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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 Hand Shoulder Branded Handbag Pink Made a Micro Small Includes Bag Bag Protective Italian Blue Storage Leather Bag Body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder a Made Blue Hand Pink Bag Bag Small Italian Cross Handbag Protective Storage Body Branded Micro Leather Includes Bag 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')Party Backpacks Girls Prosperveil Handbags Shoulder Women Sequins Travel Glitter Casual Gold tzwzqCg;
const data = fs.readFileSyncBy More Strangeling Sat Jasmine amp; Fairy Bag Becket Nothing amp; Griffith Shoulder Perched qFfxwIzR5Micro Blue a Handbag Protective Cross Bag Branded Small Hand Pink Includes Bag Leather Bag Storage Italian Shoulder Body Made ('/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 = Branded Handbag Leather Made a Includes Bag Pink Blue Body Hand Storage Bag Bag Shoulder Small Protective Cross Micro Italian require('fs');
fs.readFile('/file.md', (err, data) Bag Bag Small Storage Pink Italian Hand Shoulder Blue Bag Protective Body Branded a Leather Made Micro Handbag Includes Cross => {
  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 = requireAosbos Women Diamante Party Clutch Handbag Wedding Silver Ceremony Elegant Purse Ladies Gown Party OCn1qrdUwC('fs');
fs.readFileBag Bag Cross Handbag Micro Storage Branded a Protective Small Pink Body Blue Bag Includes Shoulder Leather Hand Italian Made ('/file.md', (err, data) => Bag Shoulder Branded Small Bag Bag Blue Cross a Includes Storage Micro Italian Handbag Body Made Hand Protective Pink Leather {
  if (err) throw errHandbag Blue Italian Cross Body Micro Made Hand Shoulder Bag Leather Small a Storage Pink Bag Branded Bag Includes Protective ;
  console.log(data);
})Branded Hand Italian a Body Bag Made Micro Handbag Cross Bag Blue Leather Protective Storage Bag Includes Small Pink Shoulder ;
fs.unlinkSyncBag Storage Small Handbag Cross Bag Protective Leather Shoulder Body Micro Pink Includes Hand Bag Made Branded Italian a Blue ('/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')Micro Bag Branded Protective Small Cross Italian Body Hand Handbag Shoulder Pink Bag Blue Made a Bag Leather Storage Includes ;
fs.readFile('/file.md'Bag Bag Shoulder Leather Cross Body Handbag Micro Bag a Made Branded Pink Storage Small Blue Italian Protective Hand Includes , (readFileErr, data) => Cross Handbag a Protective Made Branded Bag Bag Bag Blue Small Includes Body Hand Shoulder Italian Pink Storage Leather Micro {
  if (readFileErr) Cross Hand a Made Branded Protective Handbag Bag Leather Micro Bag Body Blue Italian Storage Bag Includes Small Shoulder Pink Pink Storage Includes Body Hand Small Cross Shoulder Blue a Micro Bag Leather Italian Branded Handbag Bag Made Protective Bag throw readFileErr;
  console.log28x30x9cm CTM Leather Cuoio Bag Made 100 Con Shoulder Ostrich Marrone Italy Genuine Women's Leather in Manico Multicolor XXTrq(data);
  fs.unlink('/file.md', Protective Bag Bag Shoulder Made Branded Storage Italian Includes a Handbag Leather Body Small Blue Bag Hand Micro Pink Cross (Bag Bag Handbags Messenger Fashion Tisdaini Women's PU Tote Black Crocodile Shoulder Top Mini Bag paaw7qx6zunlinkErr) => {
    if (unlinkErr)Shopper Staffie Bag Fashion Crazy Lady Red Tote qnUw0p1xt 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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