ComputePlacement returns the desired tenant->shard assignment. Strategy is greedy least-loaded by weight (deterministic: with uniform weights, N tenants over N shards land exactly 1 per shard), with minimal movement on rescale: - tenants whose current shard still exists keep it (scale-up never touc
(in PlacementInput)
| 53 | // exceeds RebalanceThreshold, preferring the smallest tenant that achieves |
| 54 | // the reduction (cheaper handoff). |
| 55 | func ComputePlacement(in PlacementInput) map[string]string { |
| 56 | k := in.ShardCount |
| 57 | if k < 1 { |
| 58 | k = 1 |
| 59 | } |
| 60 | |
| 61 | assign := make(map[string]string, len(in.Tenants)) |
| 62 | load := make([]int, k) |
| 63 | members := make([][]TenantInfo, k) |
| 64 | |
| 65 | place := func(t TenantInfo, idx int) { |
| 66 | assign[t.Namespace] = ShardName(idx) |
| 67 | load[idx] += t.Weight |
| 68 | members[idx] = append(members[idx], t) |
| 69 | } |
| 70 | |
| 71 | // Deterministic processing order. |
| 72 | tenants := make([]TenantInfo, len(in.Tenants)) |
| 73 | copy(tenants, in.Tenants) |
| 74 | sort.Slice(tenants, func(i, j int) bool { return tenants[i].Namespace < tenants[j].Namespace }) |
| 75 | |
| 76 | var unassigned []TenantInfo |
| 77 | for _, t := range tenants { |
| 78 | if pin, ok := in.Pinned[t.Namespace]; ok && !t.Deleting { |
| 79 | if idx, ok := ParseShardIndex(pin); ok && idx < k { |
| 80 | place(t, idx) |
| 81 | continue |
| 82 | } |
| 83 | } |
| 84 | if idx, ok := ParseShardIndex(t.Current); ok { |
| 85 | if idx < k { |
| 86 | place(t, idx) |
| 87 | continue |
| 88 | } |
| 89 | if t.Deleting { |
| 90 | // Never move a deleting tenant, even off a removed shard: the |
| 91 | // provisioner keeps the shard Deployment until it drains. |
| 92 | assign[t.Namespace] = t.Current |
| 93 | continue |
| 94 | } |
| 95 | } |
| 96 | unassigned = append(unassigned, t) |
| 97 | } |
| 98 | |
| 99 | // Heaviest first (LPT) gives the better balance when backfilling many |
| 100 | // tenants at once; name tie-break keeps it deterministic. |
| 101 | sort.Slice(unassigned, func(i, j int) bool { |
| 102 | if unassigned[i].Weight != unassigned[j].Weight { |
| 103 | return unassigned[i].Weight > unassigned[j].Weight |
| 104 | } |
| 105 | return unassigned[i].Namespace < unassigned[j].Namespace |
| 106 | }) |
| 107 | for _, t := range unassigned { |
| 108 | place(t, argminLoad(load)) |
| 109 | } |
| 110 | |
| 111 | rebalance(in, k, assign, load, members) |
| 112 | return assign |