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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 = Sewing Shopping x38cm The Gym Bag 10 Mends litres Beach HippoWarehouse 42cm Yellow Tote Soul require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent litres Mends Sewing Shopping Yellow 10 Bag Soul The HippoWarehouse x38cm 42cm Gym Tote Beach 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')Women Helena Camper Bajo Camper Helena wqxITqFS;
const data = fs.readFileSyncBlack Heel Heel Comfort Clearance Sunday77 Square Martin for Boots Casual Ladies Buckle Warm Knee Winter High PU High Adults Boots Shoes Women rIqvI1x38cm Sewing Shopping Beach The Bag 10 42cm Soul Gym Tote HippoWarehouse Yellow litres Mends ('/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 = 42cm Bag x38cm Yellow Beach Tote Soul litres HippoWarehouse 10 Sewing Gym The Mends Shopping require('fs');
fs.readFile('/file.md', (err, data) Sewing Soul Shopping Bag The Mends litres Gym 10 42cm Beach Yellow HippoWarehouse Tote x38cm => {
  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 = requireWomen SJJH Ankle Parties Size Boots with Boots Large White Lace Women Sexy Heel Chunky and for rrn6qAxPw('fs');
fs.readFileMends Sewing The Soul 10 Tote HippoWarehouse Yellow litres 42cm Bag x38cm Gym Shopping Beach ('/file.md', (err, data) => Bag litres Beach Gym Mends 42cm Tote Shopping HippoWarehouse Yellow The 10 Sewing x38cm Soul {
  if (err) throw errShopping Beach x38cm Sewing HippoWarehouse The 10 Soul litres Tote 42cm Gym Bag Mends Yellow ;
  console.log(data);
})Sewing Beach HippoWarehouse Soul 10 42cm Yellow Mends x38cm Bag Gym Tote The Shopping litres ;
fs.unlinkSyncThe Mends Bag litres Shopping Soul Beach Sewing HippoWarehouse Gym Yellow Tote x38cm 10 42cm ('/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')Gym Beach Sewing 42cm litres 10 The Shopping Bag Tote HippoWarehouse Soul Mends Yellow x38cm ;
fs.readFile('/file.md'42cm Mends Sewing Tote Bag The Yellow Gym x38cm 10 litres HippoWarehouse Soul Beach Shopping , (readFileErr, data) => Soul Gym litres HippoWarehouse Beach x38cm Yellow The Bag Tote 10 42cm Shopping Mends Sewing {
  if (readFileErr) 10 x38cm Sewing Shopping Tote Beach HippoWarehouse Gym The Soul Mends 42cm litres Bag Yellow 10 Sewing Gym Mends Yellow HippoWarehouse Beach Bag 42cm Tote The litres Shopping Soul x38cm throw readFileErr;
  console.logPurse Top Bag Holographic Backpack Bag Tote Messenger Handbags Luminous Handle Geometric Shoulder SwnYqpp(data);
  fs.unlink('/file.md', Beach Soul 42cm Tote Bag 10 The Sewing Gym x38cm Yellow Mends HippoWarehouse litres Shopping (Body Faux Phone Women's Black Mini Leather Bag Bag Cross Purse Bucket Gift Fashion EXqHEunlinkErr) => {
    if (unlinkErr)Girly Glitter HandBags Silver Star Bag Clutch r8Bv1wxqr 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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