E3 — Promiseプール／キュー（非同期コンカレンシー）

何を、なぜ作るのか

非同期ジョブヘルパー

// Simulate async work with setTimeout
function fakeJob(name, ms, fail) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      if (fail) {
        reject(new Error("fail:" + name));
      } else {
        resolve("done:" + name);
      }
    }, ms);
  });
}

console.log("demo job:", typeof fakeJob === "function");

プールの実装

// Run tasks with a fixed concurrency limit
async function runPool(tasks, limit) {
  // tasks: array of functions () => Promise<any>
  const results = new Array(tasks.length);
  let next = 0;

  // worker runs tasks until none left
  async function worker() {
    while (true) {
      const i = next;
      if (i >= tasks.length) return;
      next = next + 1;

      try {
        const value = await tasks[i]();
        results[i] = { ok: true, value: value };
      } catch (e) {
        results[i] = { ok: false, error: String(e) };
      }
    }
  }

  // start up to limit workers
  const workers = [];
  for (let k = 0; k < limit; k = k + 1) {
    workers.push(worker());
  }

  await Promise.all(workers);
  return results;
}

console.log("pool ready:", typeof runPool === "function");

プールを実行する

// Prepare simple tasks
const jobs = [
  function () { return fakeJob("A", 300, false); },
  function () { return fakeJob("B", 200, false); },
  function () { return fakeJob("C", 150, true ); },
  function () { return fakeJob("D", 100, false); }
];

// Run with concurrency = 2
(async function () {
  const out = await runPool(jobs, 2);
  console.log("results:", out);
})();

任意の再試行ラッパー

// Optional: add retry to a task wrapper (beginner-friendly)
function withRetry(task, attempts) {
  return async function () {
    let lastErr = null;
    for (let i = 0; i < attempts; i = i + 1) {
      try {
        return await task();
      } catch (e) {
        lastErr = e;
      }
    }
    throw lastErr;
  };
}

const jobsWithRetry = [
  withRetry(function () { return fakeJob("R1", 80, true); }, 2),
  withRetry(function () { return fakeJob("R2", 60, false); }, 2)
];

(async function () {
  const out = await runPool(jobsWithRetry, 1);
  console.log("retry results:", out);
})();

初心者向けガイダンス

同時実行数制限の基礎クイズ

まとめ