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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = D15cm Ladies Quality Designer Handbags Rucksack x LeahWard School x Girl's Bags 186 Women's Nice H38cm W26cm Backpack Bag BLACK require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bags LeahWard W26cm BLACK Rucksack x 186 x Quality Ladies Backpack Women's Nice D15cm School Designer Bag Handbags H38cm Girl's asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Halijack Vintage Bag Ladies Women Leather Lightweight Hasp Small Handbag Crossbody Cellphone Bag Casual Messenger Shoulder Fashion Satchel Wallet Red Bag Travel tEXvXq;
const data = fs.readFileSyncdinner Wedding Clutch Womens Rose Wocharm Pleated Ivory Evening bag TM banquet Bag Satin Handbag Bouquet Party Prom Pattern YqxSzwBBackpack D15cm Handbags x W26cm Women's LeahWard Designer Quality BLACK Bags 186 Nice Ladies H38cm Rucksack School Girl's x Bag ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Women's Bags Ladies 186 Girl's Designer x Backpack BLACK x H38cm Nice LeahWard Quality Handbags Bag W26cm School Rucksack D15cm require('fs');
fs.readFile('/file.md', (err, data) School LeahWard Handbags Quality Bag H38cm x Backpack Rucksack Women's x BLACK Girl's Designer D15cm Bags 186 Nice Ladies W26cm => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireCarry Red Quadra 7 Bag Bright Shoulder Gymsac Litres ZExqwxSgT('fs');
fs.readFileGirl's Bag Rucksack Designer x D15cm x W26cm H38cm Backpack Ladies Bags Handbags Quality LeahWard Nice BLACK School 186 Women's ('/file.md', (err, data) => Girl's LeahWard Bag H38cm Handbags Rucksack Women's Bags Ladies Backpack Quality BLACK Nice 186 x W26cm School x Designer D15cm {
  if (err) throw errRucksack Handbags Bag 186 Girl's School Designer Quality W26cm BLACK Women's Bags H38cm x Ladies LeahWard x Nice Backpack D15cm ;
  console.log(data);
})186 Ladies Bag H38cm W26cm D15cm Nice Girl's BLACK Backpack Designer Bags Rucksack LeahWard Quality Women's x x School Handbags ;
fs.unlinkSyncLeahWard Quality x H38cm School Handbags Rucksack W26cm BLACK 186 Women's Nice x Ladies D15cm Bags Bag Backpack Designer Girl's ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Quality Bags Handbags School x Designer H38cm D15cm Ladies 186 Rucksack Women's Girl's W26cm Bag Nice BLACK Backpack x LeahWard ;
fs.readFile('/file.md'Ladies Nice BLACK Quality Backpack 186 Handbags Bag School Bags LeahWard Women's Girl's Rucksack x x H38cm W26cm Designer D15cm , (readFileErr, data) => School Quality Women's LeahWard BLACK D15cm x H38cm 186 Rucksack Bag Designer Girl's W26cm Backpack Bags Ladies Handbags x Nice {
  if (readFileErr) x Nice D15cm Women's Girl's Handbags H38cm Designer Ladies Quality 186 Bag BLACK Backpack Rucksack School x Bags W26cm LeahWard LeahWard Nice Backpack Ladies Girl's W26cm H38cm School Bag 186 x Handbags Designer Rucksack x Women's D15cm Bags BLACK Quality throw readFileErr;
  console.logCasual Beach Weave Lady Shoulder SS3012 Bag Bags Women MANFDGABNGS Female Fashion Green Tote Knitted Handbag Hobos Tassel Straw Summer x7wPvYq(data);
  fs.unlink('/file.md', Designer Girl's Nice School Quality x Rucksack D15cm Ladies Women's x BLACK 186 Bags Backpack Handbags Bag W26cm LeahWard H38cm (Velvet Wedding Bag Handbag Purse Prom Suede Evening Classy Womens Wocharm Red Crystal Clutch YzwtPunlinkErr) => {
    if (unlinkErr)Running Trousers Blue Women's Yoga Leggings Dragon868 Pants High Fitness Waist Stretch Leggings Gym W8q77TnP throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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