This is the Programmer's and User's Guide for JGroups. It provides information about the following areas:
Installation and configuration.
Using JGroups.
Architecture and implementation of JGroups. Focus on the protocol stack and protocols.
Most of the Installation and User's Guide has been copied from what is freely available on the JGroups web site . However, the focus of this document is to introduce programmers who want to learn more about JGroups to the architecture and implementation of JGroups. I will for example go into the details of the protocol stack, how a message traverses the stack, and how protocols can process it. I will also explain the various design decisions I had to make when designing JGroups, which hopefully leads to a better understanding of why things are the way they are.
Here are a couple of points I want to abide by throughout this book:
I like brevity. I will strive to describe concepts as clearly as possible (for a non-native English speaker) and will refrain from saying more than I have to to make a point.
I like simplicity. Keep It Simple and Stupid. This is one of the biggest goals I have both in writing this book and in writing JGroups. It is easy to explain simple concepts in complex terms, but it is hard to explain a complex system in simple terms. I'll try to do the latter.
I spent 1998-1999 at the Computer Science Department at Cornell University for a post-doc, in Ken Birman's group. Ken is credited with inventing the group communication paradigm, especially the Virtual Synchrony model. At the time they were working on their third generation group communication prototype, called Ensemble. Ensemble followed Horus (written in C by Robbert VanRenesse), which followed ISIS (written by Ken Birman, also in C). Ensemble was written in OCaml, developed at INRIA, which is a functional language and related to ML. I never liked the OCaml language, which in my opinion has a hideous syntax. Therefore I never got warm with Ensemble either.
However, Ensemble had a Java interface (implemented by a student in a semester project) which allowed me to program in Java and use Ensemble underneath. The Java part would require that an Ensemble process was running somewhere on the same machine, or within the same network, and would connect to it via a bidirectional pipe. The student had developed a simple protocol for talking to the Ensemble engine, and extended the engine as well to talk back to Java.
However, I still needed to compile and install the Ensemble runtime for each different platform, which is exactly why Java was developed in the first place: portability.
Therefore I started writing a simple framework (now JChannel ), which would allow me to treat Ensemble as just another group communication transport, which could be replaced at any time by a pure Java solution. And soon I found myself working on a pure Java implementation of the group communication transport (now: ProtocolStack ). I figured that a pure Java implementation would have a much bigger impact that something written in Ensemble. In the end I didn't spend much time writing scientific papers that nobody would read anyway (I guess I'm not a good scientist, at least not a theoretical one), but rather code for JGroups, which could have a much bigger impact. For me, knowing that real-life projects/products are using JGroups is much more satisfactory than having a paper accepted at a conference/journal.
That's why, after my time was up, I left Cornell and academia altogether, and started a job in the industry: with Fujitsu Network Communications in Silicon Valley.
At around that time (May 2000), SourceForge had just opened its site, and I decided to use it for hosting JGroups. I guess this was a major boost for JGroups because now other developers could work on the code. From then on, the page hit and download numbers for JGroups have steadily risen.
In the fall of 2002, Sacha Labourey contacted me, letting me know that JGroups was being used by JBoss for their clustering implementation. I joined JBoss in 2003 and have been working on JGroups and JBossCache. My goal is to make JGroups the most widely used clustering software in the Java space...
Bela Ban, San Jose, Aug 2002, Kreuzlingen Switzerland 2006