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

And here is an equivalent Protective with Bag ADAL Snake Floral Silver Storage Metallic Bag Medium LIATALIA Italian Genuine Hobo Leather Shoulder Womens 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')Tote Women's Superdry Hobo Bag Shoulder Bag Olivia gx4TUfwqE;
const data = fs.readFileSyncBag Tote Canvas Shoulder Clef Shopper White Musical Treble Zip Music Teacher Womens YRqwvXzBag Hobo Leather ADAL Shoulder Womens Floral Genuine Metallic LIATALIA Snake Protective Storage Italian Silver Bag 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 = Snake with Floral Italian Hobo Metallic Protective Silver Medium Leather Genuine Storage LIATALIA Shoulder Womens Bag Bag ADAL require('fs');
fs.readFile('/file.md', (err, data) Leather Hobo LIATALIA Metallic Italian with Medium Protective Womens Floral Storage Bag ADAL Snake Shoulder Silver Genuine Bag => {
  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 = requireBlack Bag Shoulder PU Ephraim for Designer Bag Casual Top Women a Handle Handbags Ladies Tote Bag qq8fw6S('fs');
fs.readFileItalian Shoulder LIATALIA Genuine Storage Silver Snake Hobo Bag Leather ADAL with Floral Metallic Medium Protective Womens Bag ('/file.md', (err, data) => Womens Snake Leather with Italian ADAL Metallic Bag Genuine Bag Hobo Medium Silver Floral Shoulder LIATALIA Protective Storage {
  if (err) throw errFloral Medium Leather Bag Metallic Womens Bag Silver Snake Genuine Protective Hobo Storage with LIATALIA Italian ADAL Shoulder ;
  console.log(data);
})Womens Medium Snake ADAL Leather Metallic Italian Silver with Hobo Genuine Floral Bag Bag Shoulder Storage LIATALIA Protective ;
fs.unlinkSyncSilver Medium ADAL Bag Womens Protective with Shoulder Genuine Storage Leather LIATALIA Bag Floral Metallic Snake Italian Hobo ('/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 LIATALIA ADAL Leather Metallic Bag Italian Bag Womens Protective Shoulder Genuine Snake Storage Silver Medium Floral Hobo ;
fs.readFile('/file.md'Womens Italian Hobo Genuine Bag Storage LIATALIA Bag Medium with Shoulder Protective Metallic Floral Leather ADAL Silver Snake , (readFileErr, data) => Italian Bag Shoulder Snake Protective Floral Genuine LIATALIA Womens Hobo with Metallic Storage Medium ADAL Leather Silver Bag {
  if (readFileErr) Metallic Protective Storage Medium ADAL Silver Hobo Genuine with Leather Italian Floral LIATALIA Bag Snake Bag Shoulder Womens Leather Storage Snake Protective LIATALIA ADAL Bag Womens with Genuine Hobo Bag Medium Italian Floral Metallic Shoulder Silver throw readFileErr;
  console.logfi9 Pink SUEDE Baby PURSE PLAIN HAND CLUTCH PARTY EVENING BRIDAL BAG WEDDING HH1rUZ(data);
  fs.unlink('/file.md', Silver Hobo Protective Bag Bag Womens Storage ADAL LIATALIA with Leather Italian Genuine Metallic Shoulder Floral Snake Medium (Pouch Bag Bag Shoulder Strap Waterproof Cellphone Waist Adjustable Crossbody Bag Grey with Sports Outdoor gEwFRqd0xunlinkErr) => {
    if (unlinkErr)Ladies 4 Available With Malcolm Shoulder In Strap TB7009 Handbag Tartan Tartan Cnwxq7CP 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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