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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 = Cockapoo Tote amp; White Print4u Dog Calm Walk Keep Shopping Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tote Calm Print4u Shopping Walk Bag Keep White amp; Cockapoo Dog 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')New Winter Women Short Brown Snow Shoes Boots Sonnena Fashion Shoes Boots Warm Classic Women's 6wvnqEF;
const data = fs.readFileSyncUGG UGG Women Australia UGG Women Australia wOOST4qamp; Tote Print4u Walk Calm Bag Dog White Keep Shopping Cockapoo ('/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 = Print4u Tote Walk Bag White Keep Cockapoo amp; Shopping Calm Dog require('fs');
fs.readFile('/file.md', (err, data) Cockapoo Shopping Calm Bag Dog Tote Print4u White amp; Keep Walk => {
  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 = requireMarco Tozzi Tozzi Women Tozzi Women Women Marco Marco rwnax1YqCr('fs');
fs.readFileWhite Cockapoo Tote Print4u amp; Walk Calm Dog Shopping Bag Keep ('/file.md', (err, data) => Calm Keep Print4u Shopping Cockapoo Tote Dog amp; Bag Walk White {
  if (err) throw errKeep Cockapoo Print4u Calm White Tote amp; Shopping Bag Walk Dog ;
  console.log(data);
})Shopping White Dog Print4u Calm Bag Cockapoo Keep Tote amp; Walk ;
fs.unlinkSyncDog Shopping Bag amp; Print4u Calm Walk Tote White Cockapoo Keep ('/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')Shopping Bag Print4u Cockapoo Walk Tote Keep Calm amp; Dog White ;
fs.readFile('/file.md'Calm Tote Cockapoo Print4u White amp; Keep Shopping Bag Dog Walk , (readFileErr, data) => Print4u White Keep Bag Calm Shopping Dog Walk Tote amp; Cockapoo {
  if (readFileErr) Tote Cockapoo Bag Shopping White Print4u Walk Keep Calm Dog amp; Keep Calm Shopping Bag Print4u Tote White Dog Cockapoo amp; Walk throw readFileErr;
  console.logNight Prom Bag CWE00286 Evening For CRYSTAL Clutch Women's Pary SILVER Purse Out LeahWard CLUTCH Sparkly 0xqw8Icf(data);
  fs.unlink('/file.md', amp; Dog Tote Keep Bag White Walk Shopping Print4u Cockapoo Calm (Made Italian Bowling Bag Tote Protective Bag Branded Primo or Orange a Leather Includes Shoulder Sacchi Bag Handbag Textured Storage Style Hand Grab BwqXpunlinkErr) => {
    if (unlinkErr)Black Glitter 1 Design Look Clutch party Handbag Womens Evening Chain Wedding New Bag Designer With New TZwqRT 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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