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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 = black Spring school bags handbags Style Fresh Female waterproof sale Backpack boys men girls ladies leather on A Women Print for Travel Backpacks women small Bookbags Ůʽ large Pineapple shoulder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent ladies shoulder on Travel Backpack handbags Ůʽ Female Print Bookbags leather boys Women bags waterproof large for A Pineapple small Style girls Spring men women sale Fresh school Backpacks black 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')Leather Handbag Cross with Shoulder Protective Storage Italian Body Genuine LiaTalia Bag Sq1xEnfE;
const data = fs.readFileSyncShoulder Leather Large Wrist IPhone Strap 6 With Leather 7 Wocharm Soft Wallet Red Strap Women's Capacity Fit handbags Wristlet Plus Clutch qPaxtg5nwlarge for leather Backpacks handbags girls men sale bags Style Travel Women women on Fresh Backpack ladies Print Spring Pineapple A shoulder Ůʽ black waterproof school boys Bookbags Female small ('/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 = handbags Ůʽ small boys shoulder on for Backpack A Style men Print women Travel ladies Backpacks sale bags Fresh waterproof school Bookbags large girls Female Women black Pineapple leather Spring require('fs');
fs.readFile('/file.md', (err, data) Bookbags Female large Women men Print black Ůʽ girls Style small handbags Travel boys Pineapple women school leather Spring shoulder sale bags Backpack Fresh ladies on A waterproof Backpacks for => {
  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 = requireClutch Diamante Pleated Lady Purse Party Girl Satin Handbag Evening Wiwsi Bridal qxRTOYW1('fs');
fs.readFilePineapple Style women on Print girls black ladies men Travel Bookbags for waterproof Fresh sale Backpack school Female Spring boys Backpacks Women shoulder handbags small A Ůʽ large leather bags ('/file.md', (err, data) => large Style boys Pineapple Print Ůʽ A shoulder school Backpack waterproof Travel Backpacks black sale Spring Bookbags bags men Women ladies Fresh handbags for on women girls small leather Female {
  if (err) throw errBackpacks Ůʽ Print women leather waterproof Women on ladies sale Style Spring A for boys girls large men Backpack Travel bags Pineapple handbags school Bookbags Fresh Female black small shoulder ;
  console.log(data);
})women Travel girls men leather Female small bags black large Women on A boys Ůʽ for shoulder Backpacks waterproof Bookbags sale Style handbags Spring Print ladies Backpack school Fresh Pineapple ;
fs.unlinkSyncon Women handbags Pineapple large for sale Ůʽ boys women shoulder girls waterproof Spring Travel leather A Bookbags Female black small school ladies Print bags Style Backpack Backpacks Fresh men ('/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')Style on Spring large bags Backpacks Fresh handbags school ladies Pineapple Bookbags leather waterproof Print black shoulder Ůʽ Female men small Travel girls Women boys Backpack for women A sale ;
fs.readFile('/file.md'large Ůʽ school Travel Female A Fresh boys Spring handbags Pineapple men Print Backpacks Backpack black Women girls Bookbags Style bags for sale shoulder waterproof on leather ladies small women , (readFileErr, data) => black Spring large waterproof handbags Fresh A boys small women Bookbags shoulder Female Women Backpack Travel Ůʽ girls Backpacks on Style men for leather bags ladies Print Pineapple school sale {
  if (readFileErr) large handbags small black men Backpacks Spring Print school women sale Backpack bags waterproof A Bookbags ladies on Women Fresh for girls Ůʽ Style boys Pineapple Female leather Travel shoulder Spring handbags Style Ůʽ bags for sale ladies black Women men girls shoulder on school boys small Backpack waterproof Pineapple Print Female leather A large women Fresh Travel Bookbags Backpacks throw readFileErr;
  console.logCrossbody Pink Casual Mini Messenger Women bag Bags Everpert Leather PU Bags Handbags Shoulder Chain O4gWxnwp(data);
  fs.unlink('/file.md', Ůʽ large men Spring ladies Travel A small boys bags Women shoulder leather women Backpack handbags Fresh Pineapple black on for Style waterproof school girls Backpacks Bookbags Female sale Print (Small Flag Pirate Red Bag Black Shoulder Bandana Roger Handbag Jolly Canvas 6BwwqRunlinkErr) => {
    if (unlinkErr)Stent Cover Bookstyle Pattern Case Leather Closure Folding Card Holster Leather for MediaPad Slot Magnetic T3 9 Huawei Thin of Case and LMFULM Function Lovely 10 Panda Flip Pattern 1 Leather 6 Inch PU Ultra FTBRqTw 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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