure and SELECT into the individual fields:

* CUSEXT external file


This has the problem that you might use a field that isn't being populated. Or you can define the fields yourself, but since you don't have the fields defined, you can't use *LIKE DEFN, so you just have to manually copy the attributes:

C           MOVE CXTXLK   CXTXLK 1

And that obviously has its own set of disadvantages. At the end of the day, I consider implementation of any SQL processing to be a good excuse to convert to ILE RPG. Which is a whole additional subject in itself!

Moving On

We now have a technique for defining extension attributes in our program and accessing them via SQL. We're very close to the final step in the process, which is learning how to add additional attributes without disrupting our users. And that's in the next article!

Joe Pluta

Joe Pluta is the founder and chief architect of Pluta Brothers Design, Inc. He has been extending the IBM midrange since the days of the IBM System/3. Joe uses WebSphere extensively, especially as the base for PSC/400, the only product that can move your legacy systems to the Web using simple green-screen commands. He has written several books, including Developing Web 2.0 Applications with EGL for IBM i, E-Deployment: The Fastest Path to the Web, Eclipse: Step by Step, and WDSC: Step by Step. Joe performs onsite mentoring and speaks at user groups around the country. You can reach him at

MC Press books written by Joe Pluta available now on the MC Press Bookstore.

Developing Web 2.0 Applications with EGL for IBM i Developing Web 2.0 Applications with EGL for IBM i
Joe Pluta introduces you to EGL Rich UI and IBM’s Rational Developer for the IBM i platform.
List Price $39.95

Now On Sale

WDSC: Step by Step WDSC: Step by Step
Discover incredibly powerful WDSC with this easy-to-understand yet thorough introduction.
List Price $74.95

Now On Sale

Eclipse: Step by Step Eclipse: Step by Step
Quickly get up to speed and productivity using Eclipse.
List Price $59.00

Now On Sale

More Articles By This Author
Related Articles


Support MC Press Online





  • White Paper: Node.js for Enterprise IBM i Modernization

    SB Profound WP 5539

    If your business is thinking about modernizing your legacy IBM i (also known as AS/400 or iSeries) applications, you will want to read this white paper first!

    Download this paper and learn how Node.js can ensure that you:
    - Modernize on-time and budget - no more lengthy, costly, disruptive app rewrites!
    - Retain your IBM i systems of record
    - Find and hire new development talent
    - Integrate new Node.js applications with your existing RPG, Java, .Net, and PHP apps
    - Extend your IBM i capabilties to include Watson API, Cloud, and Internet of Things

    Read Node.js for Enterprise IBM i Modernization Now!


  • Profound Logic Solution Guide

    SB Profound WP 5539More than ever, there is a demand for IT to deliver innovation.
    Your IBM i has been an essential part of your business operations for years. However, your organization may struggle to maintain the current system and implement new projects.
    The thousands of customers we've worked with and surveyed state that expectations regarding the digital footprint and vision of the companyare not aligned with the current IT environment.

    Get your copy of this important guide today!


  • 2022 IBM i Marketplace Survey Results

    Fortra2022 marks the eighth edition of the IBM i Marketplace Survey Results. Each year, Fortra captures data on how businesses use the IBM i platform and the IT and cybersecurity initiatives it supports.

    Over the years, this survey has become a true industry benchmark, revealing to readers the trends that are shaping and driving the market and providing insight into what the future may bring for this technology.

  • Brunswick bowls a perfect 300 with LANSA!

    FortraBrunswick is the leader in bowling products, services, and industry expertise for the development and renovation of new and existing bowling centers and mixed-use recreation facilities across the entertainment industry. However, the lifeblood of Brunswick’s capital equipment business was running on a 15-year-old software application written in Visual Basic 6 (VB6) with a SQL Server back-end. The application was at the end of its life and needed to be replaced.
    With the help of Visual LANSA, they found an easy-to-use, long-term platform that enabled their team to collaborate, innovate, and integrate with existing systems and databases within a single platform.
    Read the case study to learn how they achieved success and increased the speed of development by 30% with Visual LANSA.


  • The Power of Coding in a Low-Code Solution

    LANSAWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed.
    Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

    • Discover the benefits of Low-code's quick application creation
    • Understand the differences in model-based and language-based Low-Code platforms
    • Explore the strengths of LANSA's Low-Code Solution to Low-Code’s biggest drawbacks



  • Why Migrate When You Can Modernize?

    LANSABusiness users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.
    In this white paper, you’ll learn how to think of these issues as opportunities rather than problems. We’ll explore motivations to migrate or modernize, their risks and considerations you should be aware of before embarking on a (migration or modernization) project.
    Lastly, we’ll discuss how modernizing IBM i applications with optimized business workflows, integration with other technologies and new mobile and web user interfaces will enable IT – and the business – to experience time-added value and much more.


  • UPDATED: Developer Kit: Making a Business Case for Modernization and Beyond

    Profound Logic Software, Inc.Having trouble getting management approval for modernization projects? The problem may be you're not speaking enough "business" to them.

    This Developer Kit provides you study-backed data and a ready-to-use business case template to help get your very next development project approved!

  • What to Do When Your AS/400 Talent Retires

    FortraIT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators is small.

    This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn:

    • Why IBM i skills depletion is a top concern
    • How leading organizations are coping
    • Where automation will make the biggest impact


  • Node.js on IBM i Webinar Series Pt. 2: Setting Up Your Development Tools

    Profound Logic Software, Inc.Have you been wondering about Node.js? Our free Node.js Webinar Series takes you from total beginner to creating a fully-functional IBM i Node.js business application. In Part 2, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Attend this webinar to learn:

    • Different tools to develop Node.js applications on IBM i
    • Debugging Node.js
    • The basics of Git and tools to help those new to it
    • Using as a pre-built development environment



  • Expert Tips for IBM i Security: Beyond the Basics

    SB PowerTech WC GenericIn this session, IBM i security expert Robin Tatam provides a quick recap of IBM i security basics and guides you through some advanced cybersecurity techniques that can help you take data protection to the next level. Robin will cover:

    • Reducing the risk posed by special authorities
    • Establishing object-level security
    • Overseeing user actions and data access

    Don't miss this chance to take your knowledge of IBM i security beyond the basics.



  • 5 IBM i Security Quick Wins

    SB PowerTech WC GenericIn today’s threat landscape, upper management is laser-focused on cybersecurity. You need to make progress in securing your systems—and make it fast.
    There’s no shortage of actions you could take, but what tactics will actually deliver the results you need? And how can you find a security strategy that fits your budget and time constraints?
    Join top IBM i security expert Robin Tatam as he outlines the five fastest and most impactful changes you can make to strengthen IBM i security this year.
    Your system didn’t become unsecure overnight and you won’t be able to turn it around overnight either. But quick wins are possible with IBM i security, and Robin Tatam will show you how to achieve them.

  • Security Bulletin: Malware Infection Discovered on IBM i Server!

    SB PowerTech WC GenericMalicious programs can bring entire businesses to their knees—and IBM i shops are not immune. It’s critical to grasp the true impact malware can have on IBM i and the network that connects to it. Attend this webinar to gain a thorough understanding of the relationships between:

    • Viruses, native objects, and the integrated file system (IFS)
    • Power Systems and Windows-based viruses and malware
    • PC-based anti-virus scanning versus native IBM i scanning

    There are a number of ways you can minimize your exposure to viruses. IBM i security expert Sandi Moore explains the facts, including how to ensure you're fully protected and compliant with regulations such as PCI.



  • Encryption on IBM i Simplified

    SB PowerTech WC GenericDB2 Field Procedures (FieldProcs) were introduced in IBM i 7.1 and have greatly simplified encryption, often without requiring any application changes. Now you can quickly encrypt sensitive data on the IBM i including PII, PCI, PHI data in your physical files and tables.
    Watch this webinar to learn how you can quickly implement encryption on the IBM i. During the webinar, security expert Robin Tatam will show you how to:

    • Use Field Procedures to automate encryption and decryption
    • Restrict and mask field level access by user or group
    • Meet compliance requirements with effective key management and audit trails


  • Lessons Learned from IBM i Cyber Attacks

    SB PowerTech WC GenericDespite the many options IBM has provided to protect your systems and data, many organizations still struggle to apply appropriate security controls.
    In this webinar, you'll get insight into how the criminals accessed these systems, the fallout from these attacks, and how the incidents could have been avoided by following security best practices.

    • Learn which security gaps cyber criminals love most
    • Find out how other IBM i organizations have fallen victim
    • Get the details on policies and processes you can implement to protect your organization, even when staff works from home

    You will learn the steps you can take to avoid the mistakes made in these examples, as well as other inadequate and misconfigured settings that put businesses at risk.



  • The Power of Coding in a Low-Code Solution

    SB PowerTech WC GenericWhen it comes to creating your business applications, there are hundreds of coding platforms and programming languages to choose from. These options range from very complex traditional programming languages to Low-Code platforms where sometimes no traditional coding experience is needed.
    Download our whitepaper, The Power of Writing Code in a Low-Code Solution, and:

    • Discover the benefits of Low-code's quick application creation
    • Understand the differences in model-based and language-based Low-Code platforms
    • Explore the strengths of LANSA's Low-Code Solution to Low-Code’s biggest drawbacks



  • The Biggest Mistakes in IBM i Security

    SB Profound WC Generic The Biggest Mistakes in IBM i Security
    Here’s the harsh reality: cybersecurity pros have to get their jobs right every single day, while an attacker only has to succeed once to do incredible damage.
    Whether that’s thousands of exposed records, millions of dollars in fines and legal fees, or diminished share value, it’s easy to judge organizations that fall victim. IBM i enjoys an enviable reputation for security, but no system is impervious to mistakes.
    Join this webinar to learn about the biggest errors made when securing a Power Systems server.
    This knowledge is critical for ensuring integrity of your application data and preventing you from becoming the next Equifax. It’s also essential for complying with all formal regulations, including SOX, PCI, GDPR, and HIPAA
    Watch Now.

  • Comply in 5! Well, actually UNDER 5 minutes!!

    SB CYBRA PPL 5382

    TRY the one package that solves all your document design and printing challenges on all your platforms.

    Produce bar code labels, electronic forms, ad hoc reports, and RFID tags – without programming! MarkMagic is the only document design and print solution that combines report writing, WYSIWYG label and forms design, and conditional printing in one integrated product.

    Request your trial now!

  • Backup and Recovery on IBM i: Your Strategy for the Unexpected

    FortraRobot automates the routine tasks of iSeries backup and recovery, saving you time and money and making the process safer and more reliable. Automate your backups with the Robot Backup and Recovery Solution. Key features include:
    - Simplified backup procedures
    - Easy data encryption
    - Save media management
    - Guided restoration
    - Seamless product integration
    Make sure your data survives when catastrophe hits. Try the Robot Backup and Recovery Solution FREE for 30 days.

  • Manage IBM i Messages by Exception with Robot

    SB HelpSystems SC 5413Managing messages on your IBM i can be more than a full-time job if you have to do it manually. How can you be sure you won’t miss important system events?
    Automate your message center with the Robot Message Management Solution. Key features include:
    - Automated message management
    - Tailored notifications and automatic escalation
    - System-wide control of your IBM i partitions
    - Two-way system notifications from your mobile device
    - Seamless product integration
    Try the Robot Message Management Solution FREE for 30 days.

  • Easiest Way to Save Money? Stop Printing IBM i Reports

    FortraRobot automates report bursting, distribution, bundling, and archiving, and offers secure, selective online report viewing.
    Manage your reports with the Robot Report Management Solution. Key features include:

    - Automated report distribution
    - View online without delay
    - Browser interface to make notes
    - Custom retention capabilities
    - Seamless product integration
    Rerun another report? Never again. Try the Robot Report Management Solution FREE for 30 days.

  • Hassle-Free IBM i Operations around the Clock

    SB HelpSystems SC 5413For over 30 years, Robot has been a leader in systems management for IBM i.
    Manage your job schedule with the Robot Job Scheduling Solution. Key features include:
    - Automated batch, interactive, and cross-platform scheduling
    - Event-driven dependency processing
    - Centralized monitoring and reporting
    - Audit log and ready-to-use reports
    - Seamless product integration
    Scale your software, not your staff. Try the Robot Job Scheduling Solution FREE for 30 days.