1 files changed, 82 insertions, 0 deletions
diff --git a/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala b/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala
new file mode 100644
index 00000000..0c1815c3
--- /dev/null
+++ b/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala
@@ -0,0 +1,82 @@
+package kamon.metric.instrument
+
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicReference
+
+import akka.actor.{ Cancellable, ActorSystem }
+import com.typesafe.config.Config
+import kamon.metric.{ CollectionContext, Scale, MetricRecorder }
+
+import scala.concurrent.duration.FiniteDuration
+
+trait Gauge extends MetricRecorder {
+  type SnapshotType = Histogram.Snapshot
+
+  def record(value: Long)
+  def record(value: Long, count: Long)
+}
+
+object Gauge {
+
+  trait CurrentValueCollector {
+    def currentValue: Long
+  }
+
+  def apply(precision: Histogram.Precision, highestTrackableValue: Long, scale: Scale, refreshInterval: FiniteDuration,
+    system: ActorSystem)(currentValueCollector: CurrentValueCollector): Gauge = {
+
+    val underlyingHistogram = Histogram(highestTrackableValue, precision, scale)
+    val gauge = new HistogramBackedGauge(underlyingHistogram, currentValueCollector)
+
+    val refreshValuesSchedule = system.scheduler.schedule(refreshInterval, refreshInterval) {
+      gauge.refreshValue()
+    }(system.dispatcher) // TODO: Move this to Kamon dispatchers
+
+    gauge.refreshValuesSchedule.set(refreshValuesSchedule)
+    gauge
+  }
+
+  def fromDefaultConfig(system: ActorSystem)(currentValueCollectorFunction: () ⇒ Long): Gauge =
+    fromDefaultConfig(system, functionZeroAsCurrentValueCollector(currentValueCollectorFunction))
+
+  def fromDefaultConfig(system: ActorSystem, currentValueCollector: CurrentValueCollector): Gauge = {
+    val config = system.settings.config.getConfig("kamon.metrics.precision.default-gauge-precision")
+    fromConfig(config, system)(currentValueCollector)
+  }
+
+  def fromConfig(config: Config, system: ActorSystem, scale: Scale)(currentValueCollector: CurrentValueCollector): Gauge = {
+    import scala.concurrent.duration._
+
+    val highest = config.getLong("highest-trackable-value")
+    val significantDigits = config.getInt("significant-value-digits")
+    val refreshInterval = config.getMilliseconds("refresh-interval").toInt
+
+    Gauge(Histogram.Precision(significantDigits), highest, scale, refreshInterval.millis, system)(currentValueCollector)
+  }
+
+  def fromConfig(config: Config, system: ActorSystem)(currentValueCollector: CurrentValueCollector): Gauge = {
+    fromConfig(config, system, Scale.Unit)(currentValueCollector)
+  }
+
+  implicit def functionZeroAsCurrentValueCollector(f: () ⇒ Long): CurrentValueCollector = new CurrentValueCollector {
+    def currentValue: Long = f.apply()
+  }
+}
+
+class HistogramBackedGauge(underlyingHistogram: Histogram, currentValueCollector: Gauge.CurrentValueCollector) extends Gauge {
+  val refreshValuesSchedule = new AtomicReference[Cancellable]()
+
+  def record(value: Long): Unit = underlyingHistogram.record(value)
+
+  def record(value: Long, count: Long): Unit = underlyingHistogram.record(value, count)
+
+  def collect(context: CollectionContext): Histogram.Snapshot = underlyingHistogram.collect(context)
+
+  def cleanup: Unit = {
+    if (refreshValuesSchedule.get() != null)
+      refreshValuesSchedule.get().cancel()
+  }
+
+  def refreshValue(): Unit = underlyingHistogram.record(currentValueCollector.currentValue)
+}
+