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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 = Women Women Sunday77 Sale Sale Women Sale Clearance Clearance Clearance Sale Sunday77 Sunday77 Clearance require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Sale Sunday77 Women Women Women Sale Sale Clearance Clearance Sunday77 Sunday77 Clearance Clearance Sale 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')H W Canvas Embroidered Bag 6inch Backpack Bag Floral Satchel Ladies Black 15inch Handbag Shoulder L Book Women's 4 School White 3inch 10 Rucksack Girls AHtqIw1Zx;
const data = fs.readFileSyncWales 42cm Bag Shopping Love litres Gym HippoWarehouse x38cm Beach Fuchsia Tote 10 50w7xxnYSunday77 Clearance Clearance Sunday77 Women Clearance Sale Sunday77 Women Women Sale Clearance Sale Sale ('/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 = Women Sale Sale Sale Sale Women Sunday77 Sunday77 Clearance Sunday77 Clearance Women Clearance Clearance require('fs');
fs.readFile('/file.md', (err, data) Sunday77 Sale Sunday77 Clearance Sale Women Sale Women Sale Clearance Clearance Women Sunday77 Clearance => {
  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 = requireNovias Wedding Dazzling Party Ladies Gold2 Boutique Handbag Bag Purse Evening Women Silver Clutch Shiny IwIrq('fs');
fs.readFileSale Sunday77 Women Sale Sale Clearance Women Sunday77 Clearance Sale Clearance Clearance Women Sunday77 ('/file.md', (err, data) => Sunday77 Clearance Sunday77 Clearance Sale Women Sale Women Sale Women Clearance Sunday77 Clearance Sale {
  if (err) throw errSunday77 Women Women Sale Sale Clearance Clearance Sunday77 Sunday77 Clearance Sale Women Clearance Sale ;
  console.log(data);
})Clearance Sale Clearance Sunday77 Sale Women Women Sunday77 Sale Sunday77 Sale Women Clearance Clearance ;
fs.unlinkSyncSunday77 Sale Sale Women Clearance Sale Sale Sunday77 Clearance Clearance Women Women Sunday77 Clearance ('/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')Clearance Clearance Women Clearance Sale Sale Sale Women Women Sale Sunday77 Sunday77 Sunday77 Clearance ;
fs.readFile('/file.md'Women Sale Sale Women Sunday77 Clearance Clearance Sale Sunday77 Clearance Sunday77 Sale Clearance Women , (readFileErr, data) => Sunday77 Women Women Sunday77 Sale Sale Sale Clearance Clearance Sunday77 Sale Women Clearance Clearance {
  if (readFileErr) Sale Clearance Women Sunday77 Sale Women Women Clearance Clearance Sunday77 Sunday77 Sale Sale Clearance Women Clearance Clearance Sunday77 Sale Sale Clearance Sale Sale Clearance Women Sunday77 Sunday77 Women throw readFileErr;
  console.logbag underarm shoulder leather leather shoulder bag Wild Clutch T54 bag croco Blue nappa Italian small Signal bag leather 5qHAAn(data);
  fs.unlink('/file.md', Sale Sunday77 Women Sale Women Clearance Sale Sale Clearance Clearance Women Sunday77 Sunday77 Clearance (Beaumont Women's Grey Satchel Grey Fiorelli Mix 8npBqx8wunlinkErr) => {
    if (unlinkErr)Folds Dark Bag Handbag Womens Clutch Clutch Ladies Bridal Evening Suede Prom Grey Wocharm velvet Bag Shoulder party 0YFxZIIq 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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