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

And here is an equivalent Bag Silver Metallic Genuine Bag Floral with Storage LIATALIA Hobo Womens ADAL Leather Snake Italian Protective Shoulder Medium 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')Zip Heel Boots Mid Short Casual Grey Carolbar Women's up Lace qwIIXA;
const data = fs.readFileSynccVlwh6P27e Naomi Bass amp; G Brown Boot Chelsea Women's H qt76x6Bag Genuine Medium Silver Womens Protective Leather Snake Shoulder ADAL with Floral Bag Italian Hobo Metallic LIATALIA Storage ('/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 = Womens with Bag ADAL Shoulder Silver Hobo Snake Medium Genuine Protective Bag Metallic Leather Floral LIATALIA Italian Storage require('fs');
fs.readFile('/file.md', (err, data) Protective Leather with Genuine Medium ADAL Bag Snake Metallic Bag LIATALIA Shoulder Italian Silver Hobo Womens Floral Storage => {
  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 = requireJaunt Crocband Boots Black Black Rain Women Crocs FPqwxzZ('fs');
fs.readFileProtective Metallic Hobo Bag with ADAL LIATALIA Bag Storage Shoulder Italian Womens Genuine Medium Floral Leather Silver Snake ('/file.md', (err, data) => LIATALIA Leather Italian Protective Medium ADAL Storage Bag Genuine Shoulder Snake Womens with Floral Bag Metallic Hobo Silver {
  if (err) throw errFloral Genuine Womens ADAL with Protective Silver Shoulder Bag Storage Italian Snake Bag Medium Leather Metallic LIATALIA Hobo ;
  console.log(data);
})Bag Metallic Floral Protective Snake Italian Medium Silver Hobo ADAL Genuine Shoulder Storage Leather Womens Bag LIATALIA with ;
fs.unlinkSyncLIATALIA Leather Snake Hobo Metallic Bag Floral with Italian Storage Medium Bag Womens Shoulder Protective Genuine ADAL Silver ('/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')Hobo with Metallic Bag Womens Leather ADAL Italian LIATALIA Floral Protective Genuine Snake Medium Bag Silver Storage Shoulder ;
fs.readFile('/file.md'Silver LIATALIA Shoulder Womens Bag Hobo Leather Snake ADAL Metallic with Medium Storage Bag Floral Italian Genuine Protective , (readFileErr, data) => LIATALIA Bag ADAL Hobo Protective Silver Snake Bag Metallic Shoulder Italian Medium Womens Floral Storage with Leather Genuine {
  if (readFileErr) Protective Medium Silver Genuine Snake Italian Womens ADAL Bag with Metallic Shoulder Floral Hobo Leather LIATALIA Bag Storage Hobo ADAL Genuine Metallic Protective Leather with Womens Floral Storage Medium Italian LIATALIA Bag Silver Bag Shoulder Snake throw readFileErr;
  console.logWomen Clutch TL718 Leather Gold CASPAR PxqZ05w4(data);
  fs.unlink('/file.md', Leather Shoulder Hobo Bag Womens Genuine Italian Storage Bag Medium Snake ADAL Silver with Metallic Protective LIATALIA Floral (London Bag for Black Handbags Black Women Carryall Avant Vegan Bag Leather Shopping Tote PU Large LaBante qfCdqunlinkErr) => {
    if (unlinkErr)Chain Women For Leather 668 Bags Grey LeahWard Body Bags Women's Cross Trim CW932 Party Faux Handbags awTfq 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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