Building configurable applications

As it usually is the case recently I've been reviewing options to build web applications that can be easily transferred between environments. I found some very interesting examples on what can be done to achieve some quite interesting results. But let's start at the beginning..

The project has been released to the public as com.github.testdriven:cfgagent:1.0.0

The problem

The problem usually is that the application we develop tend to have too many configuration options. Database connection string, SMTP server settings, file locations (more than one), configuration of connection pool... just to name a few. As the number of options grows (the project I'm working on now has about 200 of those) passing them all in the command line using JAVA_OPTS is just not possible. And I don't mean like "not practical" but plain and simple not possible due to the limits of what a command-line can take. There has to be a better way to do it.

There actually is a mechanism that one can use with Tomcat called catalina.properties (and I'm sure one would find similar ones in other containers) but that is not going to fly if we want to select which set of options we want to use for this particular run or if the values should come from environmental variables (for example from Docker or Heroku).

JVM Agent to the rescue

I've been looking for a while for a nice, pluggable solution to this problem. My idea was roaming about something that could be fixed but parameterized at the same time, could be specified at execution time (just like JAVA_OPTS are) and in general to be just easy to use regardless of the execution environment.

Looking at it from different angles I turned my attention to what is executed before the main method. As it turns out there is such a thing and it is the concepts of Java Agents.

Without further due let's see the solution:

The code is pretty much self explanatory. First we load the properties from a file name given to the agent (default: system.properties), then we replace all the {placeholders} with environment variable values.

Example usage

At first we create a Docker (instantly cool) container with Oracle. Then we start Tomcat with system properties configured based on the system.properties.

Conclusion

This small utility can easily be used with any type of process where the _OPTS part is getting way too long to be maintained. I've tried it with JBoss, Tomcat, command-line and it worked great every single time :) It also has this nice property that you compose behaviors instead of hard coding them in every application and your products depend only on the concept of system properties (System.getProperty()) that is already present in Java

Happy coding!

It feels great to be blogging again :)