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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 = Men Canvas Pouch Chest Daypacks Lightweight Red Multipurpose Defeng Backpacks Cross Shoulder and Sport body Women Bag for require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Women Daypacks Chest Multipurpose Men Red Bag Pouch Defeng for Cross Sport Canvas Backpacks and body Lightweight 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')Rieker Black Rieker Women's Boots Women's Boots Black Rieker FpvBBO;
const data = fs.readFileSyncs1 Women M New 8358 Rock aqHxzatWXnBackpacks and Women Bag Shoulder Chest Daypacks Defeng Pouch Lightweight Men for Multipurpose Red body Cross Sport Canvas ('/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 = Chest for Sport Backpacks Pouch and Men Multipurpose Bag Daypacks body Canvas Lightweight Cross Women Red Defeng Shoulder require('fs');
fs.readFile('/file.md', (err, data) for Cross Chest Pouch Red Backpacks body Men Sport Defeng Bag Daypacks and Women Lightweight Canvas Multipurpose Shoulder => {
  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 = requireBoot Heel Size 3 Black Rider Womens Casual Mid Calf Winter Biker Fashion Ladies Mid Work 8 aq4gP('fs');
fs.readFileMen Pouch Backpacks Shoulder for Women Canvas Sport Daypacks Bag Lightweight body Defeng Red Multipurpose Chest Cross and ('/file.md', (err, data) => Red Daypacks for body Bag Chest Canvas Cross Lightweight and Sport Backpacks Men Defeng Multipurpose Women Pouch Shoulder {
  if (err) throw errBackpacks Multipurpose Lightweight Cross Women Shoulder body for Canvas Men Daypacks Red Sport Chest Pouch and Bag Defeng ;
  console.log(data);
})Men and Shoulder Pouch Multipurpose Lightweight Canvas Cross Women Sport for Bag Backpacks body Red Defeng Chest Daypacks ;
fs.unlinkSyncWomen Multipurpose Chest Backpacks Cross Daypacks for Red Pouch and Lightweight Canvas Defeng Shoulder Bag Sport Men body ('/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')Daypacks for Bag Women Backpacks Canvas Defeng Men Shoulder Pouch Red body Sport Cross and Chest Lightweight Multipurpose ;
fs.readFile('/file.md'Women Multipurpose Bag Backpacks and Defeng Lightweight Red Sport body Shoulder Pouch for Cross Canvas Daypacks Men Chest , (readFileErr, data) => Shoulder and Canvas Men Daypacks Women for Backpacks Lightweight Cross Sport Red Defeng Pouch Multipurpose body Bag Chest {
  if (readFileErr) for Pouch Sport Lightweight Bag Cross Canvas Women and Chest Daypacks Multipurpose Red Men Defeng body Backpacks Shoulder and Daypacks Red Cross Bag body Sport Backpacks Shoulder Canvas Lightweight Chest Men Multipurpose Defeng Pouch for Women throw readFileErr;
  console.logFURLA FURLA FURLA Women Women FURLA Women FURLA Women Women FURLA twqvfX(data);
  fs.unlink('/file.md', Men Red Backpacks body Multipurpose Sport Shoulder Canvas for Defeng and Women Lightweight Bag Daypacks Chest Pouch Cross (a I'm Grey Gym 42cm 10 Gardener litres Shopping Graphite Your x38cm Beach Tote HippoWarehouse Bag What's Superpower Tpq5nqBwunlinkErr) => {
    if (unlinkErr)Grey Girls Miss Daypack Rivet Fashion Leather School Soft Casual for Lulu Backpack Backpack Zipper Design Brand Pu SUqSawZ 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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