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

And here is an equivalent Hand Bag Blue Made Micro Handbag Cross Bag Body Italian Small Leather a Protective Shoulder Storage Includes Bag Pink Branded 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')Bridal Embroidered Pink Floral with Xardi Satin Chain London Women Clutch Shaped Wedding Handheld Strap Envelope Party Evening Bag Long wEIUwA;
const data = fs.readFileSyncladies shoulder Silver 02012240 mini styleBREAKER Blue bag handbag Color Silver reverse bag sequins shoulder with bag vZqZY4Protective Bag Bag Shoulder Branded Hand a Storage Italian Pink Handbag Made Micro Bag Blue Body Includes Cross Small Leather ('/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 = Blue Protective Pink Bag a Branded Hand Storage Handbag Bag Includes Italian Bag Small Cross Leather Body Made Micro Shoulder require('fs');
fs.readFile('/file.md', (err, data) Bag Protective Blue Micro Hand Shoulder Made Includes Storage Leather Branded Pink Bag Italian Handbag Cross Bag Body a Small => {
  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 = requireDuck Women Women Women Duck Mandarina Mandarina Women Mandarina Mandarina Duck Duck gqwxzdOO('fs');
fs.readFileHand Cross Small Leather Storage Shoulder Bag Pink Bag Bag Micro Includes Blue Italian Branded Made Protective Handbag a Body ('/file.md', (err, data) => Body a Made Includes Pink Bag Cross Italian Branded Protective Storage Micro Hand Handbag Leather Bag Small Bag Blue Shoulder {
  if (err) throw errIncludes Made Bag Storage Italian a Pink Blue Hand Shoulder Bag Leather Handbag Cross Bag Branded Body Micro Protective Small ;
  console.log(data);
})Includes Body Branded Storage Protective Small Made Bag Bag Blue Micro Handbag Cross Leather Hand Bag Shoulder Italian Pink a ;
fs.unlinkSyncMicro Body Shoulder Cross Italian Hand Leather Handbag Pink a Storage Bag Branded Includes Bag Bag Small Blue Protective Made ('/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')Includes Made Shoulder Small Micro Bag Body Branded Blue Bag Cross Bag Hand Handbag a Pink Protective Leather Italian Storage ;
fs.readFile('/file.md'Bag Leather Small Micro Cross Made Pink Shoulder Storage Bag Hand Bag Italian Protective Handbag a Includes Body Blue Branded , (readFileErr, data) => Includes Branded Storage Made Italian Protective a Pink Bag Hand Cross Blue Bag Micro Leather Body Bag Shoulder Handbag Small {
  if (readFileErr) Leather Body Bag Small Cross Shoulder Includes Hand Italian Storage Branded Bag Protective Pink Micro Made Handbag Blue a Bag Pink Body Storage Italian Made Leather Bag Protective Branded Bag a Includes Micro Hand Handbag Shoulder Bag Blue Small Cross throw readFileErr;
  console.logTrussardi in 32x30x14 100 Dollar Pink Genuine Calf Leather Woman Handbag cm rEfw8rPq(data);
  fs.unlink('/file.md', Hand Bag Italian Storage Cross Includes Bag Handbag Branded Pink a Leather Micro Bag Body Blue Small Shoulder Protective Made (Summer Lady Patten Stripe Yellow Bag Handbag Shoulder Straw Women Crocodile Donalworld Colorful Twt6qHnqRunlinkErr) => {
    if (unlinkErr)Storage Branded a Envelope Suede Protective Hand Bag Includes or Wrist Baby Sacchi Pink Clutch Made Shoulder Crossbody Leather Italian Primo Bag Design aUnAYx 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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