Trifork Blog

Axon Framework, DDD, Microservices

Category ‘Custom Development’

Using Spring Session for concurrent session control in a clustered environment

April 8th, 2016 by

For a long time, Spring Security has provided support to limit the number of sessions a single user can have concurrently. This prevents users from being logged in from many different devices at the same time, for example to ensure that they won’t share their credentials to a paid site with their friends and family.

My former colleague Quinten Krijger has blogged about this feature beforeNote the last paragraph, which explains how this support is limited to single-node applications.

Although running on a single node may suffice for many applications, there are plenty applications running in a clustered environment that should be able to benefit from concurrent session control as well. As hinted in the aforementioned blog, this requires both implementing a custom SessionRegistry as well as ensuring that expiring a session is propagated to all nodes in the cluster.

This is exactly what I’ve done recently using Spring Session, a framework that allows you to take control over managing sessions using a shared external registry like Redis. In this post I’d like to walk you through the code, which can be found here:


Based on the code I wrote for this blog I’ve opened a pull request for Spring Session. That request is scheduled for inclusion in Spring Session 1.3, but the code works just fine with the upcoming 1.2 release and removes the limitation of not providing an expiry notification after exceeding the maximum number of sessions.

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GOTO Amsterdam – The ideas behind the program

March 24th, 2016 by

We recently had our program release party with Jim Webber, where we published our program for GOTO Amsterdam 2016. Over the last weeks we filled every missing spot and the exciting program is now complete. But we haven’t shared our thoughts on the program decisions itself.

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From The Trenches: LoRa, LoRaWAN tutorial with the LoRaBee

March 4th, 2016 by

Once in a while you stumble across an interesting technology which is an enabler of new solutions, and new business cases. At Trifork Eindhoven we believe LoRa / LoRaWan might be such a technology. We are currently working on several Internet of Things solutions which could directly benefit from long distance communication, with a low energy footprint.

In this document I will describe the things that I learnt while working with the LoRaBee development kit. I will also provide some basic tutorials on how to set it up, how to start a connection and how to connect it with different platforms.
The hardware that I am using:

  • SODAQ Mbili with Atmel 1284p Microcontroller x 2
  • SODAQ LoRaBee which contains the Microchip RN2483 Module x 2
  • A set of peripherals for data acquisition ( sensors, buttons, LEDs, serial interfaces… )

Quick intro into LoRa

According to Wikipedia:

LoRaWAN (Long Range Wide Area Network) is a low power wireless networking protocol designed for low-cost secure two-way communication in the Internet of Things (IoT). LoRaWANs use of sub-GHz ISM bands also means the network can penetrate the core of large structures and subsurface deployments within a range of 2 km.[1]

The technology utilized in a LoRaWAN network is designed to connect low-cost, battery-operated sensors over long distances in harsh environments that were previously too challenging or cost prohibitive to connect. With its penetration capability, a LoRa gateway deployed on a building or tower can connect to sensors more than 10 miles away or to water meters deployed underground or in basements. The LoRaWAN protocol offers unique and unequaled benefits in terms of bi-directionality, security, mobility and accurate localization that are not addressed by other LPWAN technologies. These benefits will enable the diverse use cases and business models in deployments of LPWAN IoT networks globally.”

In the following picture, you can see the typical LoRaWAN network:

As you can see, all three classes are required for a fully working LoRaWAN network. In the tutorial, we used two LoRa end node devices for a P2P communication; however that is a simple radio transmit/receive between the LoRa modules and not the LoRaWAN protocol.

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Spring-AMQP and payload validation: some notes from the trenches

February 29th, 2016 by

It’s been a while since I’ve written one of our from-the-trenches blogs: that’s mostly because I’ve been very busy in those trenches developing systems for our customers.

This week I completed a Spring Boot-based microservice which is responsible for interacting with some 3rd party SOAP service: its own clients communicate with it by sending request message over RabbitMQ, and the service then sends back a response to a response queue after handling the SOAP response message.

Of course I used Spring-AMQP to build this service. Spring-AMQP supports a nice annotation-based listener programming model, based on Spring’s generic Message support.
That allows writing listener methods like this:

@RabbitListener(queues = REQUEST_QUEUE) 
public DeclarationResponse submitDeclaration(DeclarationRequest request) { 
  // handle the request and return a response 

The request parameter here is the result of converting the incoming AMQP message using a Spring-AMQP MessageConverter, after which it is considered to be the payload of the message (even when headers are used in the conversion as well).

The request messages that the clients send have some required fields: without those fields, the service can’t make the SOAP calls. While reading the RabbitListener JavaDoc I noticed that Spring-AMQP allows you to apply validation to message payload parameters by annotating it. When using this, you also have to add the @Payload annotation (which is optional without validation if your method doesn’t have any other arguments), so the result looks like this:

@RabbitListener(queues = REQUEST_QUEUE)
public DeclarationResponse submitDeclaration(@Valid @Payload DeclarationRequest request) { … }

By the way, Spring’s own @Validated (even as a meta-annotation) and in fact every annotation whose name starts with “Valid” are supported for this purpose as well.

Now we can add some JSR-303 Bean Validation annotations to the fields in our DeclarationRequest, like @NotNull, to express our validation constraints.

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Personalised city trip itinerary using integer linear programming

February 1st, 2016 by


As a research project I have developed an itinerary service. The idea started when I was doing a hackathon with colleagues for the city of Amsterdam (see earlier post). I wanted to recommend an itinerary to a tourist visiting the city of Amsterdam. Furthermore, I wanted to make the itinerary based on the user’s interests to recommend interesting places and activities for him in the city. If the user is interested in modern art for example, the recommendation scores for modern art museums will increase for that user.

The tool divides the duration of the tourist’s stay into separate time slots. For example, a single day could be divided in 3 time slots: morning, afternoon, evening. POIs get a different recommendation score for each time slot they can be visited. The Vondelpark for example could be less recommended on Monday morning because of expected rain or because of expected crowds. On Tuesday morning the Vondelpark could be recommended because of an interesting event or nice weather.

The itinerary tool will try to limit travel time between the recommended POIs (point of interests). In this way the tourist will not waste time on travailing. The user can also set a budget for the entire itinerary.

By taking all these considerations into account the tool should be able to aid the user in making good decisions about which places to visit and when to visit them.

Here is a screenshot of the user interface of the itinerary tool that was created as part of the research project:


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Program Release Party for GOTO Amsterdam on January 21

January 12th, 2016 by

On Thursday, January 21 the GOTO Amsterdam Team hosts a free event to release the program of GOTO Amsterdam 2016.

The Amsterdam Program Committee has been working very intensively to release the schedule for this year sooner than for any other year. This new approach will be implemented for all of the GOTO conferences with the goal to have the program ready half a year before a conference takes place.

This way our participants have enough time to register early and profit from the early bird rates that we offer. This is the most honest and best way to meet our customers. Read the rest of this entry »


December 9th, 2015 by

8 days, 7 hours, 39 minutes and 30 seconds!

Go and break records copy

In crossing the Atlantic, I learned a few things that I would like to share with you. First of all, I can tell you that the ocean is really big, with plenty of space for everyone. If you believe that the world is overpopulated and you want to be alone for a while, the ocean is a good place to go. In eight days, we only saw five other ships or sailboats and only for 5-10 minutes at a time. We saw a few dolphins and were attacked by a lot of flying fish. And yes, being slapped by a flying fish is an unpleasant surprise.

Being together with 14 other guys was actually easier than you would expect. They are really great guys, and the long discussions about life and other business during our recovery time were really great.

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Booting your Microservices Architecture with Spring and Netflix: the aftermath

November 26th, 2015 by

On 25 November Trifork hosted a webinar in which I gave a short overview of Spring Cloud and its support for the Netflix OSS stack, focusing on Spring Cloud Config and the support for Netflix’s Eureka, Ribbon and Hystrix.

We’ve been investigating this stack over the last couple of months and are using parts of it in production already: we have found that a lot of the common concerns that you need to tackle as you’re moving into a distributed systems architecture are nicely covered and, in many cases, even abstracted by the Spring Cloud platform. In a typical Spring fashion, this allows you as an application developer to focus more on your business logic while letting the framework handle the concerns related to things like accessing shared configuration, working with service registries, handling failing downstream services, etc.

This blog provides you with background info to accompany the webinar, which has been recorded and can be found on our YouTube channel.
The code has been published on GitHub, as well as the accompanying config repository, in case you’d like to code along with the video.

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Controlling Java with the Leap Motion

November 17th, 2015 by

Leap Motion Controller

The Leap Motion Controller is a device that uses two cameras to track the hands and fingers. This makes it possible to use gestures for controlling the computer or applications. It is possible to buy or download applications through the Leap Motion app store, but there is also an SDK for different languages available to integrate the controller in your own application.

With this article I aim to give an insight in the usability of the Leap Motion Controller in combination with Java. For this I describe the controller and Java API itself and have written an example application which uses the controller. The application is written in Java and is available on github.

Functionality of the controller

The basic functionality for the controller and API is working without problems. This makes it possible to make the interaction with devices and computers more intuitive. In the next screenshot an example is shown from the supplied Visualizer application with the detected hands and fingers.
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City-wide crowd management in Amsterdam

November 10th, 2015 by

As most residents and visitors of Amsterdam know, every year more people are visiting Amsterdam, city wide events like GayPride, Koningsdag and MuseumNacht are getting bigger and more frequent, putting more strain on the city’s infrastructure and all people living in the city center.

That’s why this November 7th, Amsterdam Marketing organized the Museumn8 hackathon to allow developers to come up with creative and innovative solutions for improving improving mobility, navigation and crowd management in the city. Twenty teams eventually participated.

Trifork (Rienk Prinsen, Marleine van Kampen, Marijn van Zelst) and weCity (David Kat, Luc Deliance) teamed up and joined the hackathon to give their take on solving this problem. Their solution:


By transforming the advertisement billboards of JCDecaux into large information screens displaying real-time information, visitors can get informed about activities and interesting places in the vicinity of the billboard. They receive live crowd information, travel times to and queue lengths at museums and even recommendations where to go next. Read the rest of this entry »