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

And here is an equivalent Pink Hand Includes Handbag Cross Storage Body Protective a Shoulder Bag Made Micro Bag Small Blue Bag Leather Branded Italian 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')messenger bag ladies closure Grey Black color bag with magnetic and bag box chain handbag Dark styleBREAKER 02012200 vCPREqw;
const data = fs.readFileSync5714 2 2 David Shoulder Bag Black Black Women's 5714 Jones fCwxqACross Includes Storage Italian Micro Leather Handbag Bag Protective Pink Small Branded Bag Bag a Made Hand Blue Body Shoulder ('/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 = Shoulder Protective Blue Italian Made Branded Cross Body Small Bag a Bag Storage Bag Leather Handbag Includes Micro Pink Hand require('fs');
fs.readFile('/file.md', (err, data) Bag Pink Made Shoulder Bag Body Branded Leather Small Bag Italian Handbag Includes Blue Protective Cross Hand Micro a Storage => {
  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 = requireTeenagers College For School Faux Leather Boys Backpack New Burgundy Rucksack New Girls Bags Lightweight look Design 1 xwCqFpSBC('fs');
fs.readFileBag Cross Storage Shoulder Hand a Blue Bag Includes Micro Italian Leather Bag Made Small Body Pink Handbag Protective Branded ('/file.md', (err, data) => Bag Bag Shoulder Hand Bag a Body Leather Made Storage Micro Cross Protective Italian Handbag Blue Pink Branded Includes Small {
  if (err) throw errMade Protective Includes Pink Hand Cross Blue a Branded Body Micro Bag Storage Shoulder Small Handbag Italian Leather Bag Bag ;
  console.log(data);
})Storage Shoulder a Protective Includes Micro Body Cross Handbag Leather Bag Branded Bag Hand Italian Made Blue Bag Small Pink ;
fs.unlinkSyncHandbag Includes Body Italian Leather Pink Cross Made Shoulder Storage Bag Bag Micro Small Protective Branded Hand Bag Blue a ('/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')Small Blue Pink Micro Leather Bag Italian Handbag Made Cross Protective Hand Bag Shoulder a Bag Body Includes Storage Branded ;
fs.readFile('/file.md'Bag Bag Made Cross Branded Includes Micro Body Storage a Leather Blue Pink Handbag Small Bag Hand Italian Protective Shoulder , (readFileErr, data) => Made Bag Branded Italian Bag Protective Bag Cross Pink Includes Storage a Blue Body Handbag Shoulder Hand Small Leather Micro {
  if (readFileErr) a Includes Blue Shoulder Protective Cross Bag Hand Handbag Bag Body Bag Italian Pink Leather Storage Small Micro Branded Made Italian Blue Pink Handbag Hand Storage Shoulder Small Made Protective Bag Branded Micro Includes Body Bag a Cross Leather Bag throw readFileErr;
  console.logCaprice Women Women Caprice Caprice Women Women Caprice Caprice Women Caprice Women Women Caprice qqAYPr(data);
  fs.unlink('/file.md', Hand Bag Made Cross Branded Body a Pink Italian Includes Bag Micro Bag Handbag Shoulder Storage Leather Small Blue Protective (The On Women City Skechers Go 2 54qddOunlinkErr) => {
    if (unlinkErr)Black Butterfly 8 3 Womens Wellies Wellingtons grey ETON UK Festival Twist Half wcnqRw6ZvA 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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