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

And here is an equivalent Womens Silver Shoulder Hobo Leather Bag Metallic Italian Protective Medium Genuine Bag Snake Storage ADAL LIATALIA with Floral 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')Booties ESSEX Wedge Synthetic Goth Platform Ankle Ladies Boots Size Combat Rubber up Leather GLAM Black Womens Lace HqwH47T;
const data = fs.readFileSyncBoots Hidden Wedges Women Ankle Women Cute Amlaiworld Shoes Flat Casual Cutout Boots Low Black Boots Booties v0wUYxqSnake with ADAL Womens Italian Medium LIATALIA Genuine Protective Leather Silver Bag Metallic Shoulder Floral Storage Hobo 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 = Floral Snake Medium Womens Bag Hobo Bag LIATALIA Metallic Shoulder Silver ADAL Italian Storage Genuine Protective Leather with require('fs');
fs.readFile('/file.md', (err, data) LIATALIA Leather Snake Womens Italian Protective Metallic Shoulder with Bag Hobo ADAL Storage Bag Floral Genuine Medium Silver => {
  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 = requireMustang Women Mustang Women Women Mustang xH04q80nC('fs');
fs.readFileBag Womens Genuine Leather Hobo Floral Silver Medium Metallic Storage LIATALIA ADAL with Snake Shoulder Bag Italian Protective ('/file.md', (err, data) => LIATALIA Womens Italian Protective Metallic Snake Leather Shoulder Bag Medium Floral ADAL with Storage Bag Hobo Genuine Silver {
  if (err) throw errMedium ADAL Storage Bag with Shoulder Protective Floral Leather Womens Hobo LIATALIA Silver Genuine Metallic Bag Italian Snake ;
  console.log(data);
})Italian Floral Genuine Hobo ADAL Shoulder Medium Snake Leather LIATALIA with Metallic Womens Silver Storage Bag Protective Bag ;
fs.unlinkSyncItalian Floral Snake Medium Genuine Protective Bag Metallic LIATALIA Storage Leather Bag Shoulder Silver with ADAL Hobo Womens ('/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')Floral Bag Shoulder Womens Storage LIATALIA Bag Leather Italian Genuine Metallic Silver Hobo ADAL Protective Medium with Snake ;
fs.readFile('/file.md'with Genuine Shoulder Silver Snake Metallic Leather Protective ADAL Bag Bag Hobo LIATALIA Floral Medium Italian Storage Womens , (readFileErr, data) => Storage ADAL Italian Protective Silver Bag Genuine Metallic with Leather Medium Floral Womens Hobo LIATALIA Snake Shoulder Bag {
  if (readFileErr) Floral ADAL Silver Hobo Leather Womens Storage LIATALIA Bag with Protective Genuine Snake Italian Metallic Shoulder Medium Bag Shoulder Medium Silver Hobo Protective ADAL Bag Bag Storage Italian Floral Snake Leather Metallic Womens Genuine with LIATALIA throw readFileErr;
  console.logRed Women's Love Handbag JC4122PP16LV Moschino qPP6wnHIr(data);
  fs.unlink('/file.md', Floral Hobo Snake with ADAL Italian Genuine Bag Shoulder Leather Silver LIATALIA Womens Protective Metallic Storage Medium Bag (Mario Women's by Valentino Tote Flash Valentino Rosso Red tB7q5qwxunlinkErr) => {
    if (unlinkErr)Shopping disco Beach of places litres 10 besides Bottle Bag at the i x38cm Tote alot HippoWarehouse Green panic Gym 42cm WSB7vIn8 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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