How is Ostrich used for configuration?

Question

I need a way to configure my scala application. Configgy seemed the way to go in Scala but it is deprecated https://github.com/robey/configgy#readme and now this functionality is in Ostrich.

Is there a code example on how to use Ostrich only for configuration? I'm not interested in collecting the statistics.

Answer 1

I'd like to know the official answer as well, but nobody answered so I decided to poke around. Sorry if this answer isn't quite comprehensive.

The best example I found was in com.twitter.ostrich.admin.RuntimeEnvironment, especially if you look mainly at loadConfig.

Say you want to configure an instance of class T. The basic idea is as follows:

• Get a java.io.File that contains Scala source code that evaluates to a com.twitter.util.Config[T].
• In order to be a valid Config[T] you'll need to have a def apply(): T method. To keep the implementation details out of the config file, you'll want to define a class in your project that extends Config[T]. You can also use this class to define default/required fields.
• Instantiate a new com.twitter.util.Eval instance, and call apply(file) to get a Config[T] instance.
• Call config.validate() to throw the proper exceptions for malformed config files.
• Call config.apply() to get your fully configured instance of T.

Here is a simple example, where I configure a new WidgetService:

class WidgetService(val port: Int)

class WidgetConfig extends com.twitter.util.Config[WidgetService] {
  var port = required[Int]
  def apply(): WidgetService = {
    new WidgetService(port)
  }
}

object MyApp extends App {
  val configFile = new java.io.File("mywidget_config.scala")
  val eval = new com.twitter.util.Eval
  val config = eval[com.twitter.util.Config[WidgetService]](configFile)
  config.validate()
  val widgetService = config()
  println(widgetService.port)
}

And here is mywidget_config.scala:

new WidgetConfig {
  port = 8000
}

Note: you may have to make modifications if you put this in a package. I did everything in the default package for brevity.

To get the dependencies to work, I added this to my SBT config:

libraryDependencies += "com.twitter" % "util" % "1.10.1"