Why OSGi needs to come of age

The team I'm lucky enough to be part of is currently working on a project with GridGain, which I've mentioned before, once or twice. In our case, we are invoking map / reduce jobs via RESTful web services. It works well. The problems start cropping up with the way classes are loaded (or not) inside Tomcat.

GridGain is (in our case), effectively, deployed in a war file. This is because we want to communicate with a layer around GridGain via HTTP (and Spring MVC, specifically). In the interest of time and simplicity, we chose to start up the grid from within Tomcat, although, a cleaner design would be to queue tasks from within Tomcat, run our GridGain app as a daemon, and dequeue tasks to be executed there (maybe using JMS or something similar). It's a lot of jumping through hoops, really, because at that point, you may as well speak JMS directly to the GridGain-enabled daemon. Too much bloat.

Most of the complexity is really introduced simply because of the environment required to receive HTTP requests. Really, when you think about it, we've had to - simply to support HTTP invocation - change how we build, package, deploy, start and stop services, and deal with 3rd party dependencies. Worst of all, Tomcat has to deal with the servlet specification and how class loading works, therein. As it turns out, GridGain plus Tomcat equals yuck.

Now, I won't really go into specifics (in this post) about my thoughts on class loading in Java, peer class loading with GridGain, and the myriad of issues we've seen and run into. What I do want to touch on is the more general theme that, even after all of this time, this stuff (i.e. static and dynamic class loading in containers) is still way too thick, intrusive, delicate, under-documented, and buggy. We need evolution, if not revolution.

I dropped a war file containing our code, and approximately 100+ jars worth of dependencies to make Spring, Hibernate, AspectJ, and GridGain happy. One hundred, plus. Smells like disaster already. GridGain starts up, Spring MVC does its thing, life looks good. I make a request to a controller that invokes a task via GridGain and things get interesting.

The task we want to start is looked up in a Spring config, and instantiated using reflection. It makes it easy to add new types of tasks without mucking around with the core grid assembly and packaging, but it creates an interesting case. The jar(s) containing the grid tasks need to now live in the war file. The alternative seemed to be storing them in a directory accessible to the Tomcat shared class loader, but during testing, we found that Tomcat couldn't handle this (for some reason that is still admittedly a little unclear to me). The class simply wasn't found. The class loader hierarchy is modified in Tomcat due to the servlet spec so the order should be bootstrap, system (JDK), the war classes and internal libs, then the common and shared class loaders (if I remember correctly). This means that when we move our grid tasks to the shared class loader, they would come after the web app class loader. Normally this shouldn't be a problem, but I think the issue we ran into was related to the fact that the grid task extended a class that was present in the war's class loader, not in the shared class loader. When we move the grid task jar into WEB-INF/lib, it magically starts working. Sigh.

I'm a fan of OSGi. It (mostly) makes sense to me. It's explicit (painfully so, in many cases) and direct. My feeling is that, in a case like we have here, at least it would be obvious that class loaders needed to be wired together to make this work. The main grid application could have fragments containing the grid tasks, for instance, to push additional task plugins into the core application, making the classes available. Alternatively, a more direct way would be to simply list the packages as dependencies in the manifest of the core application. This would obviously couple the core app to the tasks, which isn't very nice.

The GridGain team, unfortunately, hasn't embraced OSGi. In fact, Ivanov seems to summarily disregard it in the comments of this post. I was a bit let down to see this kind of hard-line stance on a subject that more and more people seem to be interested in, especially with the SpringSource folks driving at it. While Nikita makes an excellent point about distributed class loading and OSGi being a less than nice match, I do think there's room for GridGain with peer class loading disabled in an OSGi container. For some of us who don't care about the hassle of deploying grid tasks (and GridJob implementations, specifically) on all nodes, at least having the option of GridGain in an OSGi container like Spring dm would be nice. It's possible that this can be done, but when I tried, things didn't go well. There's no doubt that Ivanov and the GridGain team have far more experience with the class loading details than I do, but the stance taken does still let me down, at least not without a better explanation and documentation.

OSGi isn't perfect. It's far from it, obviously. What it does begin to chip away at, though, is the big fat bundle-it-all-in-one-giant-file issue that plagues Java-land. Honestly, Java is thick and we have to change that. OSGi is one way to cut it down to a reasonable footprint while still allowing for decoupling and service-like component design. Sure, permgen errors are a dirty word and the class loading mechanics may be too simplistic but it scares me to think we'd be stuck in war-land for the rest of the foreseeable future.

The Cloud - Not a Panacea

Everyone loves The Cloud. I mean, why not? There's no hardware to own, no storage systems to maintain, no networking hardware to deal with, it's infinitely scalable to the billionth degree; it's perfect!

Ok, so no one gushes about it that much (do they?). It has its notable benefits, but they aren't quite as obvious or simple as people sometimes make them out to be.

The crux of the issue is simple; simply moving traditional services, applications, or systems to a cloud-like environment yields little. Traditional systems with no knowledge of a cloud (or grid, or virtualized infrastructure, or...) environment can't take advantage of the dynamicity that it offers. Many systems (and arguably, people) don't understand what the true benefits of the cloud are.

A Common Cloud Case

Let's take a concrete - and common - example: an RDBMS on the cloud. I'll talk specifically about MySQL because it's on the tip of my tongue, but it will apply to many similar systems.

So there you are, running MySQL on Amazon EC2 on your Linux distro of choice. That was easy enough. You cron a nightly export to S3. You've taken advantage of some of the shared resources and that is good, to be sure. You can use EBS for large volumes. You lose some performance, but you can afford to get a larger instance so maybe you can increase some of the buffers and keep more in memory. Depending on your situation, it may all work out in the end. We won't get into the nuances of exactly what computational resources you get because it's damn near impossible to measure accurately and consistently. Let's call it a wash.

Perhaps the most important part of this is that you can easily set up one or more replicas. That's pretty damn nice.

That's a lot. The problem is that there isn't much one couldn't do in a traditional data center. Sure, something like EC2 can help jumpstart a start up - which is great - but for medium sized and larger companies, this isn't a concern. The question is what does the cloud do that the traditional data center (and approach) does not? The cloud is dynamic and on demand for a reason and this isn't it.

What the Cloud Does Well

Applications that natively and intrinsically know about the cloud and its properties and can actually react to changing conditions are the true candidates for cloud computing. For instance, stateless web serving is something that can take advantage of this kind of environment (with some additional functionality). The reasoning here is simple: with some simple measuring of load and knowledge of capacity, additional web server instances can be forked off and run, independently. These additional web servers (or their IPs) could be added to an external load balancing system to make them available to the public. There's no significant dependency here. Content to be served by the additional web servers can be made available with little fanfare and, provided you have enough resources, additional connections can be made to resources such as relational databases, caching servers like memcached, and so forth.

That's, of course, not the native case I mentioned. That's an adaption of a traditional service to the cloud using an orchestration process or resource manager, of sorts. The best case for the cloud are applications where change of the infrastructure is built in. People tend to easily go to places like map / reduce frameworks where jobs are self contained or transport state with them. Not to trivialize that case, but it's not everyday you find the perfect fit for such a model. Many times, you find hybrids where map / reduce jobs (a computation layer) requires access to a data grid (the storage layer) which limits your flexibility and ability to deal with changing conditions. In that case, it's usually not desirable to repartition or shift your data storage based on load (although you can repopulate caches based on load and network expansion). Maybe you don't care about data affinity because the footprint is small or access is infrequent, but with massive data stores or logically partitioned data, this is prohibitively expensive.

I'm not sure I have a good conclusion to this. What I'm driving at is that, while the cloud and dynamic infrastructure is a blessing, don't think of it as a panacea. Do the cost breakdown and consider what portions of your systems make sense in such an environment. Many times, running a traditional system where dynamic setup / teardown of nodes isn't feasible, on a platform like EC2, will wind up being more expensive after a year or so given even light usage. It's a buzzy topic and everyone wants to be on the cloud. Remember that the fastest way to kill something in the eyes of business is to push it for the wrong reasons or in the wrong situations.

We're Hiring!

We at Conductor are looking for Java developers to join our engineering team here in New York City, NY, US. Please take a look at the position details and submit your resume if you fit the bill. Make sure you indicate where you heard about the position (no, I don't get a referral bonus if people come from my blog. Unless someone from our HR department is reading this - then I want my bonus).

We're very interested in people who have real world experience building large scale, highly available, distributed, performant, applications in Java with Spring. Love of open source and / or technology in general is a huge plus. Knowledge of a language like Python, Ruby, Groovy, or Javascript is also good. Experience building data mining or analysis applications is good for bonus points. Come work with us. Trust me, it's a good place to be!

A reminder: None of this is sponsored, reviewed, or endorsed by my employer. Please see the Careers section of the Conductor corporate website for details.

Principles of Architecture - Anticipate Reality

My title at work is System Architect. Actually, it's something like System Architect / Engineering Lead, but I have my suspicions that it was suggested by our business card printing company who may, or may not, get paid by the letter. That's a story for another time.

What I'm driving at is that many times, architects are thought of as different from developers. It's true that the path to software architecture is either rooted in, or tightly entwined with, software development, but it tends to be some kind of a specialization. The reason I bring this up is because, as architects, we run the risk of separation from actual, real world, implementation concerns. By removing ourselves from the nitty gritty implementation details (a phrase I've been lazy enough to toss around in certain circumstances, admittedly) we have the potential to forget or otherwise disregard the venerable minefield that is the production environment and even the real world.

Using tools like diagrams, white boards with the nice non-smelly dry erase markers, pens and paper, and even more direct methods such as defining interfaces, we're still far removed from the underbelly. We're removed from reality both conceptually and (almost) physically; the implementation - the realization of a given architecture, big or small - is not our own. In other words, we're not subject to our own dog food. And, if you're not careful, you might end up designing for the utopian world of your favorite modeling tools. The professional term for that is screwed.

By not pushing yourself back into the role of implementor either by contributing to code directly or working closely with those dealing with your precious architecture, you are robbing yourself as well as sabotaging the rest of the team, not to mention the project. If you work in an organization where this isn't feasible you can still place yourself in the shoes of the developer, the testers, tech writers, product managers, all the way up to the end users. The impact of your design decisions are greater than just the common set of technical concerns. You are bound by the goals of the project. Your design must not only be simple, elegant, and technically correct, but it must deal with the idiosyncracies of the business, production woes, maintenance and operation teams, and so on. In fact, I'm a little sad I even used the word elegant.

I spend quite a bit of time writing code at work. I do this for a few reasons. For starters, we're just in need of extra hands; we're working on some very cool projects on tight time lines and I'm always a developer regardless of title. Possibly even more important than simply generating code, though, is the need to get things right, in terms of design. By working with the rest of the team, dealing with implementation questions and concerns, all day, every day, I'm forced to constantly reconsider what is working and what isn't. Developers are the first to trip over corner cases in the design or find awkward situations that are difficult to detect in a pure design phase. In an agile environment, the constant attention to high quality and correctness means - in terms of design - following a design through to the end, through all its transformations and anticipating real world situations.

Declarative Concurrency In Java - A follow up

It's put up or shut up, right? I don't know if this is something I can do by myself, but I'm happy to try and get the ball rolling.

Some time ago, I wrote about declarative concurrency in Java. It seemed to get a good reaction from many people from different sections of the community. I wound up receiving a lot of email about how people were interested and how the idea of being able to define concurrency semantics in such a manner was appealing for a number of reasons. Well, I went ahead and stubbed out a project which I've pushed to github[1] for the world to pick at! It's minimal and there's very little there right now, but I wanted to solidify my intent to actually produce a prototype by physically creating the project and pushing it out there.

Everyone is welcome and encouraged to participate. The goal of the project will be to create a simple, open source, library in Java that will do the following.

1. Allow developers to annotate methods, indicating that they may be executed in parallel.
2. Provide a simple library that will, based on configuration and hints from the annotations, intercept invocations of the annotated methods and execute them concurrently.

I hope to get additional feedback on the concepts as well as the implementation as it evolves over time. Thanks to all who have provided feedback and encouragement thus far!

http://wiki.github.com/esammer/decothread

[1] - github.com - decothread project @ github

Principles of Architecture - Reduce and Simplify

Just a few days ago, I had a weekly one on one meeting with my boss. It's times like that where work becomes kind of like a game of Can I go a full hour without putting my foot in my mouth? Turns out, I came out unscathed this time.

Recently, around the office, we've been talking a lot about the principles behind the agile manifesto. Pav (aka John Pavley, CTO) mentioned that I probably had a similar list of principles of software architecture I operate by. He pointed out that I hadn't really vocalized what those are in any obvious way and that doing so would probably be beneficial or at least interesting. Thinking about what those principles are and needing to actually enumerate them also helps me think about what's really important and why.

Before we get into the first principle I want to discuss, I want to clarify why I'm using the term principle rather than, well, anything else. During our discussions of the principles of the agile manifesto, we used this word because, like what I hope to describe in software architecture, those items are considered intrinsic properties of software development. As Pav would say, they're discovered, not invented. I tend to think he's right and that the choice of the word principle is deliberate and intentional. This is also my intention here.

One of the first two principles that came to mind was the idea of reduction and simplicity. When designing software, we strive to reduce or eliminate complexity wherever possible. There are times where a task is inherently complicated, but the design of the system need not necessarily be complicated. If that sounds counter-intuitive, consider the separation between designing the system's architecture - the way it behaves, the layering, the major objects in play, the way it interacts with constituent systems or resources, its fault semantics, and so on - from its implementation. In many cases, what you'll find is that the implementation may have some inherent level of complexity to meet the business requirements, but that a well designed system is almost obvious and easily fits in your brain without confusing you. Let's consider something concrete.

If you were to design an SQL query execution engine... I don't even need to finish that sentence for it to sound scary. Take a few minutes to think about how you might design a query execution engine. In five or ten minutes you might actually be able to work out a simple model that makes sense (within reason). The details of how to implement that design is where one gets into the shady details of how to make the magic happen. Even the design of a compiler is simple enough, in most cases, where as the implementation is where the complexity lies. A compiler will have a grammar, a lexer, a parser, probably an AST, a chain of optimization strategies, maybe a number of output generation strategies. Within each of those major components, you could break things down further and come up with an easily understood design for a modular compiler. I'm not trying to trivialize building a compiler (try implementing a C++ compiler some day) but I do think that with some thought, the design process would effect a reasonable, intuitive result.

The point I'm trying to make is that a well designed system should be intuitive to the person or team implementing the system as well as the architect. If you find it difficult to communicate a design, there's a high chance that the implementation of that design will not make things any simpler. In fact, it's probably impossible. To be clear, some things have an inherent degree of complexity, but we should always strive for the simplest, but still most complete, design possible.

These are all relative terms; simple, complex, intuitive, complete. You'll always have to rely on your judgement, experience, and best practices of the trade. By properly deconstructing an application, its components, their components, and so on, even the most complex system can be easily understood and digested.

Some techniques I find useful for making this happen are:

• Apply Divide and Conquer. Break down components recursively until you get to easy to understand units of functionality.
• Never work alone. Statistically, you're more than likely to be surrounded by people who can contribute experiences and ideas during the design process that will yield a better result. The added benefit here is that you're constantly having to explain your thought process and ideas to other humans; the degree of complexity is probably proportional to the number of times a junior developer says Huh?
• Follow patterns and best practices. Silly questions like Does your class do one thing and do it well? have saved me from my own cleverness more than once (but admittedly not always).
• Trust your gut. If it sounds too complicated, it probably is. Take a break, look at similar problems, ask around, and try a different approach.

There's no complexity blasting ray gun of designly awesomeness. There's no third party library that you can just drop in to make it simple. Sometimes, the business requirements are as tough as they sound. Most of the time though, you can reduce and simplify.

Class Categories in Java

Class categories have existed in a few incarnations over the years. My personal knowledge of comp sci history is thin, at best, but my small amount of research into the material is that it came from Smalltalk-80. Certainly, my first exposure to class categories came from working with Objective-C on NeXTSTEP and later Mac OS X.

What a category does is relatively simple to understand. Basically, the idea is that a developer may take an existing class and, effectively, append methods to it without having access to the source code or subclassing it. This is best illustrated in code.

// A normal, and terribly boring class.
public class Foo {

  public void displayMessage(String message) {
    System.out.println("A useless message:" + message);
  }

}

// Extending the class by creating a category on it.
public class Foo (MyExtensions) {

  public String getDefaultMessage() {
    return "Hello world.";
  }

}

// ...and finally, what one would expect.
Foo f = new Foo();

f.displayMessage(f.getDefaultMessage());

Normally, when I explain class categories to Java developers, I barely get the words out before I'm hit with what you might expect. People question whether this is breaking encapsulation, if this bloats code, if it creates tight coupling, if it breaks access rules, and so on. Some of the more dynamic languages like Ruby and Perl will happily let you do things like this (albeit, sometimes safer than others), but that happens at runtime.

I'm proposing we bring categories to Java. Yep. I said it. I'm going to focus more on how this might work rather than why I think it's a good idea, although I'll try and briefly address that too. Here's how I think it could work and why.

The Basics

The syntax would work like Objective-C's syntax. Creating a category would be done by specifying the same package and class definition, in the interest of simplicity, with the addition of a category name enclosed in parenthesis (see the above example). It would not be legal to specify inheritance when creating a category; inheritance would always be defined by the original definition (i.e. the uncategorized class declaration), although there's no reason to prohibit the implementation of additional interfaces in a category. This would allow those creating categories on a class to extend an existing class to implement a new interface without modifying source code.

Access, Security, and Visibility

In many ways, the access and visibility rules of subclassing applies to categories as the result is very similar from the perspective of the original class; the new functionality is unknown and untrusted.

It would only be legal to access public or protected members of a class when creating a category. This would respect access and visibility restrictions on code developed prior to the existence of categories. Overriding a method in a category would not be permitted, although method overloading is fine. Private members within a category would not be visible outside of the category.

Category Availability

The biggest differentiation between categories in Java and class reopening in Ruby, for instance, is that the contributions made to a class via a category would be known and could be checked for at compile time. This would allow developers to see and avoid cases of competing categories or member addition during development, which is usually not possible with languages that allow for this kind of functionality.

It would not be legal to create a category on a class declared as final. This extends the meaning of declaring a class to be final, but only slightly as creating a category is similar in intention to subclassing (in theory). This rightly implies that there is no way to prevent the creation of categories, but allow subclassing as there's no obvious reason to draw a distinction as categories can only access public and protected members of an existing class, just as a subclass would.

External classes including unrelated as well as subclasses of a class with categories would see all members including those defined in categories as usual. The category information of a member should be made available via the standard reflection classes and methods. Given the above example Foo class, the following would work as expected.

/* Includes both methods from the original class declaration
 * as well as methods from categories.
 */

Methods[] methods = Foo.class.getMethods();

/* Additionally, category information should be made available
 * via reflection.
 */

Category[] categories = Foo.class.getCategories();

for (Category category : categories) {
  System.out.println("Methods in category:" + category.getName());

  for (Method method : category.getMethods()) {
    System.out.println("method name:" + method.getName());
  }
}

Some Quick Reasons Why

There are a few nice advantages to having categories available in a language, especially at compile time. There are the obvious advantages such as simple code organization. What I tend to think is more interesting, though, is creating categories to apply specialized functionality to core classes. For instance, one may want to add methods to collections to glue validation logic to core components. In cases where Spring is used, it's not uncommon to see many adapter type objects that simply exist to make an object more amenable to participate in DI. I believe that a lot of code and class structures could be greatly simplified by being able to make minor alterations to existing classes rather than resorting to multiple objects to mediate or adapt existing code to new systems and frameworks.

Like anything else, there is the obvious ability to abuse something like categories. I think there are times when more traditional approaches are the best option, and there's no replacement for good design and education, but to remove a valuable tool because some subset of the population may misuse it only serves to hurt those that could make proper use.

My plan is to attempt to draft this as a JSR and submit it for review. I don't know if I have the ability to chew through the politics (I'm assuming are) attached to that, but it might be fun to try. Clearly there's more to work out (I haven't looked deeply into what this does to the compiler and runtime at a low level, for instance), but I'm interested in what people think about categories in Java.