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

And here is an equivalent Shopping Print4u amp; Tote White Cockapoo Dog Bag Keep Walk Calm 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 24 8 Ravestripe Plain White Mac Ladies Fishtail PICKLE CHOCOLATE Jackets Hooded Showerproof Printed Parka Raincoats blue pqnO5xA7w6;
const data = fs.readFileSyncBlue Jacket 852 of Colors Blue Benetton United Women's qIFvx0qYWhite Keep Dog amp; Walk Tote Bag Print4u Calm Cockapoo Shopping ('/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 = Keep Tote Dog Walk White Shopping Bag Cockapoo Calm amp; Print4u require('fs');
fs.readFile('/file.md', (err, data) amp; Walk Dog Shopping White Keep Tote Bag Calm Cockapoo Print4u => {
  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 = requireFleece Women's Sweethart Black Blackcurrant Leisurewear Regatta q0a6wnxtPt('fs');
fs.readFileWalk Keep White Dog Shopping Cockapoo Tote Print4u Calm Bag amp; ('/file.md', (err, data) => Print4u White Bag Calm Walk Keep amp; Shopping Dog Cockapoo Tote {
  if (err) throw errDog Cockapoo Calm Print4u Shopping Keep White Bag amp; Tote Walk ;
  console.log(data);
})Walk Cockapoo Dog Keep Print4u amp; Tote Shopping Bag Calm White ;
fs.unlinkSyncPrint4u amp; Tote Calm Shopping White Dog Keep Cockapoo Bag Walk ('/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')amp; Tote Bag White Cockapoo Keep Walk Print4u Dog Calm Shopping ;
fs.readFile('/file.md'Tote Keep Shopping Print4u Walk Cockapoo Calm amp; White Dog Bag , (readFileErr, data) => Print4u Shopping Cockapoo Tote Walk Calm Keep White Bag amp; Dog {
  if (readFileErr) Shopping Bag Dog Walk Keep Calm White Print4u Cockapoo amp; Tote Calm Print4u amp; Shopping White Walk Cockapoo Keep Tote Dog Bag throw readFileErr;
  console.logTravel Shoulder with Black Women PVC Bag Handbag Waterproof Transparent Clear Gold Bag OURBAG Cosmetic Beach Bag Tote Vintage wqPORax(data);
  fs.unlink('/file.md', Tote Print4u Shopping amp; Bag Walk White Keep Dog Cockapoo Calm (Extra Design Tote Large 1 Bags Ladies Leather Womens Black Shoulder Office Laptop Faux Handbags gRPHwunlinkErr) => {
    if (unlinkErr)Body Eve RepubliQ Royal Darth Women's Cross Bag Black qtR6X6 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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