Tuesday, 8 April 2014

Tracking Exceptions - Part 4 - Spring's Mail Sender

If you've read any of the previous blogs in this series, you may remember that I'm developing a small but almost industrial strength application that searches log files for exceptions. You may also remember that I now have a class that can contain a whole bunch of results that will need sending to any one whose interested. This will be done by implementing my simple Publisher interface shown below.

public interface Publisher {

public <T> boolean publish(T report);

If you remember, the requirement was:

7 . Publish the report using email or some other technique.

In this blog I’m dealing with the concrete part of the requirement: sending a report by email.

Wednesday, 26 March 2014

Error Tracking Reports - Part 3 - Strategy and Package Private

This is the third blog in a series that's loosely looking at tracking application errors. In this series I’m writing a lightweight, but industrial strength, application that periodically scans application log files, looking for errors and, if any are found, generates and publishes a report.

If you’ve read the first blog in the series you may remember that I initially said that I needed a Report class and that “if you look at the code, you won’t find a class named Report, it was renamed Results and refactored to create a Formatter interface, the TextFormatter and HtmlFormatter classes together with the Publisher interface and EmailPublisher class”. This blog covers the design process, highlighting the reasoning behind the refactoring and how I arrived at the final implementation.

If you read on, you may think that the design logic given below is somewhat contrived. That’s because it is. The actual process of getting from the Report class to the Results class, the Formatter and Publisher interfaces together with their implementations probably only took a few seconds to dream up; however, writing it all down took some time. The design story goes like this...

Saturday, 22 March 2014

Packt Publishing: Buy One, Get One Free Offer...

I don’t normally do endorsements, adverts and recommendations from my blog, but the Guys at Packt Publishing have asked me to mention that they’re currently celebrating the release of their 2000th title with a ‘Buy One, Get One Free’ offer on eBooks. As I’ve read several of their books, and can recommend them both technical content and readability, I thought I’d highlight their offer with a link to their website, so take a look at: http://bit.ly/1j26nPN. In social media, look out for #Packt2k and #Packt. This offer ends on 26th March, so be quick...

Tuesday, 11 March 2014

Tracking Exceptions With Spring - Part 2 - Delegate Pattern

In my last blog, I started to talk about the need to figure out whether or not your application is misbehaving in it's production environment. I said that one method of monitoring your application is by checking its log files for exceptions and taking appropriate action if one is found. Obviously, log files can take up hundreds of megabytes of disk space and it's impractical and really boring to monitor them by hand.

I also said that there were several ways of automatically monitoring log files and proposed a Spring based utility that combs log files daily and sends you an email if / when it finds any exceptions.

I only got as far as describing the first class: the FileLocator, which will search a directory and it's sub-directories for log files. When it finds one, it passes it to the FileValidator.

Monday, 3 March 2014

Tracking Application Exceptions With Spring

A few weeks ago a colleague asked me to spend a week doing a support role as he needed cover whilst he took a well earned holiday and he couldn't find anyone else. As I'd just completed a particularly complex coding project and was feeling a little burnt out, I said 'yes'; after all the change would do me good.

Part of the job consisted of monitoring a collection of fairly critical backed processes, to see how well they were performing and whether or not they were going wrong.

We developers spend a lot of time and energy adding logging to our application in order to prove that it's working okay and to figure out what went wrong when an exception occurs. These log files are often used to tell us how well, or badly, our application is performing on a daily basis.

I'm ignoring other techniques such as adding probes to your application by whatever method you choose, such as HTTP or JMX. These provide immediate information on your application rather than the second level monitoring under discussion here.

There are at least three ways of monitoring log files:

Monday, 3 February 2014

Optimising Your ApplicationContext

There’s a problem with Spring, it’s been there for some time and I’ve come across it in a number of projects. It’s nothing to do with Spring, or the Guys at Spring, it’s down to Spring’s users like you and me. Let me explain… In the old days of Spring 2 you had to configure your Application Context by hand, manually creating an XML configuration file that contained all your bean definitions. The down side of this technique was that it was time-consuming to create these XML files and then, you had the headache of maintaining this increasingly complex file. I seem to remember that at the time, it was known as “Spring Config Hell”. On the upside, at least you had a central record of everything that was loaded into the context. Bowing to demand and the popular notion that annotations were the way to go, Spring 3 introduced a whole raft of stereotyping classes such as @Service, @Component, @Controller and @Repository together with an addition to the XML configuration file of the <context:component-scan/> element. This made, from a programming point of view, things a lot simpler and is a hugely popular way of constructing Spring contexts.

There is, however, a downside to using Spring annotations with wild abandon

Thursday, 2 January 2014

Publish and Subscribe with Hazelcast

A few weeks ago I wrote a blog on getting started with Hazelcast describing how ludicrously simple it is to create distributed maps, lists and queues. At the time I mentioned that Hazelcast does quite a few other things besides. This blog takes a quick look at another of Hazelcast’s features: its broadcast messaging system based on the Publish/Subscribe pattern. This takes the usual format where by the message sender app publishes messages on a certain topic. The messages aren't directed at any particular client, but can be read by any client that registers an interest in the topic.

Thursday, 5 December 2013

Investigating Memory Leaks Part 2 - Analysing the Problem

The first blog in this mini-series looked at creating a very leaky sample application, so that we can investigate techniques for solving heap based problems on server applications. It demonstrates the big problem with the Producer-Consumer pattern, namely that the consumer code has to be able to remove items from the queue at least as fast, if not faster than, the producer. The blog ended with me starting the sample code and sitting back whilst it leaked enough memory away to investigate. It’s now time to do that investigation.

If you read part 1 of this blog, you’ll know that the leaky code is part of an application1 that records stock/share orders in a dummy database using the Producer Consumer pattern. The sample code has been written to contain a very obvious flaw, namely that the OrderRecord can’t keep up with the OrderFeed. This means that the Order queue gets bigger and bigger until finally, the application runs out of heap space and falls over. The thing is, looking at my simple code, the problem should be obvious, but what if you've never seen the code before and it's huge, complex industrial strength code, plus there's no simple monitoring thread to keep an eye on the queue size or other internals? What do you do then?

Wednesday, 27 November 2013

Investigating Memory Leaks Part 1 - Writing Leaky Code

I found this little problem the other day: there’s this server that runs for a while and then falls over. It’s then restarted by its startup script and the whole process repeats itself. This doesn't sound that bad as it isn't business critical although there is a significant loss of data, so I decided to take a closer look and to find out exactly what's going wrong. The first thing to note is that the server passes all it's unit tests and a whole bunch of integration tests. It runs well in all test environments using test data, so what's going wrong in production? It's easy to guess that in production it's probably under a heavier load than test, or than had been allowed for in its design, and therefore it's running out of resources, but what resources and where? That's the tricky question.

Wednesday, 30 October 2013

Getting Started with Hazelcast

In July I wrote a blog introducing erlang to Java developers, highlighting some of the similarities and differences between the two languages. The erlang virtual machine has a number of impressive, built-in features, one of which is that they are location independent and can talk to each other. This means that that data can be synchronised between VMs by writing very few lines of code. This is really good news if you have a networked cluster of servers all doing the same thing.

You could argue that there's something lacking in the JVM if it can't even perform the most basic interprocess communication; however, Java takes the opposite view, it has a basic VM and then layers different services on top as and when required. Whether this is right is a matter of opinion and I'll leave it as a subject for a future blog, because it seems that the Hazelcast Guys have solved the problem of JVMs talking to each other; which is the point of this blog.

So, what is Hazelcast?