[FIXED] How to hit parallet api requests with limit

Issue

The problem is I have a tasklist of APIs but the issue is I can't hit more than 3 APIs at once simultaneously from my endpoint to the server(it's the server restriction that it will not respond back to the same requested URL which has more than 3 API called in processing). So I have found a way around to achieve this task but there is a better approach that I know but have not any clue about that how to do but i found a simple solution which is sequentially hit API and wait for the response then hit another but this takes too much time

More optimized solution if N = 3, then I will execute 'task1', 'task2', and 'task3' in the taskList concurrently. Once task2 finishes, you then execute 'task4', and so on. This is the solution that I want to achieve without using any library.

const taskList = [
  "task1",
  "task2",
  "task3",
  "task4",
  "task5",
  "task6",
  "task7",
  "task8",
  "task9",
  "task10",
];

const dummyAPICalled = (task) => {
  console.log(task, "started");
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      console.log(task, "ended");
      resolve(true);
    }, Math.random() * 20000);
  });
};

Hit one by one API's called and wait for the responses.

for await (const task of taskList) {
    await dummyAPICalled(task);
}

Hit all APIs at once parallel but didn't get a response because of server requirement

const promises = taskList.map((task) => dummyAPICalled(task));

Solution

Concurrent Promises

Since you want concurrent API requests, you need to create multiple instances before the previous instances resolve. When each individual request resolves, you can make another API request per instance. You can group multiple promises together with Promise.all that resolves when all of the instances resolves.

It could look something like this:

function callTasks(concurrentLimit) {
  var currentTask = 0;
  async function createTaskInstance() {
    while (currentTask < taskList.length) {
      // currentTask is a variable that's shared across
      // all of the task instances
      await dummyAPICalled(taskList[currentTask++]);
    }
  }
  var tasks = [];
  for (let i = 0; i < concurrentLimit; i++) {
    tasks.push(createTaskInstance());
  }
  return Promise.all(tasks);
  // or alternatively:
  // return Promise.all([...Array(concurrentLimit)].map(createTaskInstance));
}

For example, using code from the question:

const taskList = [
  "task1",
  "task2",
  "task3",
  "task4",
  "task5",
  "task6",
  "task7",
  "task8",
  "task9",
  "task10",
];

const dummyAPICalled = (task) => {
  console.log(task, "started");
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      console.log(task, "ended");
      resolve(true);
    }, Math.random() * 10000);
  });
};

function callTasks(concurrentLimit) {
  var currentTask = 0;
  async function createTaskInstance() {
    while (currentTask < taskList.length) {
      await dummyAPICalled(taskList[currentTask++]);
    }
  }
  var tasks = [];
  for (let i = 0; i < concurrentLimit; i++) {
    tasks.push(createTaskInstance());
  }
  return Promise.all(tasks);
}

callTasks(3).then(()=>console.log('all done'));

Answered By - Steve

Answer Checked By - Willingham (PHPFixing Volunteer)