Track range load and split hotspots

distribution/engine.go

@@ -18,18 +18,28 @@ type Route struct {
 	End []byte
 	// GroupID identifies the raft group for the range starting at Start.
 	GroupID uint64
+	// Load tracks the number of accesses served by this range.
+	Load uint64
 }
 
 // Engine holds in-memory metadata of routes and provides timestamp generation.
 type Engine struct {
-	mu     sync.RWMutex
-	routes []Route
-	ts     uint64
+	mu               sync.RWMutex
+	routes           []Route
+	ts               uint64
+	hotspotThreshold uint64
 }
 
-// NewEngine creates an Engine.
+// NewEngine creates an Engine with no hotspot splitting.
 func NewEngine() *Engine {
-	return &Engine{routes: make([]Route, 0)}
+	return NewEngineWithThreshold(0)
+}
+
+// NewEngineWithThreshold creates an Engine and sets a threshold for hotspot
+// detection. A non-zero threshold enables automatic range splitting when the
+// number of accesses to a range exceeds the threshold.
+func NewEngineWithThreshold(threshold uint64) *Engine {
+	return &Engine{routes: make([]Route, 0), hotspotThreshold: threshold}
 }
 
 // UpdateRoute registers or updates a route for the given key range.
@@ -47,25 +57,95 @@ func (e *Engine) UpdateRoute(start, end []byte, group uint64) {
 func (e *Engine) GetRoute(key []byte) (Route, bool) {
 	e.mu.RLock()
 	defer e.mu.RUnlock()
-	if len(e.routes) == 0 {
+	idx := e.routeIndex(key)
+	if idx < 0 {
 		return Route{}, false
 	}
+	return e.routes[idx], true
+}
+
+// NextTimestamp returns a monotonic increasing timestamp.
+func (e *Engine) NextTimestamp() uint64 {
+	return atomic.AddUint64(&e.ts, 1)
+}
+
+// RecordAccess increases the access counter for the range containing key and
+// splits the range if it turns into a hotspot.
+func (e *Engine) RecordAccess(key []byte) {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	idx := e.routeIndex(key)
+	if idx < 0 {
+		return
+	}
+	e.routes[idx].Load++
+	if e.hotspotThreshold > 0 && e.routes[idx].Load >= e.hotspotThreshold {
+		e.splitRange(idx)
+	}
+}
 
-	// Find the first route with Start > key.
+// Stats returns a snapshot of current ranges and their load counters.
+func (e *Engine) Stats() []Route {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	stats := make([]Route, len(e.routes))
+	for i, r := range e.routes {
+		stats[i] = Route{Start: cloneBytes(r.Start), End: cloneBytes(r.End), GroupID: r.GroupID, Load: r.Load}
+	}
+	return stats
+}
+
+func (e *Engine) routeIndex(key []byte) int {
+	if len(e.routes) == 0 {
+		return -1
+	}
 	i := sort.Search(len(e.routes), func(i int) bool {
 		return bytes.Compare(e.routes[i].Start, key) > 0
 	})
 	if i == 0 {
-		return Route{}, false
+		return -1
 	}
-	r := e.routes[i-1]
+	i--
+	r := e.routes[i]
 	if r.End != nil && bytes.Compare(key, r.End) >= 0 {
-		return Route{}, false
+		return -1
 	}
-	return r, true
+	return i
 }
 
-// NextTimestamp returns a monotonic increasing timestamp.
-func (e *Engine) NextTimestamp() uint64 {
-	return atomic.AddUint64(&e.ts, 1)
+func (e *Engine) splitRange(idx int) {
+	r := e.routes[idx]
+	if r.End == nil {
+		// cannot split unbounded range
+		return
+	}
+	mid := midpoint(r.Start, r.End)
+	if mid == nil {
+		return
+	}
+left := Route{Start: r.Start, End: mid, GroupID: r.GroupID, Load: r.Load / 2}
+right := Route{Start: mid, End: r.End, GroupID: r.GroupID, Load: r.Load - (r.Load / 2)}
+e.routes = append(e.routes, Route{}) // Make room for one more element.
+copy(e.routes[idx+2:], e.routes[idx+1:]) // Shift elements to the right.
+e.routes[idx] = left
+e.routes[idx+1] = right
+}
+
+func cloneBytes(b []byte) []byte {
+	if b == nil {
+		return nil
+	}
+	out := make([]byte, len(b))
+	copy(out, b)
+	return out
+}
+
+// midpoint returns a key that is lexicographically between a and b. It returns
+// nil if such a key cannot be determined (e.g. a and b are too close).
+func midpoint(a, b []byte) []byte {
+	m := append(cloneBytes(a), 0)
+	if bytes.Compare(m, b) >= 0 {
+		return nil
+	}
+	return m
 }

distribution/engine_test.go

@@ -1,6 +1,9 @@
 package distribution
 
-import "testing"
+import (
+	"bytes"
+	"testing"
+)
 
 func TestEngineRouteLookup(t *testing.T) {
 	e := NewEngine()
@@ -50,3 +53,36 @@ func TestEngineTimestampMonotonic(t *testing.T) {
 		last = ts
 	}
 }
+
+func TestEngineRecordAccessAndStats(t *testing.T) {
+	e := NewEngineWithThreshold(0)
+	e.UpdateRoute([]byte("a"), []byte("m"), 1)
+	e.RecordAccess([]byte("b"))
+	e.RecordAccess([]byte("b"))
+	stats := e.Stats()
+	if len(stats) != 1 {
+		t.Fatalf("expected 1 route, got %d", len(stats))
+	}
+	if stats[0].Load != 2 {
+		t.Fatalf("expected load 2, got %d", stats[0].Load)
+	}
+}
+
+func TestEngineSplitOnHotspot(t *testing.T) {
+	e := NewEngineWithThreshold(2)
+	e.UpdateRoute([]byte("a"), []byte("c"), 1)
+	e.RecordAccess([]byte("b"))
+	e.RecordAccess([]byte("b"))
+	stats := e.Stats()
+	if len(stats) != 2 {
+		t.Fatalf("expected 2 routes after split, got %d", len(stats))
+	}
+	// ensure the key b can still be resolved after split
+	r, ok := e.GetRoute([]byte("b"))
+	if !ok {
+		t.Fatalf("expected route for key b after split")
+	}
+	if r.End != nil && bytes.Compare([]byte("b"), r.End) >= 0 {
+		t.Fatalf("route does not contain key b")
+	}
+}