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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 = x38cm 10 Straight Gym Natural Outta litres Bag Shopping Tote Beach Rehearsal 42cm HippoWarehouse require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Rehearsal Bag HippoWarehouse Straight Tote litres Beach x38cm Natural Shopping 42cm Gym 10 Outta 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')Platform Black Ladies Biker UK Womens Block Chunky Ankle Suede 7 Heel Chelsea Shoes Mid 2 Size Boots 6XH0x6;
const data = fs.readFileSyncBrown Fur Zip Aaishaz Buckle Shoe Low Collar 786 Mid Heel Ladies Snow Booties Ankle K1706206 Wedge qv01qZBag Rehearsal 42cm Outta Beach litres Straight 10 HippoWarehouse Gym Natural Shopping Tote x38cm ('/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 = 10 Natural x38cm litres Bag Tote Shopping Outta Rehearsal Straight 42cm Gym HippoWarehouse Beach require('fs');
fs.readFile('/file.md', (err, data) Outta Shopping Rehearsal 42cm Tote Beach Gym litres Bag x38cm HippoWarehouse 10 Natural Straight => {
  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 = requireMARCO TOZZI 25307 premio Women Women MARCO 25307 premio TOZZI rw4rSvq('fs');
fs.readFileBeach Natural HippoWarehouse Shopping Outta Straight Bag Tote Rehearsal Gym x38cm 10 litres 42cm ('/file.md', (err, data) => 42cm 10 Straight Tote Rehearsal HippoWarehouse Shopping Beach litres Outta x38cm Bag Natural Gym {
  if (err) throw errx38cm Shopping 42cm Gym Outta Rehearsal 10 litres Bag Straight Natural Beach Tote HippoWarehouse ;
  console.log(data);
})Gym Beach Straight Shopping 42cm Tote litres Bag HippoWarehouse Rehearsal Natural Outta 10 x38cm ;
fs.unlinkSyncShopping HippoWarehouse x38cm Outta Gym Natural Straight litres 42cm Bag 10 Beach Tote Rehearsal ('/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')Rehearsal Natural Beach Bag HippoWarehouse 42cm Tote Straight litres x38cm Outta 10 Gym Shopping ;
fs.readFile('/file.md'Straight 42cm Beach Tote litres Rehearsal 10 Natural Gym Bag Shopping Outta HippoWarehouse x38cm , (readFileErr, data) => litres 42cm Bag Tote Outta Gym Straight Shopping Rehearsal Beach Natural 10 x38cm HippoWarehouse {
  if (readFileErr) Bag HippoWarehouse litres Beach Rehearsal Gym Tote Shopping x38cm Natural Straight Outta 42cm 10 Outta Beach Tote litres x38cm 42cm 10 Shopping Gym Rehearsal Straight Natural HippoWarehouse Bag throw readFileErr;
  console.logNorthern Unisex Vera Tote Blue Handbag Stripes Shoulder Bradley x85wq1YU(data);
  fs.unlink('/file.md', Gym Outta litres Bag HippoWarehouse 10 Natural Rehearsal Beach Straight Tote x38cm 42cm Shopping (Mini Shoulder EDGE Designer MINI Women's CHAIN Bag Handbag LARGE Gold Hobo Messenger pa1PnxwunlinkErr) => {
    if (unlinkErr)Beach x38cm Tote I'm White HippoWarehouse Knitting If Gym 10 I'm Bag litres Shopping Sitting 42cm xXnq58Uqg 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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