Configuration vs. convention

My buddy Dave Thomas has an interesting blog entry concerning convention vs. configuration. I’ve given it a considerable amount of thought today because it is something that hits you square in the face when switching between frameworks and languages (i.e. WebWork to Rails and back each day).

Both have upsides and downsides and nothing seems to really bring one out as a front runner. Dave’s point about ramp up time is completely accurate. But those configuration nightmares also have that same issue right? No one is really going to memorize what a WebWork validator XML file looks like. Who wants to do that? But they also suffer from double work. The work of coding and the work of configuring, which makes it difficult to compare with convention based programming where configuration is removed.

The major problem I see with convention based programming is always constraints. Of course Rails and Ruby allow you a lot of freedom to modify classes and intercept method calls and stuff like that, which alleviate some of the constraints, but there are constraints. For example, in Rails it is difficult to externalize an action chain and reuse actions for multiple events. Something like this:

Action #1
one -> two -> three

Action #2
four -> three -> five

If each of the steps are atomic units of work and simply state their outcome as success or failure, this makes sense. You can reuse three because it doesn’t know who it chains to. The chaining is externalized into some XML configuration somewhere. You can pull this off in Rails using YAML and some hacking around inside your controllers, but it can get ugly quick. In something like WebWork, this can be handled entirely in the XML configuration without the code changing at all.

Dave’s second point about maintenance seems to me to be an issue with language rather than framework. Creating crazy method-missing handlers and classes that are defined in 15 files are features of languages and this is where I see the cost of maintenance is incurred. Rails itself doesn’t really let you tweak too much. It seems to me that Ruby let’s you tweak just about anything. Of course, not being an expert in either, I won’t speak definitively about that.

Dave’s last point about Perl I’ve kinda covered already. This happens with configuration and convention. However, with IntelliJ and configuration, you get some pretty good help and code completion that can really make the re-learning much faster. You still have to recall how to code on the configuration side as well as the convention side and nothing can really make that less teadeous and error prone except documentation and good exception messages.

I’m not certain that I fully grok meta-frameworks per se. But I will say this, I have developed systems that are both configuration and convention and they always seem to make me happy. To this day I’m still floored at how few frameworks do this and even more floored and how few people have used mine even if they realize that it supports both styles. The JavaLobby link on Dave’s blog entry covers this a little bit but I think folks still lack the understanding of configurable/overridable conventions.

Take for example the Savant 2.0 alpha I released. This system I love for its use of convention and configuration. If you want to get up and running fast, you can run Savant to create a project layout for you (or just follow the convention for layouts and build one yourself) and that’s it. You can now build, clean, jar, deploy, whatever. The standard layout (the convention) is like this:

src/java/main = The main source files that will be included in the JAR
src/java/test = Unit tests for the source files (not included in the JAR)
target/classes/main = The location where the main source files are compiled to
target/classes/test = The location where the test source files are compiled to
target/jars = The location where the JARs are placed after they are built

Its simple, straight-forward and works with absolutely NO configuration. This is where it gets good though (and personally I feel really Savant excels past Maven and Ant). Let’s say you can’t use the convention or need to tweak it. Let’s say you have this project layout:


Now the Maven folks will say, “that’s three projects with separate JARs” and perhaps it is, but you can’t change it right now or you can’t change it ever and really would like a nice build system to help you out. Well, Savant to the rescue:

  <directories id="library-1">
    <source location="src/java/library-1"/>
    <output location="target/classes-1"/>

  <directories id="library-2">
    <source location="src/java/library-2"/>
    <output location="target/classes-2"/>

  <directories id="shared">
    <source location="src/java/shared"/>
    <output location="target/classes-shared"/>


This configuration overrides the default project structure and allows you to define as many source directories, build directories, test directories and JAR files as necessary. It even lets you define which JDK to use to compile each one if you want.

The moral of my story is that I think frameworks should always work based on convention right out of the box with as little configuration as humanly possible. Then if you want to tweak, slap down some configuration and go to town! Perhaps this is the meta-framework that Dave is talking about or perhaps not. Either way, this to me seems like a good solution that reduces the overhead as much as possible while still allowing the flexibility to tackle tough situations and problems.

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