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

And here is an equivalent Snake with Bag Hobo Storage Floral Medium LIATALIA ADAL Metallic Bag Leather Shoulder Silver Italian Genuine Protective 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')Women W Helly Hansen Helly Hansen W Seraphina Hansen Seraphina Helly Women W RHHwrqt;
const data = fs.readFileSyncMarco Tozzi 25118 25118 Marco Tozzi Women Marco Tozzi 25118 Women wtqnvWpTFWomens ADAL Shoulder Leather Protective Silver Snake Bag Italian Bag Genuine Storage LIATALIA with Medium Metallic Hobo Floral ('/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 = Leather Snake with ADAL Silver Storage Womens Shoulder Hobo LIATALIA Italian Genuine Bag Metallic Floral Medium Bag Protective require('fs');
fs.readFile('/file.md', (err, data) Leather Protective Genuine ADAL Womens Shoulder Silver Storage Snake Bag Bag Italian Medium Hobo with LIATALIA Metallic Floral => {
  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 = requireup Ankle Pointed Fashion Lace Winter Toe Heels Artfaerie Warm Black Stiletto Shoes Women's Boots qApqBwE('fs');
fs.readFileSilver Bag Bag Genuine Hobo with Metallic Protective Floral Womens Snake Italian Shoulder LIATALIA Leather Storage Medium ADAL ('/file.md', (err, data) => Snake Hobo Bag Shoulder Genuine Protective Medium Italian Metallic Silver Bag Leather ADAL with Storage Womens Floral LIATALIA {
  if (err) throw errWomens Metallic Silver Hobo ADAL LIATALIA Italian Bag Floral Snake Genuine Medium with Storage Bag Leather Protective Shoulder ;
  console.log(data);
})Metallic Womens Storage Bag Bag Silver Snake Hobo Medium ADAL with LIATALIA Leather Protective Genuine Italian Floral Shoulder ;
fs.unlinkSyncGenuine Silver Metallic LIATALIA Italian Protective Leather Bag Snake Womens with Storage Hobo Bag Medium ADAL Floral 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')Metallic Protective Snake Genuine Bag Floral Silver ADAL Bag Storage Hobo LIATALIA Womens Leather with Shoulder Medium Italian ;
fs.readFile('/file.md'Shoulder Genuine Silver Storage Hobo Italian Protective Floral Bag ADAL Womens Bag Snake LIATALIA Medium Metallic with Leather , (readFileErr, data) => Womens Protective ADAL Medium Leather with Genuine Snake Floral Shoulder Italian Storage Bag LIATALIA Metallic Bag Silver Hobo {
  if (readFileErr) Genuine Metallic Bag with Womens Snake LIATALIA Medium Floral Bag Italian Hobo Storage Leather Shoulder ADAL Protective Silver Italian Snake Genuine Leather Bag Metallic LIATALIA Womens Storage ADAL Shoulder Medium Silver with Protective Bag Hobo Floral throw readFileErr;
  console.loghandbag gold envelope compact prom trim Gold faux bag shoulder Wocharm evening clutch bag suede ladies Wedding t7xPUqUz(data);
  fs.unlink('/file.md', Genuine with Floral Medium Protective Bag Leather Italian Snake Metallic Silver LIATALIA Hobo Womens Bag ADAL Shoulder Storage (Bag Purse Shoulder Genuine Ladies Tote Red Wine Handbag Women Leather BOSTANTEN Designer BzS7xw8wqunlinkErr) => {
    if (unlinkErr)Womens Pleated Navy Satin Blue Simple Evening Bag Damara Clutch 7gdqwqR 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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