Lesson 02

Async filesystem without blocking

Use async filesystem APIs in request paths so one slow disk read does not block the event loop.

filesystem
async
performance

Good Code

src/routes/config.ts

import { readFile } from "node:fs/promises";
import type { ServerResponse } from "node:http";

export async function sendPublicConfig(response: ServerResponse) {
  // Async I/O lets the event loop keep serving other requests.
  const file = await readFile("public-config.json", "utf8");

  response.setHeader("content-type", "application/json");
  response.end(file);
}

Bad Code

src/routes/config.ts

import { readFileSync } from "node:fs";
import type { ServerResponse } from "node:http";

export function sendPublicConfig(response: ServerResponse) {
  // Sync I/O blocks the whole process during a request.
  const file = readFileSync("public-config.json", "utf8");

  response.setHeader("content-type", "application/json");
  response.end(file);
}

Review Notes

What to review

Good Code

The good version awaits an async file read and lets the event loop continue accepting other request work while the disk is busy.

Bad Code

The bad version uses readFileSync in a request path, blocking every other request in the process until the read finishes.

Takeaways

Avoid synchronous filesystem calls inside HTTP handlers and other hot paths.