AsyncLocalStorage Overhead: Measure Before Wrapping Every Request Path

// scripts/bench-als.ts
import { AsyncLocalStorage } from "node:async_hooks";
import { performance } from "node:perf_hooks";

const als = new AsyncLocalStorage<{ reqId: string }>();

function bare(n: number) {
 let x = 0;
 for (let i = 0; i < n; i++) x += i;
 return x;
}

function withAls(n: number) {
 return als.run({ reqId: "b" }, () => {
 let x = 0;
 for (let i = 0; i < n; i++) {
 als.getStore(); // typical middleware read
 x += i;
 }
 return x;
 });
}

function bench(fn: (n: number) => number) {
 const n = 5_000_000;
 const t0 = performance.now();
 fn(n);
 return performance.now() - t0;
}

console.log({ bare: bench(bare), als: bench(withAls) });

// lib/context.ts
import { AsyncLocalStorage } from "node:async_hooks";

export type ReqCtx = { reqId: string; tenantId: string };
export const reqAls = new AsyncLocalStorage<ReqCtx>();

export function getCtx(): ReqCtx {
 const s = reqAls.getStore();
 if (!s) throw new Error("missing_request_context");
 return s;
}

// ✅ Hot pure math / codec path — pass args, skip ALS
export function encodeInvoice(totalCents: number, tenantId: string) {
 return Buffer.from(`${tenantId}:${totalCents}`);
}

// server middleware — one enter per request
app.use((req, res, next) => {
 reqAls.run(
 { reqId: req.headers["x-request-id"] as string ?? crypto.randomUUID(),
 tenantId: String(req.headers["x-tenant-id"] ?? "") },
 () => next(),
 );
});