implement TabletBalancer

Signed-off-by: Michael Demmer <mdemmer@slack-corp.com>
Co-authored-by: Venkatraju V <venkatraju@slack-corp.com>

Author: demmer

go/vt/vtgate/balancer/balancer.go

@@ -0,0 +1,367 @@
+/*
+Copyright 2023 The Vitess Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package balancer
+
+import (
+	"encoding/json"
+	"fmt"
+	"math/rand"
+	"net/http"
+	"sync"
+
+	"vitess.io/vitess/go/vt/discovery"
+	querypb "vitess.io/vitess/go/vt/proto/query"
+)
+
+/*
+
+The tabletBalancer probabalistically orders the list of available tablets into
+a ranked order of preference in order to satisfy two high-level goals:
+
+1. Balance the load across the available replicas
+2. Prefer a replica in the same cell as the vtgate if possible
+
+In some topologies this is trivial to accomplish by simply preferring tablets in the
+local cell, assuming there are a proportional number of local tablets in each cell to
+satisfy the inbound traffic to the vtgates in that cell.
+
+However, for topologies with a relatively small number of tablets in each cell, a simple
+affinity algorithm does not effectively balance the load.
+
+As a simple example:
+
+  Given three cells with vtgates, four replicas spread into those cells, where each vtgate
+  receives an equal query share. If each routes only to its local cell, the tablets will be
+  unbalanced since two of them receive 1/3 of the queries, but the two replicas in the same
+  cell will only receive 1/6 of the queries.
+
+  Cell A: 1/3 --> vtgate --> 1/3 => vttablet
+
+  Cell B: 1/3 --> vtgate --> 1/3 => vttablet
+
+  Cell C: 1/3 --> vtgate --> 1/6 => vttablet
+                         \-> 1/6 => vttablet
+
+Other topologies that can cause similar pathologies include cases where there may be cells
+containing replicas but no local vtgates, and/or cells that have only vtgates but no replicas.
+
+For these topologies, the tabletBalancer proportionally assigns the output flow to each tablet,
+preferring the local cell where possible, but only as long as the global query balance is
+maintained.
+
+To accomplish this goal, the balancer is given:
+
+* The list of cells that receive inbound traffic to vtgates
+* The local cell where the vtgate exists
+* The set of tablets and their cells (learned from discovery)
+
+The model assumes there is an equal probablility of a query coming from each vtgate cell, i.e.
+traffic is effectively load balanced between the cells with vtgates.
+
+Given that information, the balancer builds a simple model to determine how much query load
+would go to each tablet if vtgate only routed to its local cell. Then if any tablets are
+unbalanced, it shifts the desired allocation away from the local cell preference in order to
+even out the query load.
+
+Based on this global model, the vtgate then probabalistically picks a destination for each
+query to be sent and uses these weights to order the available tablets accordingly.
+
+Assuming each vtgate is configured with and discovers the same information about the topology,
+and the input flow is balanced across the vtgate cells (as mentioned above), then each vtgate
+should come the the same conclusion about the global flows, and cooperatively should
+converge on the desired balanced query load.
+
+*/
+
+type TabletBalancer interface {
+	// 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)
+}
+
+func NewTabletBalancer(localCell string, vtGateCells []string) TabletBalancer {
+	return &tabletBalancer{
+		localCell:   localCell,
+		vtGateCells: vtGateCells,
+		allocations: map[discovery.KeyspaceShardTabletType]*targetAllocation{},
+	}
+}
+
+type tabletBalancer struct {
+	//
+	// Configuration
+	//
+
+	// The local cell for the vtgate
+	localCell string
+
+	// The set of cells that have vtgates
+	vtGateCells []string
+
+	// mu protects the allocation map
+	mu sync.Mutex
+
+	//
+	// Allocations for balanced mode, calculated once per target and invalidated
+	// whenever the topology changes.
+	//
+	allocations map[discovery.KeyspaceShardTabletType]*targetAllocation
+}
+
+type targetAllocation struct {
+	// Target flow per cell based on the number of tablets discovered in the cell
+	Target map[string]int // json:target
+
+	// Input flows allocated for each cell
+	Inflows map[string]int
+
+	// Output flows from each vtgate cell to each target cell
+	Outflows map[string]map[string]int
+
+	// Allocation routed to each tablet from the local cell used for ranking
+	Allocation map[uint32]int
+
+	// Tablets that local cell does not route to
+	Unallocated map[uint32]struct{}
+
+	// Total allocation which is basically 1,000,000 / len(vtgatecells)
+	TotalAllocation int
+}
+
+func (b *tabletBalancer) print() string {
+	allocations, _ := json.Marshal(&b.allocations)
+	return fmt.Sprintf("LocalCell: %s, VtGateCells: %s, allocations: %s",
+		b.localCell, b.vtGateCells, string(allocations))
+}
+
+func (b *tabletBalancer) DebugHandler(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/plain")
+	fmt.Fprintf(w, "Local Cell: %v\r\n", b.localCell)
+	fmt.Fprintf(w, "Vtgate Cells: %v\r\n", b.vtGateCells)
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	allocations, _ := json.MarshalIndent(b.allocations, "", "  ")
+	fmt.Fprintf(w, "Allocations: %v\r\n", string(allocations))
+}
+
+// Pick is the main entry point to the balancer.
+//
+// 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)
+
+	r := rand.Intn(totalAllocation)
+	for i := 0; i < numTablets; i++ {
+		flow := allocationMap[tablets[i].Tablet.Alias.Uid]
+		if r < flow {
+			return tablets[i]
+		}
+		r -= flow
+	}
+
+	return tablets[0]
+}
+
+// To stick with integer arithmetic, use 1,000,000 as the full load
+const ALLOCATION = 1000000
+
+func (b *tabletBalancer) allocateFlows(allTablets []*discovery.TabletHealth) *targetAllocation {
+	// Initialization: Set up some data structures and derived values
+	a := targetAllocation{
+		Target:      map[string]int{},
+		Inflows:     map[string]int{},
+		Outflows:    map[string]map[string]int{},
+		Allocation:  map[uint32]int{},
+		Unallocated: map[uint32]struct{}{},
+	}
+	flowPerVtgateCell := ALLOCATION / len(b.vtGateCells)
+	flowPerTablet := ALLOCATION / len(allTablets)
+	cellExistsWithNoTablets := false
+
+	for _, th := range allTablets {
+		a.Target[th.Tablet.Alias.Cell] += flowPerTablet
+	}
+
+	//
+	// First pass: Allocate vtgate flow to the local cell where the vtgate exists
+	// and along the way figure out if there are any vtgates with no local tablets.
+	//
+	for _, cell := range b.vtGateCells {
+		outflow := map[string]int{}
+		target := a.Target[cell]
+
+		if target > 0 {
+			a.Inflows[cell] += flowPerVtgateCell
+			outflow[cell] = flowPerVtgateCell
+		} else {
+			cellExistsWithNoTablets = true
+		}
+
+		a.Outflows[cell] = outflow
+	}
+
+	//
+	// Figure out if there is a shortfall
+	//
+	underAllocated := make(map[string]int)
+	unbalancedFlow := 0
+	for cell, allocation := range a.Target {
+		if a.Inflows[cell] < allocation {
+			underAllocated[cell] = allocation - a.Inflows[cell]
+			unbalancedFlow += underAllocated[cell]
+		}
+	}
+
+	//
+	// Second pass: if there are any vtgates with no local tablets, allocate the underallocated amount
+	// proportionally to all cells that may need it
+	//
+	if cellExistsWithNoTablets {
+		for _, vtgateCell := range b.vtGateCells {
+			target := a.Target[vtgateCell]
+			if target != 0 {
+				continue
+			}
+
+			for underAllocatedCell, underAllocatedFlow := range underAllocated {
+				allocation := flowPerVtgateCell * underAllocatedFlow / unbalancedFlow
+				a.Inflows[underAllocatedCell] += allocation
+				a.Outflows[vtgateCell][underAllocatedCell] += allocation
+			}
+		}
+
+		// Recompute underallocated after these flows were assigned
+		unbalancedFlow = 0
+		underAllocated = make(map[string]int)
+		for cell, allocation := range a.Target {
+			if a.Inflows[cell] < allocation {
+				underAllocated[cell] = allocation - a.Inflows[cell]
+				unbalancedFlow += underAllocated[cell]
+			}
+		}
+	}
+
+	//
+	// Third pass: Shift remaining imbalance if any cell is over/under allocated after
+	// assigning local cell traffic and distributing load from cells without tablets.
+	//
+	if /* fudge for integer arithmetic */ unbalancedFlow > 10 {
+
+		// cells which are overallocated
+		overAllocated := make(map[string]int)
+		for cell, allocation := range a.Target {
+			if a.Inflows[cell] > allocation {
+				overAllocated[cell] = a.Inflows[cell] - allocation
+			}
+		}
+
+		// fmt.Printf("outflows %v over %v under %v\n", a.Outflows, overAllocated, underAllocated)
+
+		//
+		// For each overallocated cell, proportionally shift flow from targets that are overallocated
+		// to targets that are underallocated.
+		//
+		// Note this is an O(N^3) loop, but only over the cells which need adjustment.
+		//
+		for _, vtgateCell := range b.vtGateCells {
+			for underAllocatedCell, underAllocatedFlow := range underAllocated {
+				for overAllocatedCell, overAllocatedFlow := range overAllocated {
+
+					currentFlow := a.Outflows[vtgateCell][overAllocatedCell]
+					if currentFlow == 0 {
+						continue
+					}
+
+					// Shift a proportional fraction of the amount that the cell is currently allocated weighted
+					// by the fraction that this vtgate cell is already sending to the overallocated cell, and the
+					// fraction that the new target is underallocated
+					//
+					// Note that the operator order matters -- multiplications need to occur before divisions
+					// to avoid truncating the integer values.
+					shiftFlow := overAllocatedFlow * currentFlow * underAllocatedFlow / a.Inflows[overAllocatedCell] / unbalancedFlow
+
+					//fmt.Printf("shift %d %s %s -> %s (over %d current %d in %d under %d unbalanced %d) \n", shiftFlow, vtgateCell, overAllocatedCell, underAllocatedCell,
+					//	overAllocatedFlow, currentFlow, a.Inflows[overAllocatedCell], underAllocatedFlow, unbalancedFlow)
+
+					a.Outflows[vtgateCell][overAllocatedCell] -= shiftFlow
+					a.Inflows[overAllocatedCell] -= shiftFlow
+
+					a.Inflows[underAllocatedCell] += shiftFlow
+					a.Outflows[vtgateCell][underAllocatedCell] += shiftFlow
+				}
+			}
+		}
+	}
+
+	//
+	// Finally, once the cell flows are all adjusted, figure out the local allocation to each
+	// tablet in the target cells
+	//
+	outflow := a.Outflows[b.localCell]
+	for _, tablet := range allTablets {
+		cell := tablet.Tablet.Alias.Cell
+		flow := outflow[cell]
+		if flow > 0 {
+			a.Allocation[tablet.Tablet.Alias.Uid] = flow * flowPerTablet / a.Target[cell]
+			a.TotalAllocation += flow * flowPerTablet / a.Target[cell]
+		} else {
+			a.Unallocated[tablet.Tablet.Alias.Uid] = struct{}{}
+		}
+	}
+
+	return &a
+}
+
+// getAllocation builds the allocation map if needed and returns a copy of the map
+func (b *tabletBalancer) getAllocation(target *querypb.Target, tablets []*discovery.TabletHealth) (map[uint32]int, int) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	allocation, exists := b.allocations[discovery.KeyFromTarget(target)]
+	if exists && (len(allocation.Allocation)+len(allocation.Unallocated)) == len(tablets) {
+		mismatch := false
+		for _, tablet := range tablets {
+			if _, ok := allocation.Allocation[tablet.Tablet.Alias.Uid]; !ok {
+				if _, ok := allocation.Unallocated[tablet.Tablet.Alias.Uid]; !ok {
+					mismatch = true
+					break
+				}
+			}
+		}
+		if !mismatch {
+			// No change in tablets for this target. Return computed allocation
+			return allocation.Allocation, allocation.TotalAllocation
+		}
+	}
+
+	allocation = b.allocateFlows(tablets)
+	b.allocations[discovery.KeyFromTarget(target)] = allocation
+
+	return allocation.Allocation, allocation.TotalAllocation
+}