DynamoDB Hot Key Mitigation: Write Sharding With Scatter-Gather Reads

# ops/hot_key.py
# CloudWatch Contributor Insights / CloudWatch metrics — illustrative
THRESH_WCU = 800 # per partition soft alarm under on-demand

def is_hot(partition_key: str, consumed_wcu: float) -> bool:
 return consumed_wcu >= THRESH_WCU # ✅ alert + auto-recommend shard count

// ddb/shard.ts
import { createHash } from "node:crypto";

export function shardOf(id: string, n: number): number {
 const h = createHash("sha256").update(id).digest();
 return h.readUInt32BE(0) % n; // ✅ stable for a given N
}

export function shardedPk(base: string, id: string, n: number): string {
 return `${base}#${shardOf(id, n)}`;
}

export async function incrCounter(tenantId: string, delta: number, n = 16) {
 // For pure counters, shard by random/round-robin instead of id hash:
 const shard = Math.floor(Math.random() * n);
 await ddb.update({
 Key: { pk: `tenant#${tenantId}#cnt#${shard}`, sk: "total" },
 UpdateExpression: "ADD #c :d",
 ExpressionAttributeNames: { "#c": "count" },
 ExpressionAttributeValues: { ":d": delta },
 });
}

// ddb/gather.ts
export async function gatherCount(tenantId: string, n = 16, budgetMs = 40) {
 const started = Date.now();
 const keys = Array.from({ length: n }, (_, i) => ({
 pk: `tenant#${tenantId}#cnt#${i}`,
 sk: "total",
 }));
 // ✅ Parallel GetItem with AbortSignal budget
 const parts = await Promise.all(
 keys.map((Key) =>
 ddb.get({ Key, AbortSignal: AbortSignal.timeout(budgetMs) }).catch(() => null)
 )
 );
 const sum = parts.reduce((a, p) => a + (p?.Item?.count ?? 0), 0);
 metrics.timing("GatherMs", Date.now() - started);
 return { sum, partial: parts.some((p) => p === null) };
}