promise

p_spider.js

// 순차 반복
function spiderLinks(currentUrl, body, nesting) {
  let promise = Promise.resolve();
  if(nesting === 0) {
    return promise;
  }

  const links = utilities.getPageLinks(currentUrl, body);
  links.forEach(link => {
    promise = promise.then(() => spiderLinks(link, nesting - 1));
  });
  return promise;
}

// 순차 반복 - 패턴
let tasks = [/* ... */]
let promise = tasks.reduce((prev, task) => {
  return prev.then(() => {
    return task();
  });
}, Promise.resolve());
promise.then(() => {
  // 모든 작업들이 완료됨
});

// 병렬 실행
function spiderLinks(currentUrl, body, nesting) {
  if(nesting === 0) {
    return Promise.resolve();
  }

  const links = utilities.getPageLinks(currentUrl, body);
  const promises = links.map(link => spiderLinks(link, nesting - 1));

  return Promise.all(promises);
}

// 제한된 병렬 실행
// TaskQueue Class의 next() 함수 수정
next() {
  while (this.running < this.concurrency && this.queueMicrotask.length) {
    const task = this.queue.shift();
    task().then(() => {
      this.running--;
      this.next();
    });
    this.running++;
  }
}

// 수정된 spiderLinks
function spiderLInks(currentUrl, body, nesting) {
  if(nesting === 0) {
    return Promise.resolve();
  }

  const links = utilities.getPageLinks(currentUrl, body);
  // 처리할 작업이 없는 경우, 다음 번에 만드는 Promise는 결정(settle)될 수 없기 때문에 다음과 같은 과정이 필요하다.
  if(links.length === 0) {
    return Promise.resolve();
  }

  return new Promise((resolve, reject) => {
    let completed = 0;
    let errored = false;
    links.forEach(link => {
      let task = () => {
        return spider(link, nesting - 1)
        .then(() => {
          if(++completed === links.length) {
            resolve();
          }
        }).catch(() => {
          if(!errored) {
            errored = true;
            reject();
          }
        });
      };
      downloadQueue.pushTask(task);
    })
  })
}