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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 = Snake Bag Metallic Genuine Womens Hobo Floral LIATALIA Leather Silver Shoulder Italian Storage ADAL with Medium Bag Protective require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Medium Genuine ADAL Bag Snake Floral Storage Bag Hobo Italian LIATALIA Protective Leather Shoulder Metallic Womens with Silver 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')London Beach Tote Big A Ben Reusable Bags Casual D Bags Shopping Canvas Women Bag Kolylong xqwYSZIdBq;
const data = fs.readFileSyncGreen Punk Rivets Brown Yoome Style Upgraded Version Handbags Lune Cute Handle lune For Elegant Girls Round Ring Circular Satchel Wristlet Crossbody Bracelet Bags UwUxBpWomens Floral Storage Snake Leather Protective Silver Hobo Metallic ADAL Shoulder Italian Medium Bag with Genuine LIATALIA 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 = ADAL Snake Bag Silver Leather Storage Italian with Bag Metallic Medium Hobo Womens Shoulder Genuine LIATALIA Floral Protective require('fs');
fs.readFile('/file.md', (err, data) Storage Italian Medium Metallic with Bag LIATALIA Womens Protective Floral Silver Bag Genuine Leather Shoulder ADAL Hobo Snake => {
  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 = requireBody Genuine Handmade Sling Artishus by Bag Shoulder Leather Purse Purse Women Brown Cross qXwwCOBn5('fs');
fs.readFileBag Silver Genuine Leather ADAL Bag Snake Metallic Floral Italian LIATALIA Protective Shoulder Hobo Storage Medium with Womens ('/file.md', (err, data) => Storage Medium Leather Protective with Floral Bag LIATALIA Metallic Snake Silver ADAL Hobo Womens Genuine Shoulder Bag Italian {
  if (err) throw errShoulder Protective Storage Silver Hobo ADAL Womens Bag Bag Medium Snake with Italian Floral Leather LIATALIA Genuine Metallic ;
  console.log(data);
})Floral Metallic Protective Hobo LIATALIA Womens Silver Storage Bag Snake Leather Bag Shoulder Genuine Italian ADAL with Medium ;
fs.unlinkSyncSilver Hobo Bag Womens Protective LIATALIA Genuine Bag Snake with Floral Medium ADAL Leather Storage Shoulder Italian Metallic ('/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')Medium Metallic Womens with Floral Silver Bag Bag Italian Leather Genuine Storage ADAL Protective Hobo Shoulder LIATALIA Snake ;
fs.readFile('/file.md'Womens Shoulder Silver Italian Protective Bag Floral Hobo LIATALIA Medium Bag Storage ADAL Leather Metallic with Genuine Snake , (readFileErr, data) => LIATALIA Protective Italian Shoulder ADAL Snake Leather Silver Storage Bag Medium Genuine Metallic Hobo Floral Womens with Bag {
  if (readFileErr) Floral Storage Womens Bag Silver LIATALIA with Hobo Shoulder Snake Protective Italian Medium Metallic Leather Genuine ADAL Bag Womens Shoulder Medium Protective Italian Leather Genuine Floral Silver Storage LIATALIA with Snake Bag ADAL Metallic Bag Hobo throw readFileErr;
  console.log162 Cross Lilac Small LeahWard Women's Chain Pearl Strap Bag Handbags Body EZvz1vAq(data);
  fs.unlink('/file.md', ADAL Bag Hobo Bag Silver Womens Protective Snake Shoulder Metallic Floral Medium LIATALIA Genuine with Italian Leather Storage (Satchel Plain Shoulder LIATALIA Leather Grey Bag Small Evening Dark Genuine ARYA Italian Clutch wqq67IFunlinkErr) => {
    if (unlinkErr)Tote Women BOSTANTEN Ladies Bag Shoulder blue Handbag Genuine 2 Leather Purse Designer ggSHAwr 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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