My weblog is now here.
IoC containers, Part 2 soon. But in the meantime, some Lisp links.
Whatever happened to Henry Baker, I wonder.
Mike Spille wrote an interesting essay about Inversion of Control containers, (with a followup here). He gives a checklist of desirable features in these containers, and surveys HiveMind, Spring, and PicoContainer. PicoContainer comes out worst, partly because it lacks some of the features from his checklist (and in fact, these omissions are deliberate on the part of the creators of PicoContainer).
I have a particular interest in IoC containers, because I developed one at work, and continue to maintain it and enhance it today. I call it the init framework (colleagues often call it components.xml, because its per-module configuration files have that name). Here, I'll just call it TIF.
In terms of generalities, TIF has a lot in common with the well-known IoC containers. It shares many ideas and some specific techniques, and the basic benefits are similar. But these similarities represent a case of convergent evolution. I began work on TIF a little over 2 years ago (the CVS logs say the first check-in was on 2002-07-01; the first lines of code must have been written a few days earlier). If those other IoC containers existed at that time, I wasn't aware of them.
(From the public CVS repositories, it seems that HiveMind originates in May 2003, PicoContainer from mid-2003, and Spring from August 2003, but it is possible that those dates are misleading: the projects may have been hosted elsewhere before moving to their current repositories. In any case, some of these ideas may well have been around earlier; I seem to remember being aware of Apache Avalon, though it was certainly not something I had any desire to imitate.)
So I find it interesting to compare TIF with the open source ones, and to consider why I made the design choices I did and why the authers of the other containers made similar or different choices. Since Mike's essay it perhaps the nearest thing I have seen to a survey of the IoC "market", it provides useful context for these comparisons.
In summary, here is how the comparison comes out:
These last two points stand out, because Mike's article, and the subsequent discussion, criticized PicoContainer for missing precisely these features. Mike's position is that these are examples where the creators of PicoContainer put their purist principles ahead of the pragmatic needs of developers who might use their container. I can't speak for the creators of PicoContainer, but my decisions had very pragmatic reasons behind them: If my container doesn't meet the needs of the real-world projects I work on, my colleagues simply won't use it!
In subsequent posts, I will consider each of these features in turn, and explain how TIF works the way it does. Then I will write another post to describe notable features of TIF not shared by other containers.
(These posts are partly a response to colleagues who have asked me why I wrote TIF when there are similar open-souce projects around, and why it doesn't support cyclical dependencies. I expect someone will eventually ask me why it doesn't support setter-based injection, and I will be able to point them here.)
The term microkernel has a specific meaning, from operating systems research, and that meaning does not seem to be relevant to NetKernel in any way. Their use of the term kernel also seems dubious, but at least that is a general term used in different ways in different parts of the software world, so they can more reasonably appropriate it to apply to their project. (Maybe all this would seem less silly if microkernels, in the true sense of the word, were more widely seen as a good thing.)
I'd like to dismiss this as the usual marketing stupidity, but the misuse of the term continues even in the technical documentation, which claims:
We have deliberately omitted technical marketing documents which present business and/or technical analysis for choosing NetKernel. If you require technical marketing literature for NetKernel please visit the 1060research.com and 1060.org sites.
You also need to dig through their web-sites and documentation to discover that, while the concept may not be specific to Java, the current implementation certainly is . But surely this is important informantion for any potential customers? My guess is that they want to avoid the question of why they don't implement apparently-relevant standards from the Java world such as the Servlets API. I hope that the answer is that they are doing innovative things that don't fit inside those standards. It would be nice to see this explained properly.
I rarely see much discussion of unit tests that acknowledges that they may not be appropriate in some contexts. But the cost/benefit function for unit tests is by no means trivial:
Even for "hard" code, unit testing is still possible, but the design must respect unit-testability. This can greatly increase the amount of work involved over the non-unit-tested case. If this level of difficulty is underestimated, the tests may end up ensuring very little, or being fragile, or creating an obstacle to maintainance and further development of the code covered by the tests.
Because of issues such as these, unit testing might best be understood as a way of catching some bugs earlier, rather than as a way to reduce overall defect rates. Thus it can make the development process more predictable.
But comprehensive unit testing has high costs, and other forms of testing may be more appropriate for some projects, or some parts of some projects. It would be nice if there was less cheerleading of unit testing, and more analyis of the pros and the cons.
I'm trying to work out if there is anything interesting in SOA. I'm hopeful that there is, but I haven't found anything written about SOA that captures it, despite the fact that a lot of clever people are writing about SOA. Of course, much of this verbiage is due to to the SOA hype machine. But once you discard the hype, what is left?
If there is any merit to Web Services, it must be in SOA, since it is certainly not in the concrete realization of SOA as XML Web Services. Each layer is flawed in various ways. XML may be a nice document interchange format, but it is a strange choice as a message serialization format for communications protocols (even if you focus on document/literal style web services). SOAP, XML Schema and WSDL all have reasonable ideas at their cores, and can all be made to work, but none of them fit their niches perfectly, and none of them are a model of simplicity and elegance.
So where is the merit in SOA?
The first two tenets seem to me to be a matter of emphasis, rather than innovations. These principles can be used to advantage in conjunction with CORBA or COM; conversely, there is nothing about Web Services that requires these principles to be observed (though some Web Services toolkits make boundaries explicit simply by having an excessively clunky programming model).
The third tenet seems like the weakest of the four. CORBA and COM do not share programming language classes; they share contracts defined in their IDLs. The relevant implementations map contracts specified using those IDLs onto machine-centric bindings and representations, rather than the textual XML representations used by Web Services. But there is little in those IDLs that mandates such a particular mapping. In any case, the debate does not seem to hinge on the superiority of XML as a data format compared to more machine-centric formats (perhaps becuase the XML generated by web-services toolkits is often little more human-readable than machine-centric representations). So my guess is that this tenet has its origin as a reaction to something else:
My conclusion is that if the first three tenets were all there is to SOA, then CORBA and DCOM would be perefectly adequate technologies for implementing SOA.
Which leaves "Service compatibility is determined based on policy". I have not yet come to any conclusion about how strong this tenet is. I suspect that there is something there, but the examples offered by WS-Policy and the related specifications are not very convincing.
This struck a chord. In my day job, I maintain a large Java code base, which is used from several other projects. Mostly those other projects use the "published" interfaces, but sometimes they try to use classes and interfaces that I regard as internal (although they are declared as public, since they are internal but widely used within my code base). Eventually I will have to document the list of published classes and interfaces. I will start by making a list of classes that are already used externally. A while ago, I wrote a Perl script that analyzes dependencies between .class files, and it shouldn't be too hard to adapt it for this purpose.
When I wrote the first entry to this diary, I planned to follow it up reasonably regularly. I didn't, but that is partly because I spent most of December on vacation. The new year seems like a good time to try again.
So to begin with, a few impressions gathered during that vacation.
New HTML Parser: The long-awaited libxml2 based HTML parser code is live. It needs further work but already handles most markup better than the original parser.
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