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Java RMI
The Scoop
Remote Method Invocation (RMI) is the object-oriented remote procedure call (ORPC) facility for distributed programming in Java, since the 1.1 days. RMI also served as motivation and a proof-of-concept for jini, javaspaces, and numerous other solid distributed networking technologies. Of course, anyone from the academic distributed programming world knows Wollrath, Waldo, and Riggs.
Yet, despite a myriad of books over the past five years on network programming, RMI always seemed to be the stepchild: relegated to a single chapter (buried on page 496, of course) that always said that RMI was "better" than sockets and "worse" than CORBA. Now, granted that RMI is operationally rather trivial compared with CORBA and was (prior to RMI/IIOP) a unilanguage distributed ORPC technology -- but still. For those of us who have to interoperate with RMI (whether welcome from the Java world or not), the lack of in-depth technical analysis (beyond the spec) has been a hindrance.
Fortunately, this trend is finally starting to buckle with the release of several in-depth RMI books including: Java RMI, Java.rmi, and Mastering RMI: Developing Enterprise Applications in Java and EJB. As evidence of this problem, Grosso states the same in his introduction – and actually pulls it off without sounding self-serving.
I chose Grosso's text because of the cute squirrel (aka the O'Reilly brand), Grosso's recent series of articles on the hashbelt algorithm, and his unadulterated academic knowledge management and mathematics bent. Fortunately, I was rewarded: this animal returns to O'Reilly's pre-bubble quality. Koodoos to both Grosso and his editors (Knudsen, Loukides, and Eckstein) for getting the train back on the track.
What's to Like
Bottom line is that Grosso simply covers the topics and does so with solid conceptual and code coherence – even by O'Reilly standards (over 40 animals grace my shelves). His prose and explanatory patterns make it clear that he has actually gotten into the real-world of RMI, and doesn't hesitate to highlight both good and bad parts. You cannot be dozing off when you read this (at least not if you expect to understand it) -- this is written by someone with solid analytic thinking skills and it shows. After too many years of "there are no caveats" journalism and publishing, this is a nice reversion. Further, I can only imagine that his current employment is a testament to his real-world knowledge of RMI.
Grosso hits on a vein which is not well-appreciated: when not smoothed over by marketing people, RMI is actually a mostly-capable ORPC technology. Certainly activation and RMI/IIOP really began to make things interesting, from Java2 and EJB respectively. Discussion of reference-counted distributed garbage collection, a feature missing from CORBA and other popular ORPC standards, also contributes a nice bonus (although Grosso's ardent attempt to debunk the "RMI doesn't scale" argument is rather weak, even going so far as to rehash the definition of Threads and threadpools – this complexity mismatch is an ugly giveaway that a well-intentioned editor went astray).
What sets this text apart is the tight focus on nitty-gritty implementation details of RMI itself. After all, these RMI texts are way too late to the game to reteach how to write "baby RMI" code: 5 years after the original spec, you either know how to write RMI or you don't. Grosso simply gives you a solid in-depth analysis of all the obscurities of the RMI runtime, custom sockets, dynamic classloading, activation, MarshalledObjects, and HTTP tunneling. In other words, all the interesting real-world topics whose official documentation is poor and which the various RMI tutorials (written many years ago) ignored.
While canonical, the single banking example followed through the text was well-executed, although authors continue to underestimate the prevalence of readers who consume textbooks non-linearly.
What's Not to Like
RMI/IIOP is shaping up to be a fascinating contributor to the "cleanup the EJB mess" discussion. Dedicating a measly 13 pages (beginning on page 503, no less) to this critical topic seems a bit of an oversight – but maybe that is just my CORBA sentiments speaking. Either way, the mechanics of CORBA are sufficiently intricate in real-world deployments that saying "if you can build an RMI system, you can build a CORBA system" (p. 511) is a bit brazen (or naïve) for my tastebuds. I can only chalk up this oversight to deadline pressure, which is probably a Good Thing, since the book was supposedly in production over almost 2 years.
A minor point: the top-level organization of the book (Part I, II, III) is arbitrary, ignore it -- use the chapter organization instead.
The Summary
Quality: solid practical insight into the nitty-gritty operational implementation details of RMI in the real-world. You simply are not going to find solid O'Reilly-quality coverage of the topics elsewhere.
Relevance: If you are responsible for making RMI actually work in production systems, this might well be the next animal on your shelf – either now or later. If you want a breezy afternoon saunter around RMI, skip this. Instead, google one (of the many) free tutorials online."
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Having come from a background of Servlet/JSP development, I was quite surprised to see that RMI lacked what I consider a basic feature of distributed computing - authentication and session tracking; this forced me to develop my own, but this is obviously a common need and should be handled by the RMI runtime. I haven't worked with CORBA beyond a couple toys to learn the system, but my understanding is that CORBA does have these feature.
The book itself is excellent - I purchased it after starting to work on a large project with RMI, and realizing that a couple online tutorials wouldn't cut it. The advanced section isn't quite as advanced as I had hoped, but overall the book is a decent reference.
take your sig and shove it
I think that most of corba/dcom/rni etc. are particularly over complicated. They place a burden on the programmer in the wrong place. It's worth checking out plan9's approach to distributed computing. Authentication is taken care of by an authentication server. Once you've got an authentication ticket then you can access networked resources by binding remote services into your local namespace. Rights are granted by the usual mechanisms of usernames & groups.
The familair paradigm of users and groups has been deliberately leveraged because that's what is familiar, after all it's stood the 30 years test of time. This extends to which processors on the network you an run processes on etc. etc.
It's worth studying if nothing else.
There are places where the networks are not touching,and there are places where they are-Boeing's Lori Gunter
I have been a java developer for some time. I had the opportunity to use RMI in java to write a distributed client-server pricing solution for a large telecom company. (I'll keep it quiet; I don't want to be sued).
The problem: Run pricing routines on very large CDR (Call Detail Record) sets. This involved a very complicated process of aggregating calls into time slots and then pricing them according to very complicated long-distance contracts. Pricing very large amounts of data sometimes took as much as two days to price and discount.
Solution: The implementation (at least in Java) of the RMI standard is simple. In order to avoid purchasing an expensive CORBA implementation we extended our c++ client program with JNI. The client program then called a local wrapper class that used RMI to spin off the pricing call on the remote RMI server. We had big-iron machines running an in-house RMI application server. The application server really just worked as a multi-threaded container that loaded RMI servers from the disk and registered them, allowing a person to order servers to be loaded and/or dropped from the registry via a console.
The solution was cheap and worked very well.
My $0.02 will always be worth more than your â0.02, so
XML-RPC is just another way to do what RMI and CORBA do--except it uses XML to encapsulate the data (including the call you are making). RMI was nice when it came out, but I haven't seen it mentioned in a long time. Where I'm working right now they would probably just use XML-RPC or SOAP simply because it is open and can be used by a wider variety of systems (like those OpenVMS machine they have sitting around). I personally use XML-RPC and SOAP and they are great if you have to parse things for the web on the other site. I used to pull a ton of RDF feeds from NewsIsFree, but they screwed it up and only started feeding the updates to the channel. I rewrote the calls using their XML-RPC engine which is much better. I then parse the tree and convert it into RDF for my website. Very slick technology.
Judging from the TOC of the book, I'm surprised that the author didn't deal more with the "why should you use RMI over some other technology?" He does cover the CORBA vs. RMI choice, but that was it. I also don't know how "new" this book is--especially if those topics (SOAP, XML-RPC) weren't covered.
-J