If you want to stay in touch with cutting-edge technology in terms of scalability of databases, high traffic sites, and large storage volumes, you should read these two articles on the new hstack.org blog.

Cosmin Lehene wrote two excellent articles on Adobe’s experiences with HBase: Why we’re using HBase: Part 1 and Why we’re using HBase: Part 2. Adobe needed a generic, real-time, structured data storage and processing system that could handle any data volume, with access times under 50ms, with no downtime and no data loss. The article goes into great detail about their experiences with HBase and their evaluation process, providing a “well reasoned impartial use case from a commercial user”. It talks about failure handling, availability, write performance, read performance, random reads, sequential scans, and consistency.

(via High Scalability)

Diese Optionen für das JDK6 sollte man kennen, wenn man dem Garbage Collector bei der Arbeit zusehen will. Speziell für das GC-Tuning sind diese Optionen unerlässlich:

-verbose:gc
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintReferenceGC

Details zu den Optionen und zur Auswertung kann man unter GC Tuning oder in der Liste der JDK-Optionen finden.

Wir hatten heute eine kurze Diskussion über Singletons und die Art und Weise ihrer Erzeugung, speziell wenn man sie faul (lazy) erzeugen möchte. Die Wikipedia hat dazu diesen schönen Eintrag – On Demand Holder Idiom:

In software engineering, the Initialization on Demand Holder idiom (design pattern) is a lazy-loaded singleton. The idiom can be implemented in both single-threaded/serial and concurrent environments, but care must be taken to correctly implement the idiom under concurrent conditions.

Ganz besondern wichtig ist die Erklärung, warum Lazy in diesem Fall so und nicht anders funktioniert:

The implementation relies on the well-specified initialization phase of execution within the Java Virtual Machine (JVM); see section 12.4 of Java Language Specification (JLS) for details.

When the class Something is loaded by the JVM, the class goes through initialization. Since the class does not have any static variables to initialize, the initialization completes trivially. The static class definition LazyHolder within it is not initialized until the JVM determines that LazyHolder must be executed. The static class LazyHolder is only executed when the static method getInstance is invoked on the class Something, and the first time this happens the JVM will load and initialize the LazyHolder class.

Heute habe ich eine wunderbare Webseite zum Thema Java Concurrency gefunden. Jakob Jenkov hat hier viele interessante Themen zur parallelen Programmierung mit Java zusammengefasst. Ein Bookmark wert und in einer ruhigen Minute unbedingt mal lesen.

Java Concurrency – A Tutorial

Java was one of the first languages to make multithreading easily available to developers. Java had multithreading capabilities from the very beginning. Therefore, Java developers often face the problems described above. That is the reason I am writing this trail on Java concurrency. As notes to myself, and any fellow Java developer whom may benefit from it.