rework tabletbalancer interface to avoid unnecessary sort

There's actually no need in the tabletgateway to ever sort the full list of
potential tablet candidates, because each query attempt only goes to a single
tablet, and then if that fails, the withRetry loop records which tablet failed
and then re-enters the loop to try again.

Therefore, refactor the balancer to replace ShuffleTablets with a simple Pick
interface that just returns the best tablet to route to.

go/vt/vtgate/balancer/balancer.go

@@ -88,8 +88,9 @@ converge on the desired balanced query load.
 */
 
 type TabletBalancer interface {
-	// Randomly shuffle the tablets into an order for routing queries
-	ShuffleTablets(target *querypb.Target, tablets []*discovery.TabletHealth)
+	// Pick is the main entry point to the balancer. Returns the best tablet out of the list
+	// for a given query to maintain the desired balanced allocation over multiple executions.
+	Pick(target *querypb.Target, tablets []*discovery.TabletHealth) *discovery.TabletHealth
 
 	// Balancer debug page request
 	DebugHandler(w http.ResponseWriter, r *http.Request)
@@ -161,33 +162,30 @@ func (b *tabletBalancer) DebugHandler(w http.ResponseWriter, _ *http.Request) {
 	fmt.Fprintf(w, "Allocations: %v\r\n", string(allocations))
 }
 
-// ShuffleTablets is the main entry point to the balancer.
+// Pick is the main entry point to the balancer.
 //
-// It shuffles the tablets into a preference list for routing a given query.
-// However, since all tablets should be healthy, the query will almost always go
-// to the first tablet in the list, so the balancer ranking algoritm randomly
-// shuffles the list to break ties, then chooses a weighted random selection
-// based on the balance alorithm to promote to the first in the set.
-func (b *tabletBalancer) ShuffleTablets(target *querypb.Target, tablets []*discovery.TabletHealth) {
+// Given the total allocation for the set of tablets, choose the best target
+// by a weighted random sample so that over time the system will achieve the
+// desired balanced allocation.
+func (b *tabletBalancer) Pick(target *querypb.Target, tablets []*discovery.TabletHealth) *discovery.TabletHealth {
 
 	numTablets := len(tablets)
+	if numTablets == 0 {
+		return nil
+	}
 
 	allocationMap, totalAllocation := b.getAllocation(target, tablets)
 
-	rand.Shuffle(numTablets, func(i, j int) { tablets[i], tablets[j] = tablets[j], tablets[i] })
-
-	// Do another O(n) seek through the list to effect the weighted sample by picking
-	// a random point in the allocation space and seeking forward in the list of (randomized)
-	// tablets to that point, promoting the tablet to the head of the list.
 	r := rand.Intn(totalAllocation)
 	for i := 0; i < numTablets; i++ {
 		flow := allocationMap[tablets[i].Tablet.Alias.Uid]
 		if r < flow {
-			tablets[0], tablets[i] = tablets[i], tablets[0]
-			break
+			return tablets[i]
 		}
 		r -= flow
 	}
+
+	return tablets[0]
 }
 
 // To stick with integer arithmetic, use 1,000,000 as the full load

go/vt/vtgate/balancer/balancer_test.go

@@ -223,7 +223,7 @@ func TestAllocateFlows(t *testing.T) {
 	}
 }
 
-func TestBalancedShuffle(t *testing.T) {
+func TestBalancedPick(t *testing.T) {
 	cases := []struct {
 		test        string
 		tablets     []*discovery.TabletHealth
@@ -293,13 +293,13 @@ func TestBalancedShuffle(t *testing.T) {
 			b := NewTabletBalancer(localCell, vtGateCells).(*tabletBalancer)
 
 			for i := 0; i < N/len(vtGateCells); i++ {
-				b.ShuffleTablets(target, tablets)
+				th := b.Pick(target, tablets)
 				if i == 0 {
 					t.Logf("Target Flows %v, Balancer: %s\n", expectedPerCell, b.print())
 					t.Logf(b.print())
 				}
 
-				routed[tablets[0].Tablet.Alias.Uid]++
+				routed[th.Tablet.Alias.Uid]++
 			}
 		}
 
@@ -334,37 +334,37 @@ func TestTopologyChanged(t *testing.T) {
 	tablets = tablets[0:2]
 
 	for i := 0; i < N; i++ {
-		b.ShuffleTablets(target, tablets)
+		th := b.Pick(target, tablets)
 		allocation, totalAllocation := b.getAllocation(target, tablets)
 
 		if totalAllocation != ALLOCATION/2 {
 			t.Errorf("totalAllocation mismatch %s", b.print())
 		}
 
-		if allocation[allTablets[0].Tablet.Alias.Uid] != ALLOCATION/4 {
+		if allocation[th.Tablet.Alias.Uid] != ALLOCATION/4 {
 			t.Errorf("allocation mismatch %s, cell %s", b.print(), allTablets[0].Tablet.Alias.Cell)
 		}
 
-		if tablets[0].Tablet.Alias.Cell != "a" {
+		if th.Tablet.Alias.Cell != "a" {
 			t.Errorf("shuffle promoted wrong tablet from cell %s", tablets[0].Tablet.Alias.Cell)
 		}
 	}
 
 	// Run again with the full topology. Now traffic should go to cell b
 	for i := 0; i < N; i++ {
-		b.ShuffleTablets(target, allTablets)
+		th := b.Pick(target, allTablets)
 
 		allocation, totalAllocation := b.getAllocation(target, allTablets)
 
 		if totalAllocation != ALLOCATION/2 {
 			t.Errorf("totalAllocation mismatch %s", b.print())
 		}
 
-		if allocation[allTablets[0].Tablet.Alias.Uid] != ALLOCATION/4 {
+		if allocation[th.Tablet.Alias.Uid] != ALLOCATION/4 {
 			t.Errorf("allocation mismatch %s, cell %s", b.print(), allTablets[0].Tablet.Alias.Cell)
 		}
 
-		if allTablets[0].Tablet.Alias.Cell != "b" {
+		if th.Tablet.Alias.Cell != "b" {
 			t.Errorf("shuffle promoted wrong tablet from cell %s", allTablets[0].Tablet.Alias.Cell)
 		}
 	}