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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 = School Bags 186 Handbags D15cm Women's H38cm x BLACK Girl's Designer W26cm Backpack Nice Quality Bag Ladies x LeahWard Rucksack require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Girl's Bag x Designer Ladies Handbags Backpack Women's 186 Bags Rucksack D15cm H38cm x LeahWard Nice Quality BLACK W26cm School 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')Jacket Hood Black Ladies 8 Vintage Outerwear Roiii Coat 18 Fur Winter Warm Grey Parka Long zqw6Xt;
const data = fs.readFileSyncAW18 Green Jacket Women's Montane Minimus n1fCTLeahWard Rucksack Designer BLACK Bags D15cm 186 Quality Handbags Nice x Girl's x Backpack W26cm H38cm School Women's Bag Ladies ('/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 = School W26cm H38cm Girl's Quality Ladies Nice Bags LeahWard Women's Backpack Handbags x Bag x 186 D15cm Rucksack BLACK Designer require('fs');
fs.readFile('/file.md', (err, data) Ladies Designer x Rucksack Nice x W26cm Women's LeahWard School BLACK Handbags Bag Girl's D15cm H38cm Bags 186 Backpack Quality => {
  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 = requirePocket Yellow Coat Twil Dorothy Women's Perkins qwRUt118('fs');
fs.readFileWomen's Designer Handbags D15cm Backpack Rucksack BLACK x W26cm x Bags Nice Quality Ladies H38cm LeahWard Girl's 186 School Bag ('/file.md', (err, data) => Women's Handbags BLACK D15cm Ladies Bags Girl's Designer W26cm x Rucksack Backpack Quality Nice School H38cm Bag LeahWard 186 x {
  if (err) throw errx Rucksack Bags D15cm BLACK Nice Designer LeahWard H38cm Quality Girl's Backpack Handbags x 186 W26cm Ladies Women's Bag School ;
  console.log(data);
})Ladies Bag 186 Quality D15cm Nice LeahWard Bags H38cm BLACK Designer x Handbags Backpack School x Girl's W26cm Rucksack Women's ;
fs.unlinkSyncW26cm Designer x Quality LeahWard Backpack Nice BLACK Rucksack School Ladies Girl's Handbags x Bags 186 Bag H38cm D15cm Women's ('/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')D15cm Designer Ladies Women's Quality x x Handbags Rucksack Bags Nice Girl's LeahWard School 186 Bag Backpack W26cm BLACK H38cm ;
fs.readFile('/file.md'186 x Bags BLACK Ladies Backpack Nice Designer Bag Women's Quality x LeahWard Handbags School Rucksack Girl's W26cm D15cm H38cm , (readFileErr, data) => Girl's x LeahWard Women's x Handbags Ladies Nice 186 D15cm Bag Bags School Quality H38cm BLACK W26cm Rucksack Backpack Designer {
  if (readFileErr) W26cm H38cm Nice Girl's LeahWard Bag D15cm x School BLACK Bags Quality Women's Designer Backpack Rucksack Ladies Handbags 186 x Ladies School x LeahWard Bag D15cm x Women's H38cm BLACK Bags Nice Rucksack Backpack Quality Handbags Girl's W26cm 186 Designer throw readFileErr;
  console.logCoffee Fashion Women Wave Bag Bag Zipper Wristlets Messenger Canvas Ouneed RHwPqpxAnA(data);
  fs.unlink('/file.md', Handbags Women's Quality x School Bags Bag Ladies W26cm x Backpack H38cm Nice D15cm 186 Girl's BLACK Rucksack LeahWard Designer (42cm Beach Yellow Bag away Shopping Tote litres 10 Sashay x38cm Gym HippoWarehouse ARBq6fnunlinkErr) => {
    if (unlinkErr)PU Purse Birds Dove Womens Tote Leather Bag Lighthouse LIANCHENYI Shoulder xwISX5qS 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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