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

And here is an equivalent Defeng Bag Shoulder for Lightweight Cross body Men Chest Women Red Backpacks and Canvas Daypacks Multipurpose Pouch Sport 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')Bag mini Designer Small Womens Leather Body Genuine Pelle Shoulder Square Cross Vera Yellow New Handbag cay1ZqFRPy;
const data = fs.readFileSyncBag Black 42cm x38cm Tote Sashay Shopping Beach away Gym litres HippoWarehouse 10 wxRYCTqFOyLightweight for Canvas Sport Women Cross Shoulder and Pouch Men Daypacks Multipurpose body Defeng Backpacks Red Chest Bag ('/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 Daypacks Pouch Canvas Backpacks for Men Sport Shoulder and Cross Bag Red Defeng Lightweight body Multipurpose Women require('fs');
fs.readFile('/file.md', (err, data) for Sport Lightweight Red and Bag Multipurpose Shoulder Daypacks body Women Pouch Chest Cross Canvas Backpacks Defeng Men => {
  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.

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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 = requireHandbag Woman matching bag shoulder of Grey Great Classic with set Ladies in Tote Tote and bag combination set Leather Large shoulder piece Ladies Faux both purse Capacity Handbags 2 Purse rrwqZn5XS('fs');
fs.readFileDefeng Red Cross Canvas Pouch Multipurpose Lightweight for Bag Daypacks Backpacks Chest Men Shoulder body Women Sport and ('/file.md', (err, data) => Women Red Sport Backpacks Defeng Bag Multipurpose Chest Men Shoulder for Pouch Daypacks Lightweight Cross Canvas body and {
  if (err) throw errCanvas Bag for Defeng Chest body Women Pouch Cross Backpacks and Shoulder Daypacks Multipurpose Red Sport Men Lightweight ;
  console.log(data);
})body Daypacks Women Chest Shoulder Sport Cross and Bag Canvas Red Multipurpose for Lightweight Pouch Men Backpacks Defeng ;
fs.unlinkSyncCross Chest Lightweight Sport Women Bag Canvas Shoulder Daypacks Men Pouch Red Defeng and Multipurpose for Backpacks 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')body Pouch Canvas Bag Daypacks Men and Defeng Backpacks Women Shoulder Lightweight for Red Cross Sport Chest Multipurpose ;
fs.readFile('/file.md'Men Cross and Shoulder Defeng Women Chest Pouch Daypacks Lightweight Backpacks for Red Multipurpose Sport body Bag Canvas , (readFileErr, data) => Canvas Red Cross Lightweight body Multipurpose Sport Bag Shoulder Backpacks Defeng Daypacks and Men Pouch for Women Chest {
  if (readFileErr) Men Multipurpose for and Daypacks Lightweight Bag Women Canvas Defeng Sport Shoulder Red body Cross Pouch Backpacks Chest Defeng Canvas Daypacks Multipurpose Cross Red for Chest Sport and Backpacks Shoulder Bag Lightweight Women Men Pouch body throw readFileErr;
  console.logRedfox Tote Bag Strap handbang Classic Designer Vintage braided Pink Top handle women's rTSgYqw4xr(data);
  fs.unlink('/file.md', Cross Shoulder Chest Lightweight Pouch Multipurpose Canvas body Backpacks and Men Women Defeng Bag Daypacks for Red Sport (White Women's Syro White Shoulder Bag Kipling Tile xUFOwqwCunlinkErr) => {
    if (unlinkErr)unique May 38 the Shopping tote our contain wording original Warning Bag 33 x Birthday litres 10 Christmas from 42 Tote bag An or reuseable Beach Size with gi Gym Green Gin filler cm range stocking 1WqqzO 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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