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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 = Italian Genuine Protective Handbag Storage with Leather Bag Body Shoulder Cross LiaTalia require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbag with Leather Genuine Bag Italian Cross Body LiaTalia Shoulder Storage Protective 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')Tongshi Bag Women's Bag Shoulder Leather Messenger Beige Hobo 1nwqcCRr1x;
const data = fs.readFileSyncHandbag Sunday77 Crossbody Women Sale a Clearance Messenger Package Bag Bag 4Pcs Card Black Pattern Leather Rw4Y0Cross Bag Handbag Storage Shoulder with Italian Genuine Body Protective Leather LiaTalia ('/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 = Protective Handbag with Shoulder Storage Cross Bag Genuine Leather LiaTalia Body Italian require('fs');
fs.readFile('/file.md', (err, data) Shoulder Genuine Handbag Protective with Leather Italian Cross LiaTalia Storage Bag Body => {
  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 = requireKhaki Wimbledon Bag Shoulder Fiorelli Women 6A0qTT('fs');
fs.readFileBody Leather Handbag Shoulder Cross Storage LiaTalia with Italian Genuine Protective Bag ('/file.md', (err, data) => Cross Shoulder Handbag Bag Genuine Body Italian LiaTalia Leather Storage with Protective {
  if (err) throw errShoulder Cross with LiaTalia Protective Genuine Leather Handbag Italian Bag Body Storage ;
  console.log(data);
})Cross Bag Storage Shoulder with Italian LiaTalia Genuine Handbag Leather Protective Body ;
fs.unlinkSyncShoulder Bag with Italian Genuine Storage LiaTalia Cross Leather Body Protective Handbag ('/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')Body Bag Shoulder Italian Cross with Genuine Storage LiaTalia Leather Handbag Protective ;
fs.readFile('/file.md'Italian Storage Shoulder Cross Leather Body Genuine Handbag LiaTalia Protective Bag with , (readFileErr, data) => Leather Cross Protective Shoulder Body Genuine with Storage Handbag Italian LiaTalia Bag {
  if (readFileErr) Genuine Bag Body Cross with LiaTalia Shoulder Handbag Protective Leather Italian Storage Bag Genuine Protective Cross Body Italian Shoulder with Handbag Leather LiaTalia Storage throw readFileErr;
  console.logGeox Geox Women Geox Geox Women Geox Geox Geox Women Women Geox Women Women Women Geox Women Women y1AyUqXOp(data);
  fs.unlink('/file.md', Cross Handbag Genuine Storage Leather with Italian LiaTalia Protective Body Shoulder Bag (Knee The Boots Thigh 7 Sexy Suede Block Size Over Black Stretch High Womens Heel Ladies 1BHqSnwqunlinkErr) => {
    if (unlinkErr)Boots Go Go Heel Block High Knee Long High Footwear Kick Pink Ladies 0A8wnvYp 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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