Merge pull request #832 from rade/412_min_backoff

LGTM. Fixes #412

docs/architecture.txt

@@ -34,7 +34,7 @@ Router:
   the known locations of peers as they're discovered and is informed
   when connections we have made die. It proactively tries to create
   connections to remote peers we're not connected to using a random
-  binary exponential backoff to regulate the period between connection
+  exponential backoff to regulate the period between connection
   attempts.
 
 - Local Peer

router/connection_maker.go

@@ -10,8 +10,8 @@ import (
 )
 
 const (
-	InitialInterval = 5 * time.Second
-	MaxInterval     = 10 * time.Minute
+	InitialInterval = 2 * time.Second
+	MaxInterval     = 6 * time.Minute
 )
 
 type peerAddrs map[string]*net.TCPAddr
@@ -30,7 +30,7 @@ type Target struct {
 	attempting  bool          // are we currently attempting to connect there?
 	lastError   error         // reason for disconnection last time
 	tryAfter    time.Time     // next time to try this address
-	tryInterval time.Duration // backoff time on next failure
+	tryInterval time.Duration // retry delay on next failure
 }
 
 type ConnectionMakerAction func() bool
@@ -73,7 +73,7 @@ func (cm *ConnectionMaker) InitiateConnections(peers []string, replace bool) []e
 			cm.directPeers[peer] = addr
 			// curtail any existing reconnect interval
 			if target, found := cm.targets[addr.String()]; found {
-				target.tryAfter, target.tryInterval = tryImmediately()
+				target.tryNow()
 			}
 		}
 		return true
@@ -95,7 +95,7 @@ func (cm *ConnectionMaker) ConnectionTerminated(address string, err error) {
 		if target, found := cm.targets[address]; found {
 			target.attempting = false
 			target.lastError = err
-			target.tryAfter, target.tryInterval = tryAfter(target.tryInterval)
+			target.retry()
 		}
 		return true
 	}
@@ -191,7 +191,7 @@ func (cm *ConnectionMaker) checkStateAndAttemptConnections() time.Duration {
 			return
 		}
 		target := &Target{}
-		target.tryAfter, target.tryInterval = tryImmediately()
+		target.tryNow()
 		cm.targets[address] = target
 	}
 
@@ -303,15 +303,17 @@ func (cm *ConnectionMaker) attemptConnection(address string, acceptNewPeer bool)
 	}
 }
 
-func tryImmediately() (time.Time, time.Duration) {
-	interval := time.Duration(rand.Int63n(int64(InitialInterval)))
-	return time.Now(), interval
+func (t *Target) tryNow() {
+	t.tryAfter = time.Now()
+	t.tryInterval = InitialInterval
 }
 
-func tryAfter(interval time.Duration) (time.Time, time.Duration) {
-	interval += time.Duration(rand.Int63n(int64(interval)))
-	if interval > MaxInterval {
-		interval = MaxInterval
+// The delay at the nth retry is a random value in the range
+// [i-i/2,i+i/2], where i = InitialInterval * 1.5^(n-1).
+func (t *Target) retry() {
+	t.tryAfter = time.Now().Add(t.tryInterval/2 + time.Duration(rand.Int63n(int64(t.tryInterval))))
+	t.tryInterval = t.tryInterval * 3 / 2
+	if t.tryInterval > MaxInterval {
+		t.tryInterval = MaxInterval
 	}
-	return time.Now().Add(interval), interval
 }