23
Sat, Nov
1 New Articles

TechTip: Use Qualified Data Structures for Checking Records

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

Database update collisions are a common problem, and qualified data structures provide an easy solution.

 

Database update collisions have been around since the advent of interactive database maintenance. First, we had no record locks. That left us with "last Enter key wins." Let's say I edit a record and you update the same record at the same time. We key in conflicting changes and press Enter. If I hit Enter first, my change is written to the database first; then your Enter is processed, and your change overwrites my change. Last Enter key wins!

 

A number of techniques have been used over the years to address this problem. Some applications used record locking, but that was ugly because the second person ended up with a hard halt, often after a relatively long wait period. Another option was to mark the record as busy by setting a busy flag in the record itself, but then jobs inevitably ended abnormally, leaving records locked and requiring periodic cleanup programs.

 

One of the cleaner techniques is the "last change check." Nobody locks the record during editing; instead a program that intends to update a record retrieves the record and saves it before displaying the data to the user. After the user changes the data and requests an update, the maintenance program then retrieves the record again, this time locking it. The program compares the newly retrieved record to the saved record. If there are no changes, the program updates the record. If the record has been changed, however, the update is canceled and the user is notified with a message along the lines of "Record has been changed by another user."

Implementing the Last Change Check

The last change check can be achieved in one of two basic ways. One is to use a "last changed" timestamp. This is relatively easy to implement and works especially well in environments where you are already storing the last change user, program, and timestamp in each record as opposed to recording a full audit journal.

 

This article, however, is going to use a different technique, the before and after image check. Image checking is done by comparing the entire contents of the saved record to that of the re-read record. Doing this in RPG hasn't always been very easy, and in fact prior to RPG IV's D-spec it was downright difficult. While you could use an externally described data structure, you couldn't have two data structures with the same field names, so it was hard to define both the before and after images. But with the advent of the qualified data structure, it has become very easy.

 

Let's look at a program that takes advantage of this RPG feature.

 

A    fCUSTMAST  UF   E           K DISK

 

B    d dsCUSTMAST1   e ds                  extname(CUSTMAST:*input) qualified

B    d dsCUSTMAST2   e ds                  extname(CUSTMAST:*input) qualified

B    d dsCUSTMAST3   e ds                  extname(CUSTMAST:*output) qualified

 

C    d myCust          s                   like(CMCUST)

C    d success         s               n

 

      /free

 

D      // Get myCust from user

 

D      chain(n) (myCust) CUSTMAST dsCUSTMAST1;

      

E      // Show data to user

E      // Get input from user

E      // Edit input

 

F      chain (myCust) CUSTMAST dsCUSTMAST2;

F      if (dsCUSTMAST1 <> dsCUSTMAST2);

         // Send "record has changed" message

G        unlock CUSTMAST;

G        success = *off;

       else;

         // Copy input fields into output data structure

H        eval-corr dsCUSTMAST3 = dsCUSTMAST2;

         // Move updated fields into dsCUSTMAST3

H        update CUSTMASTR dsCUSTMAST3;

H        success = *on;

       endif;

 

I      *inlr = *on;

 

      /end-free

 

The program is simple. In fact, it's a little more complex than it needs to be because of a quirk in the extname keyword, but I'll explain that as I go through the program.

 

A. Here I define a keyed update-capable file. Nothing unusual. Note that I haven't described a display file or anything; we're not making any assumptions as to how the program communicates with the user, only that it does.

 

B. Next, I define the data structures. Since they are qualified, I can have as many data structures based on the same file as I need. This is the first place we run into the quirk I mentioned earlier. Externally described data structures are formatted as an exact match to the buffer layout of the record named. Since records can theoretically have input-only or output-only fields, the input buffer could conceivably be different from the output buffer. Because of that, you have to tell the compiler which buffer layout you're matching: input or output. Only input data structures can be used for CHAINs and READs; only output data strcutures can be used for WRITEs and UPDATEs. We'll deal with this issue again in a moment.

 

C. Here are our work variables. They're just there to help flesh out the program a little more.

 

D. OK, the user entered a customer number (somehow). The first chain is done using the (n) extender so the record is not locked. Also notice that we read into dsCUSTMAST1.

 

E. This is where the meat of the program goes: displaying the data to the user, accepting input, editing the results, applying business rules, etc.—all the stuff that does the actual business work but doesn't pertain to the locking mechanism.

 

F. Now for the focus of this article: all I do to implement the record check is to read the record again, this time with a lock, into the second data structure, dsCUSTMAST2. Then I compare dsCUSTMAST1 to dsCUSTMAST2. That's all it takes!

 

G. If the before and after images are different, I notify the user through a message or some other method. For our example, I just set success to *off. Also, note that if you wanted to implement the timestamp technique, this is the place where you would do that. Rather than save the entire record, you need to save only the timestamp and here compare the saved timestamp with the timestamp from the second read.

 

H. If the images do match, I can do the update. This is the second time we run into the input vs. output buffer issue. Since we cannot perform an UPDATE from a data structure based on the input format, we have to move the data from one to another. We could just move one data structure to another, but let's consider the rare situation where the input and output buffers don't match; a straight data-structure-to-data-structure move would potentially give you some bad results. However, RPG provides a really nice alternative, the EVAL-CORR opcode, which moves all named fields from one data structure to the matching fields in another, all in one instruction. Very handy. So I move all the fields from the second input data strcuture to the output data structure, update the output data structure with whatever fields have changed, and then execute the UPDATE instruction.

 

I. For our simple example, it's time to get out.

 

That's it. I hope this technique provides a helpful introduction to qualified data structures and the ways they can help your programming.

as/400, os/400, iseries, system i, i5/os, ibm i, power systems, 6.1, 7.1, V7, V6R1

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 This email address is being protected from spambots. You need JavaScript enabled to view it..


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

BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$

Book Reviews

Resource Center

  • SB Profound WC 5536 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. You can find Part 1 here. In Part 2 of our free Node.js Webinar Series, Brian May teaches you the different tooling options available for writing code, debugging, and using Git for version control. Brian will briefly discuss the different tools available, and demonstrate his preferred setup for Node development on IBM i or any platform. Attend this webinar to learn:

  • 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 company are not aligned with the current IT environment.

  • SB HelpSystems ROBOT Generic IBM announced the E1080 servers using the latest Power10 processor in September 2021. The most powerful processor from IBM to date, Power10 is designed to handle the demands of doing business in today’s high-tech atmosphere, including running cloud applications, supporting big data, and managing AI workloads. But what does Power10 mean for your data center? In this recorded webinar, IBMers Dan Sundt and Dylan Boday join IBM Power Champion Tom Huntington for a discussion on why Power10 technology is the right strategic investment if you run IBM i, AIX, or Linux. In this action-packed hour, Tom will share trends from the IBM i and AIX user communities while Dan and Dylan dive into the tech specs for key hardware, including:

  • Magic MarkTRY 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. Make sure your data survives when catastrophe hits. Request your trial now!  Request Now.

  • SB HelpSystems ROBOT GenericForms of ransomware has been around for over 30 years, and with more and more organizations suffering attacks each year, it continues to endure. What has made ransomware such a durable threat and what is the best way to combat it? In order to prevent ransomware, organizations must first understand how it works.

  • SB HelpSystems ROBOT GenericIT security is a top priority for businesses around the world, but most IBM i pros don’t know where to begin—and most cybersecurity experts don’t know IBM i. In this session, Robin Tatam explores the business impact of lax IBM i security, the top vulnerabilities putting IBM i at risk, and the steps you can take to protect your organization. If you’re looking to avoid unexpected downtime or corrupted data, you don’t want to miss this session.

  • SB HelpSystems ROBOT GenericCan you trust all of your users all of the time? A typical end user receives 16 malicious emails each month, but only 17 percent of these phishing campaigns are reported to IT. Once an attack is underway, most organizations won’t discover the breach until six months later. A staggering amount of damage can occur in that time. Despite these risks, 93 percent of organizations are leaving their IBM i systems vulnerable to cybercrime. In this on-demand webinar, IBM i security experts Robin Tatam and Sandi Moore will reveal:

  • FORTRA Disaster protection is vital to every business. Yet, it often consists of patched together procedures that are prone to error. From automatic backups to data encryption to media management, Robot automates the routine (yet often complex) 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:

  • FORTRAManaging messages on your IBM i can be more than a full-time job if you have to do it manually. Messages need a response and resources must be monitored—often over multiple systems and across platforms. 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:

  • FORTRAThe thought of printing, distributing, and storing iSeries reports manually may reduce you to tears. Paper and labor costs associated with report generation can spiral out of control. Mountains of paper threaten to swamp your files. Robot 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:

  • FORTRAFor over 30 years, Robot has been a leader in systems management for IBM i. With batch job creation and scheduling at its core, the Robot Job Scheduling Solution reduces the opportunity for human error and helps you maintain service levels, automating even the biggest, most complex runbooks. Manage your job schedule with the Robot Job Scheduling Solution. Key features include:

  • LANSA Business 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.

  • 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:

  • LANSASupply Chain is becoming increasingly complex and unpredictable. From raw materials for manufacturing to food supply chains, the journey from source to production to delivery to consumers is marred with inefficiencies, manual processes, shortages, recalls, counterfeits, and scandals. In this webinar, we discuss how:

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • Profound Logic 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.

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? 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: