04
Mon, Nov
5 New Articles

An Introduction to Keyed Data Queues

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

Data queues are an efficient way for two or more programs to communicate with one another. The programs might reside on the same AS/400, or one might be on an AS/400 and another on a PC.

You might compare data queues to a mailbox. Post office personnel put things into the mailbox, and the people with keys to the box take them out. A mailbox holds letters, but a data queue holds data records called messages. Don't confuse them with the messages sent by commands like SNDMSG. They're not the same thing.

Data queues have been around since the S/38 days, but keyed data queues are a fairly recent invention. Before keyed data queues, you could retrieve entries from data queues in first-in-first-out (FIFO) or last-in-first-out (LIFO) order only.

Keyed data queues aren't any better or worse than FIFO and LIFO data queues; they're just different. They offer other capabilities, namely the ability to select and sequence the messages on a data queue. Before I talk about how to use keyed data queues, let's look at the commands and APIs you'll need to work with them.

The Create Data Queue (CRTDTAQ) command creates data queues, keyed as well as nonkeyed. To create a keyed data queue, specify SEQ(*KEYED) and provide the length of the key in the key length (KEYLEN) parameter. The following command creates a data queue named MYDTAQ in library MYLIB. The messages will be 1024 bytes long, and the key will be 8 bytes long.

 CRTDTAQ DTAQ(MYLIB/MYDTAQ) + MAXLEN(1024) SEQ(*KEYED) + KEYLEN(8) 

To place some data on the data queue, call the Send Data Queue (QSNDDTAQ) API. QSNDDTAQ usually requires only four parameters, but if you're sending data to a keyed data queue, you'll need two more-the key data and the length of the key data. You can see the parameter list for QSNDDTAQ in 1.

To place some data on the data queue, call the Send Data Queue (QSNDDTAQ) API. QSNDDTAQ usually requires only four parameters, but if you're sending data to a keyed data queue, you'll need two more-the key data and the length of the key data. You can see the parameter list for QSNDDTAQ in Figure 1.

Use the Receive Data Queue (QRCVDTAQ) API to receive an entry from the data queue. Keyed data queues require three parameters that are not permitted for *LIFO and *FIFO data queues. You'll have to specify a key search criteria, the length of the key value, and the key value itself. See 2 for the complete parameter list for QRCVDTAQ.

Use the Receive Data Queue (QRCVDTAQ) API to receive an entry from the data queue. Keyed data queues require three parameters that are not permitted for *LIFO and *FIFO data queues. You'll have to specify a key search criteria, the length of the key value, and the key value itself. See Figure 2 for the complete parameter list for QRCVDTAQ.

The key search criteria can be one of the six usual relational operators-EQ, NE, GT, LT, GE, or LE. QRCV-DTAQ uses the key value you provide to search for an appropriate entry on the data queue.

If you'd like to look at the messages on a data queue without removing them, you can use the Retrieve Data Queue Message (QMHRDQM) API. See the OS/400 System API Reference V3R1 manual for more details.

PCs running Client Access/400 can create, delete, read from, write to, and otherwise massage data queues on the AS/400, through either batch file commands or APIs. It's easier to use commands, but they're limited in what they can do. For more power, use the Client Access APIs. The Client Access/400 API reference manuals listed at the end of this article describe them and show how to call them from programs written in languages like C and Pascal.

One thing keyed data queues do is let you retrieve entries in a sorted order, regardless of how they arrived on the queue. This is a good way to prioritize the messages in a data queue.

One example of this is the use of data queues to replace data entry. It's common for an AS/400 shop to modify a data entry program to read from a data queue instead of a terminal. This program usually runs in batch, receiving the messages as they hit the data queue and processing them as soon as it can.

If messages are usually processed as soon as they arrive on the data queue, any organization of data queue-FIFO, LIFO, or KEYED-is acceptable. If messages arrive in groups, so that there are often several messages waiting on the queue, you can prioritize them with a keyed data queue.

1 contains code that places inventory transactions on a keyed data queue. The ACTION field has the value 0 if the transaction increases inventory, and 1 if the transaction decreases inventory. Placing ACTION at the beginning of the key forces the system to process transactions that increase inventory, such as receipts, before transactions that decrease inventory, like issues or scrap, when two or more messages are waiting in the queue.

Figure 1 contains code that places inventory transactions on a keyed data queue. The ACTION field has the value 0 if the transaction increases inventory, and 1 if the transaction decreases inventory. Placing ACTION at the beginning of the key forces the system to process transactions that increase inventory, such as receipts, before transactions that decrease inventory, like issues or scrap, when two or more messages are waiting in the queue.

The batch transaction job would include code like that in 2. The program waits for a transaction to arrive on the data queue, processes it, and waits for another one.

The batch transaction job would include code like that in Figure 2. The program waits for a transaction to arrive on the data queue, processes it, and waits for another one.

For more ideas, you should also read "Line Up for Data Queues" (MC, May 1992). There, you'll find a program that uses a keyed data queue to sort a subfile.

Another reason to consider key data queues is that FIFO and LIFO data queues have a take-it-or-leave-it attitude. When you ask for an entry from one of these types of queues, you have to take the next entry in line. Keyed data queues allow you to select the entries you want and ignore the others.

For example, suppose you call QRCVDTAQ with a key value of MA and a search operator of GT. QRCV-DTAQ looks for a data queue entry with a key value greater than MA. If two or more messages in the queue have key values greater than MA, QRCVDTAQ retrieves the one that arrived on the queue first.

One good use for this is sending information to different processes through a common data queue. These processes could be terminals that display messages and announcements in different warehouses (the way CRT screens in airports display flight information). The terminal in each warehouse would receive the messages for that warehouse. The processes could be running on PCs, and each PC could pick just the messages intended for it. The processes could be separately running never-ending batch jobs that send the information to different printers.

For more examples, see the articles "Easy Cooperative Processing Programming with Data Queues" (MC, October 1993) and "Event-driven Programming with Data Queues" (MC, December 1993).

The speaker at a meeting I attended told us that we all had great potential. Once he had us feeling good about ourselves, he explained that "potential means you haven't done it yet." If keyed data queues are still a potential in your shop, they're worth your consideration. The possibilities for keyed data queues are enormous, especially when keyed data queues are accessed by PCs.

Ted Holt is an associate technical editor for Midrange Computing. He can be reached by E-mail at This email address is being protected from spambots. You need JavaScript enabled to view it..


An Introduction to Keyed Data Queues

Figure 1: Sending Data Queue Messages with QSNDDTAQ

 IDQDTA DS 1024 ... C QTY IFNE *ZERO C ACTION CAT ITEM:0 KEY C MOVELQTY DQDTA C CALL 'QSNDDTAQ' C PARM 'MYDTAQ' DQNAME 10 Data queue name C PARM 'MYLIB' DQLIB 10 Data queue library C PARM 1024 DQLEN 50 Data queue length C PARM DQDTA Queued data C PARM 8 KEYLEN 30 Key length C PARM KEY 8 Key value C ENDIF 
An Introduction to Keyed Data Queues

Figure 2: Receiving Data Queue Messages with QRCVDTAQ

 ... IDQDTA DS 1024 ... C CALL 'QRCVDTAQ' C PARM 'MYDTAQ' DQNAME 10 Data queue name C PARM 'MYLIB' DQLIB 10 Data queue library C PARM 1024 DQLEN 50 Data queue length C PARM DQDTA Queued data C PARM -1 WAIT 50 Seconds to wait C PARM 'GE' KEYSLT 2 Key selection C PARM 8 KEYLEN 30 Key length C PARM *BLANKS KEY 8 Key value C PARM 0 SNDLEN 30 Sender info length C PARM SND 8 Sender info * C* If the data queue msg contains sentinel value, shut down the job. * C '$$EOJ' SCAN DQDTA 11 C *IN11 IFEQ *ON C SETON LR C RETRN C ENDIF * ... (code to process the transaction goes here) 
TED HOLT

Ted Holt is IT manager of Manufacturing Systems Development for Day-Brite Capri Omega, a manufacturer of lighting fixtures in Tupelo, Mississippi. He has worked in the information processing industry since 1981 and is the author or co-author of seven books. 


MC Press books written by Ted Holt available now on the MC Press Bookstore.

Complete CL: Fifth Edition Complete CL: Fifth Edition
Become a CL guru and fully leverage the abilities of your system.
List Price $79.95

Now On Sale

Complete CL: Sixth Edition Complete CL: Sixth Edition
Now fully updated! Get the master guide to Control Language programming.
List Price $79.95

Now On Sale

IBM i5/iSeries Primer IBM i5/iSeries Primer
Check out the ultimate resource and “must-have” guide for every professional working with the i5/iSeries.
List Price $99.95

Now On Sale

Qshell for iSeries Qshell for iSeries
Check out this Unix-style shell and utilities command interface for OS/400.
List Price $79.95

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: