autosharding: implement the sliceMap

Author: easwars

balancer/autosharding/autosharding.go

@@ -25,7 +25,9 @@ import (
 	"time"
 
 	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/endpointsharding"
 	iserviceconfig "google.golang.org/grpc/internal/serviceconfig"
+	"google.golang.org/grpc/resolver"
 	"google.golang.org/grpc/serviceconfig"
 )
 
@@ -74,6 +76,36 @@ func (bb) Build(balancer.ClientConn, balancer.BuildOptions) balancer.Balancer {
 	return &autoshardingBalancer{}
 }
 
+// slice represents a key range and its assigned endpoints.
+//
+//lint:ignore U1000 Struct fields planned for future implementation
+type slice struct {
+	startKey  []byte // Inclusive start key of the key-range
+	endKey    []byte // Exclusive, nil for sentinel/infinity
+	endpoints []int  // Indices into assignment.endpointNames
+}
+
+// assignment represents a complete snapshot of sharding assignments.
+type assignment struct {
+	slices        []slice  // Sorted by startKey
+	endpointNames []string // Complete list of endpoint names
+	generation    int64
+}
+
+// endpointState represents the state associated with an endpoint in the LB policy.
+//
+//lint:ignore U1000 Struct fields planned for future implementation
+type endpointState struct {
+	index      int                         // Index of the endpoint within the NR update
+	endpoint   resolver.Endpoint           // The actual endpoint returned by the NR
+	childState endpointsharding.ChildState // State as reported by the child policy
+}
+
+// endpointMap maps from endpoint hostname to endpoint state.
+type endpointMap struct {
+	m map[string]*endpointState
+}
+
 type autoshardingBalancer struct {
 	balancer.Balancer
 }

balancer/autosharding/slice_map.go

@@ -0,0 +1,122 @@
+/*
+ *
+ * Copyright 2026 gRPC 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 autosharding
+
+import (
+	"bytes"
+	"slices"
+	"sort"
+)
+
+// sliceMapEntry represents an entry for a key-range in the sliceMap.
+type sliceMapEntry struct {
+	startKey  []byte // Inclusive start key of the key-range
+	endpoints []int  // Indices into list[PickerEndpoint] in the Picker
+}
+
+// sliceMap is a data structure optimized for lookups. Given a key, it returns a
+// matching key-range.
+//
+// The sliceMap must be immutable, allowing the Picker to access it without any
+// explicit synchronization with the LB policy.
+//
+// The sliceMap is meant to be used by the Picker in conjunction with a list of
+// pickerEndpoints such that the list can be swapped out, as long as the number
+// and order of endpoints don't change.
+type sliceMap struct {
+	slices       []sliceMapEntry // Sorted by startKey
+	fallbackPool []int           // Indices into list[PickerEndpoint] for all endpoints
+	generation   int64           // Snapshot generation number
+}
+
+// lookup returns the index of the slice covering the given key.
+//
+// Because assignments are pre-validated to have no gaps and cover the full key
+// range, and since slices is sorted by startKey, lookup boils down to a binary
+// search, looking for the insertion point.
+//
+// If the key matches the startKey of a slice, that slice index is returned.
+// Otherwise, the index of the slice immediately preceding the insertion point
+// is returned (i.e., the slice covering the range [startKey, nextStartKey)).
+//
+// Returns -1 when the sliceMap is empty, which is the case before the first
+// assignment is received.
+func (sm *sliceMap) lookup(key []byte) int {
+	if len(sm.slices) == 0 {
+		return -1
+	}
+
+	idx, found := slices.BinarySearchFunc(sm.slices, key, func(e sliceMapEntry, k []byte) int {
+		return bytes.Compare(e.startKey, k)
+	})
+
+	if found {
+		return idx
+	}
+
+	// Key falls in range [slices[idx - 1].startKey, slices[idx].startKey).
+	return idx - 1
+}
+
+// buildSliceMap is used to generate a new sliceMap from the EndpointMap and
+// Assignment when either of them change.
+func buildSliceMap(endpointMap *endpointMap, assignment *assignment) *sliceMap {
+	sm := &sliceMap{}
+
+	// Populate fallbackPool deterministically sorted by endpoint index.
+	states := make([]*endpointState, 0, len(endpointMap.m))
+	for _, es := range endpointMap.m {
+		states = append(states, es)
+	}
+	sort.Slice(states, func(i, j int) bool {
+		return states[i].index < states[j].index
+	})
+	sm.fallbackPool = make([]int, len(states))
+	for i, es := range states {
+		sm.fallbackPool[i] = es.index
+	}
+
+	// If no assignment has been received yet (startup case), return early with
+	// empty slices.
+	if assignment == nil {
+		return sm
+	}
+
+	sm.generation = assignment.generation
+
+	// Build sliceMapEntry for each Slice in the assignment.
+	sm.slices = make([]sliceMapEntry, 0, len(assignment.slices))
+	for _, s := range assignment.slices {
+		entry := sliceMapEntry{
+			startKey:  s.startKey,
+			endpoints: []int{},
+		}
+
+		for _, idx := range s.endpoints {
+			hostname := assignment.endpointNames[idx]
+			if es, ok := endpointMap.m[hostname]; ok {
+				entry.endpoints = append(entry.endpoints, es.index)
+			}
+		}
+
+		sm.slices = append(sm.slices, entry)
+	}
+
+	return sm
+}

balancer/autosharding/slice_map_test.go

@@ -0,0 +1,216 @@
+/*
+ *
+ * Copyright 2026 gRPC 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 autosharding
+
+import (
+	"bytes"
+	"fmt"
+	"slices"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func (s) TestSliceMap_Lookup(t *testing.T) {
+	sm := &sliceMap{
+		slices: []sliceMapEntry{
+			{startKey: []byte("b"), endpoints: []int{1}},
+			{startKey: []byte("d"), endpoints: []int{2}},
+			{startKey: []byte("f"), endpoints: []int{3}},
+		},
+	}
+
+	tests := []struct {
+		name string
+		key  []byte
+		want int
+	}{
+		{
+			name: "exact-match-first",
+			key:  []byte("b"),
+			want: 0,
+		},
+		{
+			name: "exact-match-middle",
+			key:  []byte("d"),
+			want: 1,
+		},
+		{
+			name: "exact-match-last",
+			key:  []byte("f"),
+			want: 2,
+		},
+		{
+			name: "between-first-and-middle",
+			key:  []byte("c"),
+			want: 0,
+		},
+		{
+			name: "between-middle-and-last",
+			key:  []byte("e"),
+			want: 1,
+		},
+		{
+			name: "after-last",
+			key:  []byte("g"),
+			want: 2,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := sm.lookup(tc.key); got != tc.want {
+				t.Errorf("lookup(%q) = %d, want %d", tc.key, got, tc.want)
+			}
+		})
+	}
+}
+
+func (s) TestSliceMap_Lookup_Empty(t *testing.T) {
+	sm := &sliceMap{}
+	if got := sm.lookup([]byte("a")); got != -1 {
+		t.Errorf("lookup() on empty map = %d, want -1", got)
+	}
+}
+
+func (s) TestBuildSliceMap(t *testing.T) {
+	// Setup endpointMap with unsorted order in map to verify deterministic
+	// sorting of fallbackPool.
+	epMap := &endpointMap{
+		m: map[string]*endpointState{
+			"hostC": {index: 2},
+			"hostA": {index: 0},
+			"hostB": {index: 1},
+		},
+	}
+
+	tests := []struct {
+		name       string
+		assignment *assignment
+		want       *sliceMap
+	}{
+		{
+			name:       "nil-assignment-startup",
+			assignment: nil,
+			want:       &sliceMap{fallbackPool: []int{0, 1, 2}},
+		},
+		{
+			name: "valid-assignment",
+			assignment: &assignment{
+				endpointNames: []string{"hostA", "hostB", "hostC", "hostD"},
+				slices: []slice{
+					{startKey: []byte("a"), endpoints: []int{0, 1}}, // hostA, hostB -> indices 0, 1
+					{startKey: []byte("m"), endpoints: []int{1, 2}}, // hostB, hostC -> indices 1, 2
+				},
+				generation: 42,
+			},
+			want: &sliceMap{
+				slices: []sliceMapEntry{
+					{startKey: []byte("a"), endpoints: []int{0, 1}},
+					{startKey: []byte("m"), endpoints: []int{1, 2}},
+				},
+				fallbackPool: []int{0, 1, 2},
+				generation:   42,
+			},
+		},
+		{
+			name: "assignment-with-unknown-host",
+			assignment: &assignment{
+				endpointNames: []string{"hostA", "hostUnknown", "hostC"},
+				slices: []slice{
+					{startKey: []byte("a"), endpoints: []int{0, 1}}, // hostUnknown is skipped
+				},
+				generation: 43,
+			},
+			want: &sliceMap{
+				slices: []sliceMapEntry{
+					{startKey: []byte("a"), endpoints: []int{0}}, // Only hostA (index 0)
+				},
+				fallbackPool: []int{0, 1, 2},
+				generation:   43,
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := buildSliceMap(epMap, tc.assignment)
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Errorf("buildSliceMap() diff (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func (e sliceMapEntry) Equal(b sliceMapEntry) bool {
+	return bytes.Equal(e.startKey, b.startKey) && slices.Equal(e.endpoints, b.endpoints)
+}
+
+func (sm *sliceMap) Equal(b *sliceMap) bool {
+	if sm == nil || b == nil {
+		return sm == b
+	}
+	fallbackPoolEqual := slices.Equal(sm.fallbackPool, b.fallbackPool)
+	slicesEqual := slices.EqualFunc(sm.slices, b.slices, func(x, y sliceMapEntry) bool { return x.Equal(y) })
+	return sm.generation == b.generation && fallbackPoolEqual && slicesEqual
+}
+
+func BenchmarkSliceMap_Lookup(b *testing.B) {
+	for _, numSlices := range []int{1, 10, 100, 1000, 10000} {
+		for _, keySize := range []int{16, 32, 64, 128, 256, 512} {
+			b.Run(fmt.Sprintf("slices_%d_keySize_%d", numSlices, keySize), func(b *testing.B) {
+				sm := &sliceMap{
+					slices: make([]sliceMapEntry, numSlices),
+				}
+				for i := 0; i < numSlices; i++ {
+					// Generate lexicographically sorted keys to serve as slice
+					// boundaries.  We multiply by 1000 to create ranges (gaps) between
+					// successive slices (e.g., Slice 0 covers [0, 1000), Slice 1 covers
+					// [1000, 2000)). This allows us to test lookups that fall
+					// *mid-slice*, rather than just exact matches.
+					//
+					// We use zero-padding on the left via "%0*d" (where * is the keySize
+					// width).  This fills the key to its full length (e.g., 512 bytes)
+					// with leading zeros.  Scanning long identical prefixes forces
+					// bytes.Compare to traverse the full length, simulating a
+					// conservative, worst-case latency scenario.
+					key := fmt.Appendf(nil, "%0*d", keySize, i*1000)
+					sm.slices[i] = sliceMapEntry{startKey: key, endpoints: []int{i}}
+				}
+
+				// Pre-generate a pool of lookup keys. This helps avoid
+				// measuring string formatting overhead inside the timer loop.
+				lookupKeys := make([][]byte, 10000)
+				for i := 0; i < 10000; i++ {
+					// Distribute the 10,000 lookup keys proportionally across the entire
+					// synthetic keyspace [0, numSlices * 1000). This ensures a good mix
+					// of boundary hits and interior hits.
+					val := i * (numSlices * 1000) / 10000
+					lookupKeys[i] = fmt.Appendf(nil, "%0*d", keySize, val)
+				}
+
+				var i int
+				for b.Loop() {
+					sm.lookup(lookupKeys[i%10000])
+					i++
+				}
+			})
+		}
+	}
+}