Welcome!

Weblogic Authors: Yeshim Deniz, Elizabeth White, Michael Meiner, Michael Bushong, Avi Rosenthal

Related Topics: Weblogic

Weblogic: Article

Listen Very Carefully; I Shall Say This Only Once!

Listen Very Carefully; I Shall Say This Only Once!

I have a rather deaf and rather elderly grandmother. She is a lovely woman and can spend hours telling tales - sometimes fascinating and sometimes... well, less fascinating - about times past.

Communication in the reverse direction can be a bit of a challenge - sometimes a raised voice will suffice, sometimes repeating what you want to say several times will do - and sometimes combining the two techniques together finally gets the message across. I swear that she finds it easier to hear things selectively, based on how convenient they are for her. This is a very clever - not to say mystical - filtering technique, to be sure, but still, as she says, what's the point of getting old if you don't get artful! When all else fails, and her ability to hear has apparently reached zero, there's nothing like a good old-fashioned piece of paper and pen (unless she's lost her glasses, but that's another story altogether). I'm sure you'll agree that my grandmother is very similar to a lot of software systems - in her day she was a woman ahead of her time, you know... Well, okay. Maybe you won't - so I'll explain what I mean...

Nearly any development that anyone ever does will involve creating a bit of new logic, and a lot of passing messages between existing systems. These existing systems are the ones that I think of as like my gran - some very elderly, and all esoteric and rather particular about how you communicate with them. Given this, it's generally a bad idea to build a new system by close coupling all the required elements in a point-to-point fashion. The resultant system would be extremely brittle and unreliable - a kind of giant, unpredictable hairball (imagine my gran and her bridge club all jointly completing a single bingo card). At the very best, all the communication logic would be surrounded by complex retry logic and be very difficult to understand.

That's why loose coupling is king - and reliable messaging often forms the backbone of loosely coupled systems within single organizations. It is for this reason that MOM systems like IBM's MQSeries and BEA WebLogic's own JMS messaging system are used so widely in real-world deployments. With their reliable messaging infrastructures, the sender of a message can know that eventually it will be delivered, whatever happens. They are the software equivalent of my writing notes to granny, and used to decouple systems can increase availability (at the expense of a whole operation not necessarily happening in real-time).

Whenever the subject of reliable message queuing is brought up, the subject of guaranteed once and once-only delivery is usually not far behind. It's all very well knowing for sure that a message will definitely be delivered from the point of view of the sender, but from the receiver's point of view it may be important to identify if you are receiving a message that you have actually already seen before. If my grandmother found a note reminding her to buy a pint of milk every day her fridge would soon overflow; and in the same way, if an increase in an employee's base salary was actioned multiple times because the message kept arriving at the HR system it would make for happy employees, but unhappy employers. So how do you make sure that you haven't seen a message before? Well, you need to keep a note of what messages you have received, and check each apparently new one against the list of ones you already received. So off you go to write some more code...

It's here that transactions come into the picture, and save you writing all that infrastructure logic. With an xa-compliant messaging server (for example, the one supplied out-of-the-box in WebLogic Server) the transaction manager can ensure once and once-only delivery. Most software systems, whatever the business logic, receive an instruction, process it, and update their state as a result of the processing. In the situation we're looking at now, the instruction will be received via a reliable queue. In nearly all systems, the persistent state is held in a database, most of which are xa compliant. By the rules of transactions, if you can receive the message, and do the database updates in a single JTA transaction, then either the message will be consumed and the updates will be persisted - if the transaction commits - or the message will be left on the queue and the persistent state will be unaltered - if the transaction aborts. It only takes a minute's thought to tell you that the transaction manager, in concert with the xa-compliant queuing system and database, has delivered once and once-only delivery to your application, so to speak. A great example of the use of infrastructure technology - you can count the lines of "once and once-only" logic that you now don't have to write and maintain because the transaction infrastructure did it for you out of the box. It's the equivalent of paying someone to take away gran's reminder notes at the supermarket checkout.

There is, however, a slight design wrinkle that needs to be borne in mind when using transactions like this. Imagine that the supermarket is out of milk. Our guy never gets to take away the reminder note, and gran will end up repeatedly making the trip to the supermarket only to find the milk cooler empty - her short- term memory really shows the signs of her age. Whilst walking up and down the road may keep her fit, there are certainly better uses of her time.

In systems implemented on computers - rather than grandmothers - this is known as the "poison message problem". Imagine our HR system receiving a message that Fred Smyth should receive a 10% pay raise - which couldn't be effected since Fred was actually laid off last month. If the transaction was rolled back as a result of this discovery, then the next thing that would happen is that the same instruction will be received again, and fail for the same reason. It's even less useful to keep computers fit in this way than it is my gran... Clearly this is a bad situation. The logic in the HR system should put this instruction in an error queue and flag the problem to a human to rectify in order that it can continue processing useful work, rather than choking on the (hopefully rare) error. On the other hand, of course, if the pay raise fails because the database is still restarting after a failure, then trying again in a couple of minutes may well be a good idea. In EJB terms, this illustrates the difference between System exceptions (database down) and Application exceptions (logic dictates that this cannot happen) and explains the difference in transactional behavior of the EJB container under these two conditions.

In general, you should code your applications so that they don't roll back transactions for business reasons, rather committing some kind of error notification. However, WebLogic Server's JMS implementation actually has two configurable settings to help with this kind of problem in the event that you don't code for it correctly. To cater to the technical failure case, a session can have an associated redelivery delay - a pause before an attempt will be made to redeliver it - this will hopefully allow the database time to recover (or whatever other transient error condition that exists to be corrected). To keep the logic failures from causing a permanent problem (or to prevent the system from wasting lots of time in the event of a long-term technical outage) there is also a redelivery limit. If a the system attempts to redeliver a message more than this (configurable). number of times, then it will not try again. You can configure whether the message should simply be discarded, or rather sent to an error queue (or topic) for administrative processing. For systems that are concerned about message ordering, WebLogic can also ensure that messages are redelivered in their original order too. I have never needed to give my gran more than one note at a time, so I have never tried that - and it's somewhat beyond the scope of this article.

So that's it for this month. I'm off now for a milky cup of cocoa and an audience with gran. I guarantee that will not happen once and once only.

More Stories By Peter Holditch

Peter Holditch is a senior presales engineer in the UK for Azul Systems. Prior to joining Azul he spent nine years at BEA systems, going from being one of their first Professional Services consultants in Europe and finishing up as a principal presales engineer. He has an R&D background (originally having worked on BEA's Tuxedo product) and his technical interests are in high-throughput transaction systems. "Of the pitch" Peter likes to brew beer, build furniture, and undertake other ludicrously ambitious projects - but (generally) not all at the same time!

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
SYS-CON Events announced today that Evatronix will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Evatronix SA offers comprehensive solutions in the design and implementation of electronic systems, in CAD / CAM deployment, and also is a designer and manufacturer of advanced 3D scanners for professional applications.
As businesses evolve, they need technology that is simple to help them succeed today and flexible enough to help them build for tomorrow. Chrome is fit for the workplace of the future — providing a secure, consistent user experience across a range of devices that can be used anywhere. In her session at 21st Cloud Expo, Vidya Nagarajan, a Senior Product Manager at Google, will take a look at various options as to how ChromeOS can be leveraged to interact with people on the devices, and formats th...
SYS-CON Events announced today that Taica will exhibit at the Japan External Trade Organization (JETRO) Pavilion at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Taica manufacturers Alpha-GEL brand silicone components and materials, which maintain outstanding performance over a wide temperature range -40C to +200C. For more information, visit http://www.taica.co.jp/english/.
Organizations do not need a Big Data strategy; they need a business strategy that incorporates Big Data. Most organizations lack a road map for using Big Data to optimize key business processes, deliver a differentiated customer experience, or uncover new business opportunities. They do not understand what’s possible with respect to integrating Big Data into the business model.
Enterprises have taken advantage of IoT to achieve important revenue and cost advantages. What is less apparent is how incumbent enterprises operating at scale have, following success with IoT, built analytic, operations management and software development capabilities – ranging from autonomous vehicles to manageable robotics installations. They have embraced these capabilities as if they were Silicon Valley startups. As a result, many firms employ new business models that place enormous impor...
Amazon is pursuing new markets and disrupting industries at an incredible pace. Almost every industry seems to be in its crosshairs. Companies and industries that once thought they were safe are now worried about being “Amazoned.”. The new watch word should be “Be afraid. Be very afraid.” In his session 21st Cloud Expo, Chris Kocher, a co-founder of Grey Heron, will address questions such as: What new areas is Amazon disrupting? How are they doing this? Where are they likely to go? What are th...
SYS-CON Events announced today that MIRAI Inc. will exhibit at the Japan External Trade Organization (JETRO) Pavilion at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. MIRAI Inc. are IT consultants from the public sector whose mission is to solve social issues by technology and innovation and to create a meaningful future for people.
SYS-CON Events announced today that Dasher Technologies will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Dasher Technologies, Inc. ® is a premier IT solution provider that delivers expert technical resources along with trusted account executives to architect and deliver complete IT solutions and services to help our clients execute their goals, plans and objectives. Since 1999, we'v...
SYS-CON Events announced today that NetApp has been named “Bronze Sponsor” of SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. NetApp is the data authority for hybrid cloud. NetApp provides a full range of hybrid cloud data services that simplify management of applications and data across cloud and on-premises environments to accelerate digital transformation. Together with their partners, NetApp emp...
SYS-CON Events announced today that IBM has been named “Diamond Sponsor” of SYS-CON's 21st Cloud Expo, which will take place on October 31 through November 2nd 2017 at the Santa Clara Convention Center in Santa Clara, California.
SYS-CON Events announced today that TidalScale, a leading provider of systems and services, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. TidalScale has been involved in shaping the computing landscape. They've designed, developed and deployed some of the most important and successful systems and services in the history of the computing industry - internet, Ethernet, operating s...
Infoblox delivers Actionable Network Intelligence to enterprise, government, and service provider customers around the world. They are the industry leader in DNS, DHCP, and IP address management, the category known as DDI. We empower thousands of organizations to control and secure their networks from the core-enabling them to increase efficiency and visibility, improve customer service, and meet compliance requirements.
SYS-CON Events announced today that IBM has been named “Diamond Sponsor” of SYS-CON's 21st Cloud Expo, which will take place on October 31 through November 2nd 2017 at the Santa Clara Convention Center in Santa Clara, California.
Join IBM November 1 at 21st Cloud Expo at the Santa Clara Convention Center in Santa Clara, CA, and learn how IBM Watson can bring cognitive services and AI to intelligent, unmanned systems. Cognitive analysis impacts today’s systems with unparalleled ability that were previously available only to manned, back-end operations. Thanks to cloud processing, IBM Watson can bring cognitive services and AI to intelligent, unmanned systems. Imagine a robot vacuum that becomes your personal assistant tha...
In a recent survey, Sumo Logic surveyed 1,500 customers who employ cloud services such as Amazon Web Services (AWS), Microsoft Azure, and Google Cloud Platform (GCP). According to the survey, a quarter of the respondents have already deployed Docker containers and nearly as many (23 percent) are employing the AWS Lambda serverless computing framework. It’s clear: serverless is here to stay. The adoption does come with some needed changes, within both application development and operations. Tha...
In his Opening Keynote at 21st Cloud Expo, John Considine, General Manager of IBM Cloud Infrastructure, will lead you through the exciting evolution of the cloud. He'll look at this major disruption from the perspective of technology, business models, and what this means for enterprises of all sizes. John Considine is General Manager of Cloud Infrastructure Services at IBM. In that role he is responsible for leading IBM’s public cloud infrastructure including strategy, development, and offering ...
SYS-CON Events announced today that Avere Systems, a leading provider of enterprise storage for the hybrid cloud, will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Avere delivers a more modern architectural approach to storage that doesn't require the overprovisioning of storage capacity to achieve performance, overspending on expensive storage media for inactive data or the overbui...
Widespread fragmentation is stalling the growth of the IIoT and making it difficult for partners to work together. The number of software platforms, apps, hardware and connectivity standards is creating paralysis among businesses that are afraid of being locked into a solution. EdgeX Foundry is unifying the community around a common IoT edge framework and an ecosystem of interoperable components.
SYS-CON Events announced today that TidalScale will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. TidalScale is the leading provider of Software-Defined Servers that bring flexibility to modern data centers by right-sizing servers on the fly to fit any data set or workload. TidalScale’s award-winning inverse hypervisor technology combines multiple commodity servers (including their ass...
SYS-CON Events announced today that N3N will exhibit at SYS-CON's @ThingsExpo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. N3N’s solutions increase the effectiveness of operations and control centers, increase the value of IoT investments, and facilitate real-time operational decision making. N3N enables operations teams with a four dimensional digital “big board” that consolidates real-time live video feeds alongside IoT sensor data a...