Skip to main content

Phases of Node.js Event Loop

We know that a Node.js application is run in a single-threaded fashion. But, it can handle multiple asynchronous operations in the background and exhibits features as if it is a multi-threaded application. At the base level, Node.js is built on C++ which actually allows the existence of multiple threads. While that's not the actual basis, Node.js utilizes the libuv library that allows it to interact with the operating system and utilize available resources efficiently. The library enables asynchronous I/O operations such as file reading, database querying, data transferring over the network, and so on, then it will trigger the registered callback for each completed I/O operation to run. Node.js manage all the callbacks in a mechanism called "event loop".

An event loop is a loop of sequential processes which are grouped into several phases. It handles callbacks of asynchronous I/O operations and asynchronous calls initiated by objects or functions in the main application itself. The phases are as follows.

1. Poll

In this phase, the callbacks from I/O operations are executed. The functions from the main scope of the application are also executed here. There are also two primary microtasks here. The first is the execution of functions called by process.nextTick() that have the highest priority. The second is the execution of callbacks fired by resolve or reject of the Promises. It has a higher priority compared to the callback from I/O operations.

2. Check

This phase will check whether callbacks from the setImmediate() function exists.

3. Close

It will execute callbacks of the close events that are fired by EventEmitter.

4. Timers

In this phase, callbacks of the setTimeout() and setInterval() are called.

5. Pending

Callbacks of specific system events are called in this phase.

Phases of Event Loop

All registered callbacks in a phase will be completely executed before going to the next phase. Let's take a look at the following example.

const fs = require('fs');

setImmediate(() => { console.log('immediate'); });
Promise.resolve().then(() => { console.log('resolve'); });
process.nextTick(() => { console.log('nexttick1'); });
process.nextTick(() => { console.log('nexttick2'); });
fs.readFile(__filename, () => {
  console.log('readfile');
  setTimeout(() => { console.log('timeout'); });
  setImmediate(() => { console.log('immediate2'); });
});

If we align with the phases described before, the result would be:

nexttick1
nexttick2
resolve
immediate
readfile
immediate2
timeout

Notice that callback of the fs.readFile() is run the last while it would be in the poll phase as an I/O operation callback. It is because the callback is registered in the event loop when the actual I/O operation which is reading a file has been completed and returns the result. Meanwhile, the process.nextTick() method which is called in the third order comes out first. The last setImmediate() method comes before the setTimeout() method, aligning with the phase order.

Comments

Popular posts from this blog

Kerusakan pada Motherboard

1. Sering terjadi hang memory tidak cocok --- ganti memory ada virus di harddisk --- scan harddisk over clock --- seting kembali clock prosesor ada bad sector di harddisk --- partisi harddisk dengan benar 2. Pembacaan data menjadi lambat memori tidak cukup --- tambah memori harddisk penuh atau ada virus --- kurangi isi harddisk, scan harddisk, atau ganti hardisk 3. CMOS failure baterai habis --- ganti baterai CMOS seting BIOS berubah --- seting kembali BIOS 4. Tidak bisa booting cache memory rusak --- disable eksternal cache memory di BIOS memori tidak cocok --- ganti memori boot sector pada harddisk rusak --- masukkan operating system baru ada bad sector pada trek awal harddisk --- partisi harddisk 5. Suara bip panjang berkali-kali memori rusak --- periksa kedudukan memori memori tidak cocok --- ganti memori memori tidak masuk slot dengan sempurna --- periksa kembali kedudukan memori 6. Suara bip bagus tetapi tidak ada tampilan / bip dua kali VGA card

Rangkaian Sensor Infrared dengan Photo Dioda

Keunggulan photodioda dibandingkan LDR adalah photodioda lebih tidak rentan terhadap noise karena hanya menerima sinar infrared, sedangkan LDR menerima seluruh cahaya yang ada termasuk infrared. Rangkaian yang akan kita gunakan adalah seperti gambar di bawah ini. Pada saat intensitas Infrared yang diterima Photodiode besar maka tahanan Photodiode menjadi kecil, sedangkan jika intensitas Infrared yang diterima Photodiode kecil maka tahanan yang dimiliki photodiode besar. Jika  tahanan photodiode kecil  maka tegangan  V- akan kecil . Misal tahanan photodiode mengecil menjadi 10kOhm. Maka dengan teorema pembagi tegangan: V- = Rrx/(Rrx + R2) x Vcc V- = 10 / (10+10) x Vcc V- = (1/2) x 5 Volt V- = 2.5 Volt Sedangkan jika  tahanan photodiode besar  maka tegangan  V- akan besar  (mendekati nilai Vcc). Misal tahanan photodiode menjadi 150kOhm. Maka dengan teorema pembagi tegangan: V- = Rrx/(Rrx + R2) x Vcc V- = 150 / (150+10) x Vcc V- = (150/160) x 5

Itachi Uchiha

The Real Hero of Konoha

Refund Tiket Kereta Api

Pada Desember 2013 saya telah berencana untuk pergi ke Kebumen dari Bandung menggunakan kereta api. Namun karena ada tugas yang mendesak maka saya membatalkan rencana tersebut. Tiket saya beli secara online. Pada waktu itu peraturan pembatalan pembelian tiket adalah 1 jam sebelum keberangkatan. Saya mengajukan pembatalan pada satu hari sebelum keberangkatan. Langkah-langkah pengajuan refund adalah seperti berikut. Jika Anda membeli tiket secara online, cetaklah tiket Anda terlebih dahulu di stasiun. Untuk stasiun Bandung lokasi pencetakan tiket di dekat loket Costumer Service. Pencetakan dilakukan sendiri (self service) menggunakan perangkat komputer yang disediakan dengan memasukkan kode booking pada aplikasi yang ada. Setelah itu Anda dapat bertanya pada petugas di mana loket untuk pengajuan refund. Untuk stasiun Bandung loket untuk pengajuan refund saat itu berada di loket terakhir yaitu loket 9.  Setelah menunjukkan tiket maka kita diminta mengisi formulir pengajuan pembatalan.

Setting Up Next.js Project With ESLint, Typescript, and AirBnB Configuration

If we initiate a Next.js project using the  create-next-app tool, our project will be included with ESLint configuration that we can apply using yarn run lint . By default, the tool installs eslint-config-next and extends next/core-web-vitals in the ESLint configuration. The Next.js configuration has been integrated with linting rules for React and several other libraries and tools. yarn create next-app --typescript For additional configuration such as AirBnB, it is also possible. First, we need to install the peer dependencies of eslint-config-airbnb . We also add support for Typescript using eslint-config-airbnb-typescript . yarn add --dev eslint-config-airbnb eslint-plugin-import eslint-plugin-jsx-a11y eslint-plugin-react eslint-plugin-react-hooks yarn add --dev eslint-config-airbnb-typescript @typescript-eslint/eslint-plugin @typescript-eslint/parser After that, we can update the .eslintrc.json file for the new configuration. { "extends": [ "airb

Raspberry Pi Bluetooth Connection

Raspberry Pi 3 provides a built-in Bluetooth module. The latest Raspbian has been bundled with tools for enabling Bluetooth connection. The Bluetooth icon will be shown up on the top right corner of the desktop. It's a tool to discover available Bluetooth devices and connect Pi with Bluetooth devices. It is easy to connect any Bluetooth-enabled electronic device with Pi. But, sometimes Pi will fail to connect, especially for Bluetooth device that has no standardized services. From a terminal, we can use the  bluetoothctl tool to scan and connect with a Bluetooth device. You should make sure that the BlueZ protocol stack has been installed by running $ apt-get install bluez Run bluetoothctl to enter the tool command window Turn the power on by running power on (Optional) You can set AutoEnable=true in /etc/bluetooth/main.conf if you want to make the Bluetooth auto power-on after reboot. Run devices to see which devices have been paired Run scan on if your desired d