Practical RPG: Queuing, Part I: Data Queues

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

One of the most basic functions of inter-process communication is the use of the queue. This first queuing article will introduce the simplest of the IBM i queuing mechanisms, the data queue.


In programming, queuing is a technique used to transfer data asynchronously between two software components. It differs from a parameter in that the sender can place a message on a queue, go on processing, and then optionally check later to see if the message was processed.


The asynchronous nature of queuing is very different from the synchronous style of passing data via one or more parameters directly to another procedure or program and waiting until that data is processed. Asynchronous processing provides a wide spectrum of capabilities, ranging from simple flow control to sophisticated concepts like store and forward, subscribe and publish, and guaranteed delivery. And since there are so many different uses for queuing, the IBM i has several different queue implementations. This article introduces you to the data queue.

Data Queues vs. Message Queues

I consider data queues the simplest queuing mechanism on the IBM i, although I suppose an argument could be made that message queues are "simpler" than data queues because they have more built-in support. The problem is that message queues are designed for a very specific function: communicating operational information from one application component to another and ultimately to human beings. Because of this focus, the message queue infrastructure has several features specific to textual data and locales, such as (human) language support and substitution variables. Data queues, on the other hand, tend to be locale-independent and more centered on transporting binary or at least machine-targeted data (I use "machine-targeted" when I refer to encoding such as XML, which is nominally human-readable but really intended to pass information between programming tiers).


That doesn't make message queues any less important; in fact, message queues are fundamental to programming on the IBM i, especially in an ILE environment. I'll try to address message queues and their uses in another installment. And if we have time, I'll also go in the other direction and talk about user queues, an even more basic data transport mechanism. But for now, let me focus on the data queue.

Creating and Deleting

Creating and deleting data queues is relatively straightforward: the CRTDTAQ and DLTDTAQ commands provide those capabilities. And while you might expect (rightly) that the DLTDTAQ command doesn't need many parameters, it might not be so intuitive that the base CRTDTAQ command is also light on parameters. DLTDTAQ first:


Data queue . . . . . . . . . . .                 Name, generic*        

  Library  . . . . . . . . . . .     *LIBL       Name, *LIBL, *CURLIB...


The command has exactly one parameter, the qualified name of the data queue to delete. This is the standard for nearly all Delete commands on the IBM i. The CRTDTAQ command, at least in its simplest form, isn't much more complex:


Data queue . . . . . . . . . . .                 Name                

  Library  . . . . . . . . . . .     *CURLIB     Name, *CURLIB       

Type . . . . . . . . . . . . . .   *STD          *STD, *DDM          

Maximum entry length . . . . . .                 1-64512             

Force to auxiliary storage . . .   *NO           *NO, *YES           

Sequence . . . . . . . . . . . .   *FIFO         *FIFO, *LIFO, *KEYED


The qualified data queue name is the same as on the Delete command. The type parameter can be either *STD or *DDM. I won't talk about DDM queues today except to point out that remote data queues are a really nifty way to add multi-tier support quickly.


But let me talk about the other couple of parameters. MAXLEN (Maximum entry length) is typically the only parameter I mess with; it identifies the maximum length of the largest message you plan to put on the queue. I nearly always have a single fixed length for my queue messages so my maximum length is usually the length of my messages, but that's not a requirement; data queues allow you to put a message of any length up to the maximum length onto the queue. Force to auxiliary storage is a sort of persistence mechanism; it makes sure that messages that get written stay written in the case of a system crash. Finally, you have the sequence. I usually use *FIFO, I never use *LIFO, and I occasionally use *KEYED. Each has its uses, but for today's discussion we'll stick with First-In, First-Out, or *FIFO. FIFO queues order their entries in arrival sequence: the first message to be pushed into a queue is the first to be popped off the stack by a read operation.


Note that after you populate this part of the command, you get the second half, which contains some more technical entries such as sender ID and queue size parameters. The parameters can be important, but I won't be covering them in this article; maybe I can get to them in a more advanced data queue article.

So Now It's Time to Read and Write!

Having created the queue using CRTDTAQ, it's now time to send some data and receive it. First let me set you up with some prototypes.


     D SendData        PR                  ExtPgm('QSNDDTAQ')

     D   Dtaqnam                     10a   const

     D   Dtaqlib                     10a   const

     D   Dtaqlen                      5p 0 const

     D   Data                              const like(myMessage)


     D ReceiveData     PR                  ExtPgm('QRCVDTAQ')

     D   Dtaqnam                     10a   const

     D   Dtaqlib                     10a   const

     D   Dtaqlen                      5p 0

     D   Data                              like(myMessage)

     D   WaitTime                     5p 0 const


SendData and ReceiveData are the prototypes for the two primary APIs, QSNDDTAQ and QRCVDTAQ. Both of these APIs can have far more complex calls with many more parameters. But the parameters I've prototyped here are all that you need to use the basic functionality of a FIFO data queue. Technically, these prototypes would also work fine with a LIFO queue, but I don't often have a use for a LIFO queue (also known as a "stack"). Just remember that these are for non-keyed queues. You may have noticed that that I use "like(myMessage)" wherever I have to actually define the data. That's because I usually use the same size buffer for all the messages in a given application so that it's easy to define a field or data structure named myMessage and then use that in the prototype of the API call.


Having said all that, it's time to use the prototypes:


SendData( 'APPQ': 'APPLIB': %size(myMessage): myMessage);


This puts a message onto the data queue named APPQ in library APPLIB (which you would have already created with the CRTDTAQ command). Another program would then read that message, using this call:


ReceiveData( 'APPQ': 'APPLIB': lenReceived: myMessage: 60);


 That's really all there is to it. Obviously, we could spend hours going over the various application infrastructure settings. In this case, I set the timeout for 60 seconds, but other applications might have shorter wait times or even no wait time at all. Or you could go to the other extreme and have a very long wait time. That's an application issue. Just remember to check the lenReceived variable; if it's zero, then you didn't get a message from the queue.


So, the purpose here wasn't to make you a data queue expert. It was to show you that just a couple of commands and a few lines of code are all you need to take advantage of data queues. In future articles, we'll take a look at keyed queues and how they are used (hint: think multi-line transactions) and other queue types as well, including user queues and message queues. Until then, keep on queuing!


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



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.


  • Progressive Web Apps: Create a Universal Experience Across All Devices

    LANSAProgressive Web Apps allow you to reach anyone, anywhere, and on any device with a single unified codebase. This means that your applications—regardless of browser, device, or platform—instantly become more reliable and consistent. They are the present and future of application development, and more and more businesses are catching on.
    Download this whitepaper and learn:

    • How PWAs support fast application development and streamline DevOps
    • How to give your business a competitive edge using PWAs
    • What makes progressive web apps so versatile, both online and offline



  • 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



  • Node Webinar Series Pt. 1: The World of Node.js on IBM i

    SB Profound WC GenericHave 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.
    Part 1 will teach you what Node.js is, why it's a great option for IBM i shops, and how to take advantage of the ecosystem surrounding Node.
    In addition to background information, our Director of Product Development Scott Klement will demonstrate applications that take advantage of the Node Package Manager (npm).
    Watch Now.

  • 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.