5' on IT-Architecture: four laws of robust software systems

Murphy's Law ("If anything can go wrong, it will") was born at Edwards Air Force Base in 1949 at North Base. It was named after Capt. Edward A. Murphy, an engineer working on Air Force Project MX981, (a project) designed to see how much sudden deceleration a person can stand in a crash. One day, after finding that a transducer was wired wrong, he cursed the technician responsible and said, "If there is any way to do it wrong, he'll find it."

5' on IT-Architecture: three laws of good software architecture

The issue with architectural decisions is that they effect the whole system and/or you often need to make them early in the development process. It means a lot effort if you change that decision a couple of months later. From an economic standpoint architectural decisions are often irrevocable. Good architecture is one that allows an architect to make late decisions without superior effect on efforts and costs. Let's put that on record.

Java 7: NIO.2 I/O operations on asynchronous channels are not atomic

This part of my NIO.2 series wasn't on schedule when I started writing about NIO.2 aasynchronous file channels. It deals with an important detail: read and write operations are not atomic. What that means is that AsynchronousFileChannel -> write() does not garantee to write all bytes passed as parameter to the destination file. Instead it returns the number of bytes written as return parameters of the corresponding I/O operations and the client needs to deal with situations where the bytes written isn't equal to the remaining bytes in the passed ByteBuffer.

Java 7: Closing NIO.2 file channels without loosing data

Closing an asynchronous file channel can be very difficult. If you submitted I/O tasks to the asynchronous channel you want to be sure that the tasks are executed properly. This can actually be a tricky requirement on asynchronous channels for several reasons. The default channel group uses deamon threads as worker threads, which isn't a good choice, cause these threads just abandon if the JVM exits. If you use a custom thread pool executor with non-deamon threads (see last part of this series) you need to manage the lifecycle of your thread pool yourself. If you don't the threads just stay alive when the main thread exits. Hence, the JVM actually does not exit at all, what you can do is kill the JVM.

Threading stories: ThreadLocal in web applications

This week I spend reasonable time to eliminate all our ThreadLocal variables in our web applications. The reason was that they created classloader leaks and we coudn't undeploy our applications properly anymore. Classloader leaks happen when a GC root keeps referencing an application object after the application was undeployed. If an application object is still referenced after undeploy, then the whole class loader can't be garbage collected cause the considered object references your applications class file which in turn references the classloader. This will cause an OutOfMemoryError after you've undeployed and redeployed a couple of times.

Java 7: NIO.2 File Channels on the test bench - Part 2 - Applying custom thread pools

Asynchronous file processing isn't a green card for high performance. In my last post I have demonstrated that conventional I/O can be faster then asynchronous channels. There are some additional important facts to know when applying NIO.2 file channels. The Iocp class that performs all the asynchronous I/O tasks in NIO.2 file channels is, by default, backed by a so called "cached" thread pool. That's a thread pool that creates new threads as needed, but will reuse previously constructed threads *when* they are available. Look at the code of the ThreadPool class held by the Iocp.

Java 7: NIO.2 File Channels on the test bench - Part 1 - Introduction

Another blog post about new JDK 7 features. This time I am writing about the new AnsynchronousFileChannel class. I am analyzing the new JDK 7 features in depth for a couple of weeks now and I have decided to number my posts consecutively. Just to make sure I don't get confused :-) Here is my 7th post about Java 7 (I admit that - by coincidence - this was also a little confusing). Using NIO.2 asynchronous file channels effectively is a wide topic. There are some things to consider here. I have decided to devide the stuff into four posts. In this first part I will introduce the involved concepts when you use asynchonous file channels. Since these file channels work asynchronously, it is interesting to look at their performance compared to conventional I/O. The second part deals with issues like memory and CPU consumption and explains how to use the new NIO.2 channels safely in a high performance scenario. You also need to understand how to close asynchronous channels without loosing data, that's part three. Finally, in part four, we'll take a look into concurrency.