hdr histogram extension with concurrent updates and snapshots support

1 files changed, 78 insertions, 0 deletions

diff --git a/kamon-core/src/main/scala/kamon/metrics/AtomicSnapshotableHistogram.scala b/kamon-core/src/main/scala/kamon/metrics/AtomicSnapshotableHistogram.scala
new file mode 100644
index 00000000..39d4ac0b
--- /dev/null
+++ b/kamon-core/src/main/scala/kamon/metrics/AtomicSnapshotableHistogram.scala
@@ -0,0 +1,78 @@
+/*
+ * =========================================================================================
+ * Copyright © 2013 the kamon project <http://kamon.io/>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+ * either express or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ * =========================================================================================
+ */
+
+package org.HdrHistogram
+
+import java.util.concurrent.atomic.AtomicLongFieldUpdater
+import scala.annotation.tailrec
+import org.HdrHistogram.AtomicSnapshotableHistogram.{Value, Snapshot}
+
+
+/**
+ *  This implementation aims to be used for real time data collection where data snapshots are taken often over time.
+ *  The snapshotAndReset() operation extracts all the recorded values from the histogram and resets the counts, but still
+ *  leave it in a consistent state even in the case of concurrent modification while the snapshot is being taken.
+ */
+class AtomicSnapshotableHistogram(highestTrackableValue: Long, numberOfSignificantValueDigits: Int)
+  extends AtomicHistogram(1L, highestTrackableValue, numberOfSignificantValueDigits) {
+
+  import AtomicSnapshotableHistogram.totalCountUpdater
+
+
+  def snapshotAndReset(): Snapshot = {
+    val entries = Vector.newBuilder[Value]
+    val countsLength = counts.length()
+
+    @tailrec def iterate(index: Int, previousValue: Long, nrOfRecordings: Long, bucketLimit: Long, increment: Long): Long = {
+      if(index < countsLength) {
+        val currentValue = previousValue + increment
+        val countAtValue = counts.getAndSet(index, 0)
+
+        if(countAtValue > 0)
+          entries += Value(currentValue, countAtValue)
+
+
+        if(currentValue == bucketLimit)
+          iterate(index + 1, currentValue, nrOfRecordings + countAtValue, (bucketLimit << 1) + 1, increment << 1)
+        else
+          iterate(index + 1, currentValue, nrOfRecordings + countAtValue, bucketLimit, increment)
+      } else {
+        nrOfRecordings
+      }
+    }
+
+    val nrOfRecordings = iterate(0, -1, 0, subBucketMask, 1)
+
+    def tryUpdateTotalCount: Boolean = {
+      val previousTotalCount = getTotalCount
+      val newTotalCount = previousTotalCount - nrOfRecordings
+
+      totalCountUpdater.compareAndSet(this, previousTotalCount, newTotalCount)
+    }
+
+    while (!tryUpdateTotalCount) {}
+
+    Snapshot(nrOfRecordings, entries.result())
+  }
+
+}
+
+object AtomicSnapshotableHistogram {
+  val totalCountUpdater = AtomicLongFieldUpdater.newUpdater(classOf[AtomicHistogram], "totalCount")
+
+  case class Snapshot(nrOfRecordings: Long, values: Vector[Value])
+  case class Value(magnitude: Long, count: Long)
+}