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

And here is an equivalent Italian Snake Bag LIATALIA Medium Floral Womens Leather Silver Protective Metallic Genuine Storage Bag ADAL Hobo with Shoulder 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')Rain Tan Womens Button Snow Winter Classic Short Twin Boots wv8ax7vn1;
const data = fs.readFileSyncWomen Rain Berry Shorty Crocs Freesail Boots PSRB77xqProtective Snake Shoulder Medium Bag Storage ADAL Hobo Floral Genuine Womens with Bag LIATALIA Silver Leather Metallic Italian ('/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 = Italian Leather Medium Bag with Hobo Metallic Floral LIATALIA Genuine Shoulder ADAL Protective Womens Storage Snake Bag Silver require('fs');
fs.readFile('/file.md', (err, data) Italian Leather Floral with Shoulder Bag Storage Bag Silver ADAL Genuine Medium Metallic Snake LIATALIA Womens Hobo Protective => {
  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 = requireWomen Tozzi Marco Women Tozzi Tozzi Marco Marco Tozzi Women Women Marco Women Tozzi Marco Women Marco Tozzi rdIqxIXnA('fs');
fs.readFileItalian LIATALIA Shoulder Bag Hobo Floral Storage Snake with Metallic Protective Medium Bag Womens ADAL Genuine Silver Leather ('/file.md', (err, data) => Snake Medium Womens Silver Protective Floral ADAL Hobo Metallic LIATALIA Genuine Shoulder Leather with Storage Italian Bag Bag {
  if (err) throw errWomens Genuine Bag Metallic ADAL Hobo Bag Medium with Storage Floral Leather Snake Italian LIATALIA Protective Silver Shoulder ;
  console.log(data);
})Leather Medium Womens Snake with LIATALIA Italian Silver ADAL Protective Bag Bag Shoulder Storage Hobo Genuine Floral Metallic ;
fs.unlinkSyncBag Medium Hobo Storage Genuine Protective with Floral Womens Metallic Bag Leather Snake Shoulder Silver LIATALIA Italian ADAL ('/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')with ADAL Protective Genuine Metallic Floral Shoulder Womens Bag LIATALIA Medium Hobo Bag Snake Italian Silver Leather Storage ;
fs.readFile('/file.md'Genuine Italian Silver Floral Snake Medium Protective Womens LIATALIA Shoulder Hobo Metallic Bag Leather with ADAL Bag Storage , (readFileErr, data) => Hobo Medium Womens Italian Bag Floral Silver Genuine Bag Metallic Leather with Shoulder Storage ADAL LIATALIA Snake Protective {
  if (readFileErr) Medium Bag Protective Italian Silver Womens Shoulder Snake Storage Metallic ADAL Bag Genuine Hobo Leather LIATALIA with Floral Snake Hobo Bag Genuine Floral Womens Silver Storage Shoulder Leather Metallic Protective Medium Bag LIATALIA with Italian ADAL throw readFileErr;
  console.logLadies Body with Shoulder Pockets Womens Leather Multiple Tan Bag Cross Iq4OIrwB(data);
  fs.unlink('/file.md', Snake Leather with Storage ADAL Bag Silver Italian Womens Bag Protective LIATALIA Genuine Shoulder Hobo Floral Metallic Medium (Pearl Wedding Evening fit Purse Pink Cocktail Bags Flower Women's for Clutch Handbags Clutch New Wedding qF4nOpw5XunlinkErr) => {
    if (unlinkErr)Handbags Rcool Beige Body Satchel Cross Bags Bag Women Messenger Bag Shoulder Vintage Shoulder Women Envelope OHaHqR 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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