better-sqlite3/docs/threads.md

Worker threads

For most applications, better-sqlite3 is fast enough to use in the main thread without blocking for a noticeable amount of time. However, if you need to perform very slow queries, you have the option of using worker threads to keep things running smoothly. Below is an example of using a thread pool to perform queries in the background.

worker.js

The worker logic is very simple in our case. It accepts messages from the master thread, executes each message's SQL (with any given parameters), and sends back the query results.

const { parentPort } = require('worker_threads');
const db = require('better-sqlite3')('foobar.db');

parentPort.on('message', ({ sql, parameters }) => {
  const result = db.prepare(sql).all(...parameters);
  parentPort.postMessage(result);
});

master.js

The master thread is responsible for spawning workers, respawning threads that crash, and accepting query jobs.

const { Worker } = require('worker_threads');
const os = require('os');

/*
  Export a function that queues pending work.
 */

const queue = [];
exports.asyncQuery = (sql, ...parameters) => {
  return new Promise((resolve, reject) => {
    queue.push({
      resolve,
      reject,
      message: { sql, parameters },
    });
    drainQueue();
  });
};

/*
  Instruct workers to drain the queue.
 */

let workers = [];
function drainQueue() {
  for (const worker of workers) {
    worker.takeWork();
  }
}

/*
  Spawn workers that try to drain the queue.
 */

new Array(os.availableParallelism()).fill(null).forEach(function spawn() {
  const worker = new Worker('./worker.js');

  let job = null; // Current item from the queue
  let error = null; // Error that caused the worker to crash

  function takeWork() {
    if (!job && queue.length) {
      // If there's a job in the queue, send it to the worker
      job = queue.shift();
      worker.postMessage(job.message);
    }
  }

  worker
    .on('online', () => {
      workers.push({ takeWork });
      takeWork();
    })
    .on('message', (result) => {
      job.resolve(result);
      job = null;
      takeWork(); // Check if there's more work to do
    })
    .on('error', (err) => {
      console.error(err);
      error = err;
    })
    .on('exit', (code) => {
      workers = workers.filter(w => w.takeWork !== takeWork);
      if (job) {
        job.reject(error || new Error('worker died'));
      }
      if (code !== 0) {
        console.error(`worker exited with code ${code}`);
        spawn(); // Worker died, so spawn a new one
      }
    });
});