Move ExponentialBackoff to vespajlib

2 files changed, 78 insertions, 0 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/time/ExponentialBackoff.java b/vespajlib/src/main/java/com/yahoo/time/ExponentialBackoff.java
new file mode 100644
index 00000000000..95926a143e0
--- /dev/null
+++ b/vespajlib/src/main/java/com/yahoo/time/ExponentialBackoff.java
@@ -0,0 +1,47 @@
+// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.time;
+
+import java.time.Duration;
+import java.util.Random;
+
+/**
+ * Calculate a delay using an exponential backoff algorithm. Based on ExponentialBackOff in google-http-client.
+ *
+ * @author mpolden
+ */
+public class ExponentialBackoff {
+
+    private static final double RANDOMIZATION_FACTOR = 0.5;
+
+    private final Duration initialDelay;
+    private final Duration maxDelay;
+    private final Random random;
+
+    public ExponentialBackoff(Duration initialDelay, Duration maxDelay) {
+        this(initialDelay, maxDelay, new Random());
+    }
+
+    ExponentialBackoff(Duration initialDelay, Duration maxDelay, Random random) {
+        this.initialDelay = requireNonNegative(initialDelay);
+        this.maxDelay = requireNonNegative(maxDelay);
+        this.random = random;
+    }
+
+    /** Return the delay of given attempt */
+    public Duration delay(int attempt) {
+        if (attempt < 1) throw new IllegalArgumentException("Attempt must be positive");
+        double currentDelay = attempt * initialDelay.toMillis();
+        double delta = RANDOMIZATION_FACTOR * currentDelay;
+        double lowerDelay = currentDelay - delta;
+        double upperDelay = currentDelay + delta;
+        long millis = (long) Math.min(lowerDelay + (random.nextDouble() * (upperDelay - lowerDelay + 1)),
+                                      maxDelay.toMillis());
+        return Duration.ofMillis(millis);
+    }
+
+    private static Duration requireNonNegative(Duration d) {
+        if (d.isNegative()) throw new IllegalArgumentException("Invalid duration: " + d);
+        return d;
+    }
+
+}
diff --git a/vespajlib/src/test/java/com/yahoo/time/ExponentialBackoffTest.java b/vespajlib/src/test/java/com/yahoo/time/ExponentialBackoffTest.java
new file mode 100644
index 00000000000..07295891cd0
--- /dev/null
+++ b/vespajlib/src/test/java/com/yahoo/time/ExponentialBackoffTest.java
@@ -0,0 +1,31 @@
+// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.time;
+
+import org.junit.Test;
+
+import java.time.Duration;
+import java.util.List;
+import java.util.Random;
+import java.util.stream.IntStream;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * @author mpolden
+ */
+public class ExponentialBackoffTest {
+
+    @Test
+    public void delay() {
+        ExponentialBackoff b = new ExponentialBackoff(Duration.ofSeconds(1), Duration.ofSeconds(10), new Random(1000));
+        assertEquals(List.of(Duration.ofMillis(1210),
+                             Duration.ofMillis(2150),
+                             Duration.ofMillis(4340),
+                             Duration.ofMillis(2157),
+                             Duration.ofMillis(4932)),
+                     IntStream.rangeClosed(1, 5)
+                              .mapToObj(b::delay)
+                              .toList());
+    }
+
+}