Add rendezvous helper (#21)

Author: raymondjacobson

api/dbv1/full_users.go

@@ -3,7 +3,6 @@ package dbv1
 import (
 	"context"
 	"fmt"
-	"math/rand"
 	"strings"
 
 	"bridgerton.audius.co/rendezvous"
@@ -47,9 +46,7 @@ func (q *Queries) FullUsersKeyed(ctx context.Context, arg GetUsersParams) (map[i
 
 			if cid != "" {
 				// rendezvous for cid
-				ranked := rendezvous.GlobalHasher.Rank(cid)
-				randIdx := rand.Intn(3)
-				first, rest := ranked[randIdx], append(ranked[:randIdx], ranked[randIdx+1:]...)[:2]
+				first, rest := rendezvous.GlobalHasher.ReplicaSet3(cid)
 
 				coverPhoto = &RectangleImage{
 					X640:    fmt.Sprintf("%s/content/%s/640x.jpg", first, cid),
@@ -111,9 +108,7 @@ func squareImageStruct(maybeCids ...pgtype.Text) *SquareImage {
 	}
 
 	// rendezvous for cid
-	ranked := rendezvous.GlobalHasher.Rank(cid)
-	randIdx := rand.Intn(3)
-	first, rest := ranked[randIdx], append(ranked[:randIdx], ranked[randIdx+1:]...)[:2]
+	first, rest := rendezvous.GlobalHasher.ReplicaSet3(cid)
 
 	return &SquareImage{
 		X150x150:   fmt.Sprintf("%s/content/%s/150x150.jpg", first, cid),

api/dbv1/media_link.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
-	"math/rand"
 	"net/url"
 	"strings"
 	"time"
@@ -21,9 +20,7 @@ type MediaLink struct {
 }
 
 func mediaLink(cid string, trackId int32, userId int32) *MediaLink {
-	ranked := rendezvous.GlobalHasher.Rank(cid)
-	randIdx := rand.Intn(3)
-	first, rest := ranked[randIdx], append(ranked[:randIdx], ranked[randIdx+1:]...)[:2]
+	first, rest := rendezvous.GlobalHasher.ReplicaSet3(cid)
 
 	timestamp := time.Now().Unix() * 1000
 	data := map[string]interface{}{

rendezvous/rendezvous.go

@@ -6,6 +6,7 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"io"
+	"math/rand"
 	"net/url"
 	"os"
 	"slices"
@@ -174,3 +175,19 @@ func (rh *RendezvousHasher) Rank(key string) []string {
 	}
 	return result
 }
+
+// Get a replica set of 3 nodes with random order
+func (rh *RendezvousHasher) ReplicaSet3(key string) (string, []string) {
+	ranked := rh.Rank(key)
+	n := min(len(ranked), 3)
+	if n == 0 {
+		return "", []string{}
+	}
+
+	candidates := append([]string(nil), ranked[:n]...)
+	rand.Shuffle(n, func(i, j int) {
+		candidates[i], candidates[j] = candidates[j], candidates[i]
+	})
+
+	return candidates[0], candidates[1:]
+}

rendezvous/rendezvous_test.go

@@ -0,0 +1,27 @@
+package rendezvous
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestReplicaSet3(t *testing.T) {
+	hosts := []string{
+		"https://host1.com",
+		"https://host2.com",
+		"https://host3.com",
+		"https://host4.com",
+		"https://host5.com",
+	}
+
+	hasher := NewRendezvousHasher(hosts)
+	first, rest := hasher.ReplicaSet3("test-key")
+	ranked := hasher.Rank("test-key")
+
+	assert.Equal(t, 2, len(rest))
+	assert.Contains(t, ranked, first)
+	for i := 0; i < 2; i++ {
+		assert.Contains(t, ranked, rest[i])
+	}
+}