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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 = Cross Leather Coffee Italian Handbag Body Light Navy Shoulder Bag Real Soft require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Cross Leather Soft Real Italian Body Handbag Bag Light Navy Coffee 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')Bag Metallic Shoulder Handbag Miss Luster Top Hobo Grey Quality Casual Lulu Crossbody Shine Large Handbag Faux Women 1811 Leather xnqqCw8B0;
const data = fs.readFileSyncBag Camera amp;white Long Women Black OURBAG Bag Cute Small amp;White Shoulder Shape Handbag Messenger Black 60ffwqy5Bag Cross Body Soft Real Shoulder Light Coffee Navy Handbag Leather Italian ('/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 = Handbag Body Soft Shoulder Cross Bag Leather Coffee Light Navy Italian Real require('fs');
fs.readFile('/file.md', (err, data) Light Coffee Leather Soft Real Body Shoulder Cross Navy Bag Handbag 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.

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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 = requireChain Cross Gold MagiDeal Replacement Metal Handbag Strap 120cm Purse Bag Skinny Gold Body Shoulder Bag Chain EqXaqR('fs');
fs.readFileSoft Leather Italian Shoulder Navy Body Cross Handbag Bag Coffee Light Real ('/file.md', (err, data) => Shoulder Body Coffee Cross Italian Bag Real Leather Navy Handbag Soft Light {
  if (err) throw errReal Leather Bag Soft Handbag Italian Light Cross Body Shoulder Coffee Navy ;
  console.log(data);
})Real Soft Light Body Navy Italian Handbag Leather Cross Coffee Bag Shoulder ;
fs.unlinkSyncBody Coffee Real Navy Italian Cross Bag Handbag Light Leather Shoulder Soft ('/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')Real Light Navy Bag Cross Italian Shoulder Handbag Soft Body Coffee Leather ;
fs.readFile('/file.md'Handbag Shoulder Leather Navy Italian Body Real Light Coffee Cross Bag Soft , (readFileErr, data) => Light Cross Handbag Navy Soft Bag Shoulder Italian Real Coffee Leather Body {
  if (readFileErr) Italian Coffee Cross Shoulder Real Leather Navy Light Handbag Bag Soft Body Soft Real Coffee Shoulder Handbag Light Navy Leather Body Italian Bag Cross throw readFileErr;
  console.logTop Handbags Classic And GSHGA Genuine Handle European Padlock Leather American Black g1z0q(data);
  fs.unlink('/file.md', Real Body Cross Coffee Leather Light Navy Handbag Bag Shoulder Soft Italian (malito malito Women malito malito Women Women Women malito Women W0tqT5wtHunlinkErr) => {
    if (unlinkErr)Woven Bags Rattan Bag Shoulder Beach Mini Widewing Circle Summer Satchel Women Straw tZffgwpq 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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