]> Matthias Wimmer (溫雅石)

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Reactive Programming with RxJava

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RxJava is an implementation of “Reactive Extensions” (Rx) in Java. But what is this? Originally Rx are an implementation of Microsoft to address the increasing complexity of software. It is a model to build large scale asynchronous service architectures.

Another company that has to be mentioned is Netflix: they implemented the Rx in Java which resulted in RxJava.

On the programming side Rx looks very similar to how streams (java.util.stream.Stream) work. The main difference in my opinion is that streams are pull and lazy. They produce only as much data as someone is reading from them. RxJava on the other side is more push style. The source of data is pushing events through a pipe of operators similar to what you know from streams (filter(), map(), and so on) to the sink. And as I know from this book, Java streams are the wrong tool to parallelize anything that is not CPU bound–like network requests. There reason for this is, that (parallel) streams are executed on a thread pool that is shared with several other features of Java. This thread pools is limited to have only that many workers as the system has CPU cores. Therefore this pool gets exhausted very soon, when threads in it get blocked by I/O operations. Any thread waiting for an I/O operation effectively results in a processor core not used by your program.

RxJava on the other hand is not limited to a fixed thread pool. Any source (Observable) and sink (Subscriber) of data can be bound declaratively to user defined Schedulers. As well as Rx favours a model in which I/O operations are done asynchronously and non-blocking. Therefore resulting in a need for much fewer threads. In a traditional model of using one thread per network connection, threads become very soon the first thing that limits scalability.

What is really great about this book

The best part of this book for me were the reflections on Relational Database Access in chapter 5. While as a developer you might be tempted to convert everything to the reactive model, this part of the book shows where it doesn't make sense to do so.

By converting the access to your relational database to an asynchronous model you won't gain anything. Whatever you are doing on the client side, let's say for example your PostgreSQL will run all of your concurrent requests in different processes. This results in a noticeable limit on the number of parallel queries you're able to run. You cannot lift this limit by becoming asynchronous on the client side.

Links to the book

ZooKeeper, Distributed Process Coordination

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ZooKeeper is a component that facilitates building distributed applications. It is:

The data managed by ZooKeeper is presented in a file system like manner with directories and files whose names get separated by slashes (/). The difference to a file system is, that you can store information in the directories as well. Or seen differently: directories are files at the same time. Based on this simple abstraction, users of ZooKeeper can implement things like leader election in a cluster of software instances.

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Mein Weg zur testgetriebenen Entwicklung

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Testgetriebene Entwicklung (TDD) funktioniert – überall. Sie scheitert nicht am Kunden, den nicht Qualität sondern Tempo interessiert. Sie steht nicht im Widerspruch zum jungen Projekt mit wenig Budget, das schnell Kunden gewinnen muss um zu überleben. Sie hindert nicht daran schnell Features umsetzen zu können. Ja, sie macht mir auch nicht (mehr) langsamer.

Es ist keine neun Monate her, dass ich das nicht glauben konnte. Mehrere Anläufe hatte ich unternommen neben Code auch Tests zu schreiben. Länger als zwei Monate ging das nie gut:

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Java Webstart und WELD

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Für meinen Arbeitgeber habe ich ein Programm entwickelt, das wir nutzen Zeiten zu erfassen und in unser Projektmanagement einzukoppeln. Um in der Firma einfach auf allen Rechnern immer die neuste Softwareversion zur Verfügung zu haben, habe ich mich dafür entschieden Java Webstart einzusetzen. Diese Technik hatte ich auch bereits für einige andere Java-Anwendungen von meinen Kunden eingesetzt.

Neu dieses mal war nun allerdings, dass ich nicht Sprint sondern CDI für die Dependency Injection gewählt habe. CDI implementiert durch WELD ist bei uns auf der Arbeit hierzu meist der Standard der Wahl.

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