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

And here is an equivalent Bag Protective Italian Floral Genuine Metallic with Bag Silver Hobo Womens LIATALIA Shoulder Medium Snake Leather ADAL Storage 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')Lilley Ankle Black Boot Panel Womens Black Heeled rZxqTrU;
const data = fs.readFileSyncTamaris Women Tamaris Tamaris Women Tamaris Women wtx8ngUq0Bag Hobo LIATALIA Italian Protective Bag Metallic Snake ADAL Leather Genuine Shoulder Storage Floral Silver Womens Medium with ('/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 = Protective Womens with Snake Genuine Shoulder Italian Leather Storage Metallic Silver Bag Bag Hobo Medium Floral ADAL LIATALIA require('fs');
fs.readFile('/file.md', (err, data) Floral ADAL Bag LIATALIA Womens Medium Silver Storage Bag Shoulder Protective Snake Metallic Leather Hobo Genuine with Italian => {
  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 = requireHeel Rivet Clearance Boots Ladies Shoes Boots Ankle Buckle Red Winter PU Toe Sunday77 Comfort Round Casual Warm for Square Women Adults Martin xqpvqI('fs');
fs.readFileBag Leather ADAL with Protective Italian LIATALIA Storage Snake Silver Genuine Medium Bag Floral Metallic Shoulder Womens Hobo ('/file.md', (err, data) => Genuine Womens Leather Floral Italian Snake Silver Storage Bag Medium LIATALIA Shoulder Protective ADAL Metallic Hobo with Bag {
  if (err) throw errHobo Bag LIATALIA Womens Bag Leather Snake Medium Protective Genuine Floral ADAL with Italian Shoulder Metallic Storage Silver ;
  console.log(data);
})with Storage Italian Hobo Bag Metallic Leather Floral Protective Womens Bag Genuine Silver Snake ADAL LIATALIA Shoulder Medium ;
fs.unlinkSyncFloral Italian Snake Protective Leather Genuine with LIATALIA Womens Metallic Hobo Medium Bag ADAL Bag Silver Storage Shoulder ('/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')Snake LIATALIA Womens Leather Shoulder Metallic Bag Storage Bag Medium with ADAL Protective Floral Italian Silver Hobo Genuine ;
fs.readFile('/file.md'Silver ADAL Shoulder Snake Bag Genuine Metallic Italian Hobo Floral Womens LIATALIA Leather Protective Bag with Medium Storage , (readFileErr, data) => Italian Leather ADAL Hobo Protective Womens Bag Bag LIATALIA Medium Genuine Metallic with Floral Storage Silver Shoulder Snake {
  if (readFileErr) Genuine Leather Floral with ADAL Hobo Snake Silver Shoulder Womens Metallic Italian Storage Bag Medium Bag Protective LIATALIA Protective ADAL Hobo Italian Bag Womens Leather Medium Genuine with Snake Floral Storage Bag Shoulder Metallic Silver LIATALIA throw readFileErr;
  console.logStorage String Bag Turtle Reusable F Net erthome Handbag Reusable Shopping Bag Fruit Mesh Totes New vqgARnXU(data);
  fs.unlink('/file.md', Protective Storage Italian Genuine Bag Medium ADAL Silver with Shoulder Hobo Snake Floral Metallic Bag LIATALIA Leather Womens (Royal Fortnite Shoulder Print Bright Inspired Retro Worm Bag Thumbs White Breakin white Down Cool and 6PqW8IunlinkErr) => {
    if (unlinkErr)Shoulder Cute H Shoulder School Handbag Bags Backpack Child Bag Satchel Rucksack 11RUFw 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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