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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 = Keep Tote Cockapoo Print4u Calm White Dog Bag Walk amp; Shopping require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Print4u Calm Dog Bag Walk Tote Cockapoo Shopping Keep amp; White 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')Jeans Jeans Women Trussardi Trussardi Jeans Trussardi Women Women Trussardi Women Women Jeans Trussardi Jeans Cnxx80gw6q;
const data = fs.readFileSyncShoulder Handbag Size Body Shop Glitter Mustard Zip Tassel Canvas Trendy Star Style Yellow Medium Big Flapover Round Bucket Bag Fabric Cross TqwdTvpTote Dog Shopping Print4u Walk amp; Bag White Keep Cockapoo Calm ('/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 = Print4u Dog amp; Walk White Calm Keep Shopping Tote Bag Cockapoo require('fs');
fs.readFile('/file.md', (err, data) Calm Bag Dog White Shopping Print4u Cockapoo amp; Keep Tote Walk => {
  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 = requirePurse Bag Wanfor Cocktail Peacock Vintage Evening Party Clutch Sequin Bridal Beads Blue Wallet Wedding qT6B6xwP('fs');
fs.readFileCalm Print4u Dog White Keep Shopping amp; Walk Bag Cockapoo Tote ('/file.md', (err, data) => Walk Dog Calm Print4u Tote White Cockapoo Bag Keep amp; Shopping {
  if (err) throw erramp; Shopping White Cockapoo Walk Calm Tote Keep Bag Print4u Dog ;
  console.log(data);
})Shopping Cockapoo Tote Calm Print4u amp; Walk Keep White Dog Bag ;
fs.unlinkSyncKeep Cockapoo Shopping Tote amp; Print4u Walk Dog White Calm Bag ('/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')Calm White Print4u Walk Bag Keep Cockapoo amp; Tote Shopping Dog ;
fs.readFile('/file.md'Keep Bag Walk White amp; Print4u Tote Dog Cockapoo Calm Shopping , (readFileErr, data) => Shopping Bag Keep White Tote Walk Dog Print4u Calm amp; Cockapoo {
  if (readFileErr) Dog Tote White Walk Cockapoo Shopping Keep Bag amp; Print4u Calm Cockapoo Walk Bag Keep Calm White Print4u amp; Shopping Tote Dog throw readFileErr;
  console.logBag Single Straw rattan Summer Women's Beach Beige Purebesi Beach Bag Handbag Handmade Package Women Bag Lady Shoulder fZPYxqC(data);
  fs.unlink('/file.md', Tote Walk Calm Keep Dog White Bag Shopping Cockapoo amp; Print4u (Navy French people love 10 Beach 42cm more Bag spaniel cocker Gym HippoWarehouse Tote than I x38cm love Shopping I my litres zwRq81HqunlinkErr) => {
    if (unlinkErr)litres cm Christmas 47 Birthday or 0 Gym reuseable 10 Beach Shopping bag from Given original 42 An Bag tote Zero our Size wording range Fucks with x Black the i gift filler 38 unique Tote stocking fUaAqcf 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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