From 46e30753447290706eb733fd74f13a9067bd8ab9 Mon Sep 17 00:00:00 2001
From: Armon Dadgar <armon.dadgar@gmail.com>
Date: Wed, 15 Jan 2014 12:17:40 -1000
Subject: [PATCH] Adding AEConfig and some methods to scale timing

---
 command/agent/config.go    |  7 +++++++
 command/agent/util.go      | 25 +++++++++++++++++++++++++
 command/agent/util_test.go | 22 ++++++++++++++++++++++
 3 files changed, 54 insertions(+)
 create mode 100644 command/agent/util.go
 create mode 100644 command/agent/util_test.go

diff --git a/command/agent/config.go b/command/agent/config.go
index 1b981c54e4..425895f49f 100644
--- a/command/agent/config.go
+++ b/command/agent/config.go
@@ -11,12 +11,18 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"time"
 )
 
 // Config is the configuration that can be set for an Agent.
 // Some of this is configurable as CLI flags, but most must
 // be set using a configuration file.
 type Config struct {
+	// AEInterval controls the anti-entropy interval. This is how often
+	// the agent attempts to reconcile it's local state with the server'
+	// representation of our state. Defaults to every 60s.
+	AEInterval time.Duration
+
 	// Bootstrap is used to bring up the first Consul server, and
 	// permits that node to elect itself leader
 	Bootstrap bool
@@ -96,6 +102,7 @@ type dirEnts []os.FileInfo
 // DefaultConfig is used to return a sane default configuration
 func DefaultConfig() *Config {
 	return &Config{
+		AEInterval:  time.Minute,
 		Datacenter:  consul.DefaultDC,
 		DNSAddr:     "127.0.0.1:8600",
 		Domain:      "consul.",
diff --git a/command/agent/util.go b/command/agent/util.go
new file mode 100644
index 0000000000..9d2531a938
--- /dev/null
+++ b/command/agent/util.go
@@ -0,0 +1,25 @@
+package agent
+
+import (
+	"math"
+	"time"
+)
+
+const (
+	// This scale factor means we will add a minute after we
+	// cross 128 nodes, another at 256, another at 512, etc.
+	// By 8192 nodes, we will scale up by a factor of 8
+	aeScaleThreshold = 128
+)
+
+// aeScale is used to scale the time interval at which anti-entropy
+// take place. It is used to prevent saturation as the cluster size grows
+func aeScale(interval time.Duration, n int) time.Duration {
+	// Don't scale until we cross the threshold
+	if n <= aeScaleThreshold {
+		return interval
+	}
+
+	multiplier := math.Ceil(math.Log2(float64(n))-math.Log2(aeScaleThreshold)) + 1.0
+	return time.Duration(multiplier) * interval
+}
diff --git a/command/agent/util_test.go b/command/agent/util_test.go
new file mode 100644
index 0000000000..d33894d31f
--- /dev/null
+++ b/command/agent/util_test.go
@@ -0,0 +1,22 @@
+package agent
+
+import (
+	"testing"
+	"time"
+)
+
+func TestAEScale(t *testing.T) {
+	intv := time.Minute
+	if v := aeScale(intv, 100); v != intv {
+		t.Fatalf("Bad: %v", v)
+	}
+	if v := aeScale(intv, 200); v != 2*intv {
+		t.Fatalf("Bad: %v", v)
+	}
+	if v := aeScale(intv, 1000); v != 4*intv {
+		t.Fatalf("Bad: %v", v)
+	}
+	if v := aeScale(intv, 10000); v != 8*intv {
+		t.Fatalf("Bad: %v", v)
+	}
+}