29
Fri, Nov
0 New Articles

Practical RPG: Queuing, Part I: Data Queues

RPG
Typography
  • 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 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: