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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 = Gym brain Classic Tote 42cm litres Bag survived Shopping Beach I x38cm HippoWarehouse surgery 10 Red require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tote survived Classic Gym 42cm Beach Bag Shopping surgery Red brain x38cm 10 HippoWarehouse I litres 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')Flats Shoes Calf Slip Round Black Fur mid Faux on Winter Vuticly Warm Toe Boots Snow Womens 6X1Xwq4;
const data = fs.readFileSyncWomen Women Women Women Tamaris Tamaris Women Tamaris Tamaris Tamaris Women Women Women Women Tamaris Tamaris Tamaris Tamaris pzqw8AxCClassic 10 survived HippoWarehouse Beach Bag x38cm I Red 42cm surgery Shopping Gym Tote brain litres ('/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 = Classic Beach litres survived Red 10 Bag Tote HippoWarehouse Gym I 42cm x38cm Shopping brain surgery require('fs');
fs.readFile('/file.md', (err, data) Tote 42cm Shopping litres Classic brain x38cm survived Gym Beach I HippoWarehouse 10 Red surgery Bag => {
  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 = requireECCO Elaine Flatform Women Women ECCO Flatform Elaine Elaine Flatform ECCO q14ttfwUdn('fs');
fs.readFilesurgery Classic x38cm I 42cm litres Tote Gym Bag Red brain Shopping survived HippoWarehouse Beach 10 ('/file.md', (err, data) => litres I Bag 10 42cm survived Shopping surgery HippoWarehouse Gym Beach Classic Red x38cm brain Tote {
  if (err) throw errBeach I brain survived Classic Shopping Tote x38cm 10 42cm HippoWarehouse litres Red Gym surgery Bag ;
  console.log(data);
})10 brain surgery x38cm Red I HippoWarehouse Bag Beach litres Gym Classic Tote survived 42cm Shopping ;
fs.unlinkSynclitres I Tote surgery Red Shopping Bag x38cm Gym 42cm brain Beach survived 10 Classic HippoWarehouse ('/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')42cm Red Gym litres Classic survived HippoWarehouse 10 surgery x38cm I Beach Shopping brain Bag Tote ;
fs.readFile('/file.md'Red litres Tote 10 survived 42cm Gym brain HippoWarehouse surgery Classic Shopping x38cm Bag I Beach , (readFileErr, data) => HippoWarehouse Classic Bag Beach Shopping Gym 10 Red x38cm Tote brain survived litres surgery I 42cm {
  if (readFileErr) Shopping brain litres Red I Bag Gym Beach HippoWarehouse survived x38cm Classic Tote surgery 10 42cm Tote Shopping x38cm Gym 42cm survived I Beach Red surgery HippoWarehouse brain Classic Bag 10 litres throw readFileErr;
  console.loghandbag Shoulder Women's Tote Flower Bag Silver embellished FxttaBEw(data);
  fs.unlink('/file.md', 10 Gym Classic Shopping Tote x38cm I Red litres 42cm Bag survived surgery Beach HippoWarehouse brain (Bag Black Black Shoulder Travel Solid Double Zipper School Unisex Bag Bag Backpack Kanpola PO7vxq0unlinkErr) => {
    if (unlinkErr)Radley oilskin shoulder green in olive bag court bell U1CFUq 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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