Testing.md

Testing

Fresh registries

During unit testing it is common to instantiate the service class that is being tested many times. This is a problem if your service defines a gauge or a health check; you can not register multiple of those under the same name.

Here is an example. The service class Example defines a gauge:

class Example(db: Database) extends nl.grons.metrics4.scala.DefaultInstrumented {
  // Define a gauge with a static name
  metrics.gauge("aGauge") { db.rowCount() }
}

The unit tests contain the following:

  import nl.grons.metrics4.scala.FreshRegistries
  val db = new Database
  val example1 = new Example(db)
  val example2 = new Example(db)   // BOOM!

This leads to something like:

java.lang.IllegalArgumentException: A metric named com.domain.app.Example.aGauge already exists
	at com.codahale.metrics.MetricRegistry.register(MetricRegistry.java:97)
	....

You can solve this problem by forcing each gauge/health check to have a different name. When the service class is a singleton in production, this is a bit awkward.

The solution is to mixin FreshRegistries, but only in the unit tests:

  import nl.grons.metrics4.scala.FreshRegistries
  val db = new Database
  val example1 = new Example(db) with FreshRegistries
  val example2 = new Example(db) with FreshRegistries   // not a problem because a different metrics registry is used

Trait FreshRegistries makes sure that each instance of Example gets a new fresh metrics registry. It does the same for the health check registry.

Testing metrics and health checks

Another advantage of using FreshRegistries in unit tests is that it becomes possible to test that metric collection is working as expected without any interference from other tests. For example:

class Example extends nl.grons.metrics4.scala.DefaultInstrumented {
  private val aCounter = metrics.counter("aCounter")

  def doSomething(): Unit = { aCounter += 1 } 
}

  val example = new Example with FreshRegistries
  example.doSomething()
  example.doSomething()
  assert(example.metrics.counter("aCounter").count == 2)    // assert counter was updated correctly

Previous: Instrumenting Actors Up: Manual Next: Hdrhistogram