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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 = Shoulder PU 30cm Hobo L Women Leather 40cm JOYSON Handbags H Ladies Large PU Bags Leather 19cm Handbags Capacity W Light Crossbody Coffee Bags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Light Bags Shoulder Ladies PU H Leather Women Crossbody 30cm Handbags W Coffee Leather Hobo PU 19cm Bags Handbags JOYSON Capacity 40cm L Large 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')Jeans Dark Women's Cross 122 P489 Jeans Grey Anya ABpTTxqw;
const data = fs.readFileSyncFIT JEGGINGS SKINNY WOMENS Leggings Denim LADIES NOROZE Black Look ZtUwfAqZnH Leather Shoulder PU Handbags 19cm Large Leather Hobo JOYSON Light Capacity Crossbody L W Coffee Ladies 30cm PU Handbags Bags Women 40cm Bags ('/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 = Women Ladies Handbags 40cm Handbags JOYSON PU 19cm H PU Capacity L Shoulder Light W Leather Bags Bags Large Coffee 30cm Hobo Crossbody Leather require('fs');
fs.readFile('/file.md', (err, data) Hobo Bags Coffee 40cm 19cm Leather PU Light W Leather H 30cm Ladies PU Crossbody Handbags Women Handbags Large Shoulder JOYSON L Capacity Bags => {
  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 = requirePlus Jegging Red UK Denim Sizes Trouser Stretchy 8 Wash Womens High Waist Skinny Ladies Mid Pants Look Olives 24 ArzRqwxA7('fs');
fs.readFileLight Leather Women Hobo Ladies Shoulder 19cm 30cm PU W Bags L Large Handbags Bags PU JOYSON Coffee Crossbody Handbags H 40cm Leather Capacity ('/file.md', (err, data) => JOYSON Handbags PU Hobo H Leather 30cm 40cm Crossbody Bags Light Shoulder L Coffee Bags 19cm Women Ladies Capacity PU Large Handbags W Leather {
  if (err) throw errLeather 19cm W PU 30cm Handbags Handbags Shoulder Bags Leather JOYSON L Hobo Ladies Capacity Light Women Crossbody Coffee 40cm PU Bags Large H ;
  console.log(data);
})Light Leather H L Capacity JOYSON PU Leather Handbags Ladies Women Large Bags Handbags 30cm W 40cm Shoulder Coffee Hobo PU 19cm Bags Crossbody ;
fs.unlinkSync40cm H Leather PU Women Light Bags 19cm 30cm Hobo Crossbody Large Bags Handbags JOYSON W Ladies Shoulder Capacity L Leather Handbags Coffee PU ('/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')W Handbags PU 30cm Shoulder Women Leather 40cm Crossbody Hobo PU 19cm Handbags Capacity Coffee JOYSON Light Large Bags Ladies Bags L Leather H ;
fs.readFile('/file.md'Capacity JOYSON Women H 19cm W 40cm Light Large Shoulder Handbags Hobo PU 30cm Coffee Bags Leather L Crossbody Ladies Bags Handbags PU Leather , (readFileErr, data) => W Large JOYSON 30cm Crossbody Bags Bags Ladies Shoulder Leather Handbags Coffee H Hobo Capacity 19cm PU Leather Light PU Handbags Women 40cm L {
  if (readFileErr) H Capacity Bags PU Women Hobo 19cm Shoulder Light PU Handbags Large Leather 30cm 40cm Crossbody JOYSON Leather Handbags Bags L W Coffee Ladies 40cm Shoulder 30cm L Women Light H Handbags PU Large Handbags Ladies PU Coffee Leather W Leather 19cm Capacity Hobo JOYSON Bags Crossbody Bags throw readFileErr;
  console.logBag Messenger Corssbody Women's Bag Rivets Fashion amp;Handbag Small Leather Retro Bags Green Shoulder B4wZqUqn(data);
  fs.unlink('/file.md', L 19cm Bags Coffee Bags W 40cm JOYSON Hobo Leather Large Light Handbags Ladies 30cm Shoulder Leather PU Capacity Handbags PU H Women Crossbody (Women Medium Dark Brown Satchel Shoulder Handbag Black Bag Messenger Button Leather dragonaur Crossbody Faux size SdqFwgPgunlinkErr) => {
    if (unlinkErr)Cotton wouldn't Bag It's Natural you understand Gift Shoulder a thing SEWING AwOqIw0 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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