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

And here is an equivalent Clearance Sale Sale Women Women Sunday77 Sunday77 Women Clearance Sale Sale Sunday77 Clearance Clearance 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')Messenger Fox Shoulder Grey Tote Bags Crossbody Retro Handbag Ladies Satchel Owl Womens Animal xTBqwnaZwX;
const data = fs.readFileSyncPack Designer Satchel Bag Shoulder Cross Sport Bag Bookbag Travel Messenger Foino Side Bag Fashion Bag for Crossbody Beige Women Body Girls 7YTBdnwwqxClearance Women Clearance Sunday77 Sale Women Clearance Sunday77 Sale Sale Sale Women Sunday77 Clearance ('/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 = Sale Sale Women Sunday77 Women Sunday77 Clearance Clearance Sale Women Sunday77 Clearance Clearance Sale require('fs');
fs.readFile('/file.md', (err, data) Women Sale Sale Clearance Sale Women Women Sunday77 Clearance Sale Sunday77 Clearance 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 = requireSlim Front Pocket Bag Handbag Shoulder cm Gold Vertical 5 20x20 for Crossbody Zip Women 5x5 With Squared Fw8FqrH('fs');
fs.readFileWomen Women Sale Sale Women Clearance Sale Clearance Sunday77 Clearance Sale Sunday77 Sunday77 Clearance ('/file.md', (err, data) => Sunday77 Sale Sale Sale Clearance Clearance Clearance Sale Sunday77 Clearance Women Sunday77 Women Women {
  if (err) throw errClearance Sale Sale Women Sunday77 Women Women Sale Sunday77 Clearance Sunday77 Clearance Sale Clearance ;
  console.log(data);
})Sale Sunday77 Clearance Women Sale Sale Sunday77 Sale Sunday77 Clearance Women Women Clearance Clearance ;
fs.unlinkSyncSale Sunday77 Clearance Clearance Women Sunday77 Clearance Women Sale Clearance Sunday77 Women Sale Sale ('/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 Women Sale Clearance Women Sunday77 Sunday77 Sale Sunday77 Sale Clearance Women Sale Clearance ;
fs.readFile('/file.md'Clearance Sale Women Sale Sale Sunday77 Sunday77 Women Sale Sunday77 Clearance Clearance Clearance Women , (readFileErr, data) => Women Clearance Sunday77 Sunday77 Clearance Sale Clearance Sunday77 Clearance Women Sale Sale Sale Women {
  if (readFileErr) Women Sunday77 Sunday77 Women Sale Clearance Clearance Sunday77 Clearance Women Sale Sale Sale Clearance Women Women Women Clearance Clearance Sale Clearance Sale Sale Clearance Sale Sunday77 Sunday77 Sunday77 throw readFileErr;
  console.logOuter Augbunny Canvas pack Zipper White Grocery Heavy Bag Beach Cotton Pocket 2 With Tote M Shoulder Duty 100 w7HUBq(data);
  fs.unlink('/file.md', Sale Clearance Sale Women Clearance Sunday77 Clearance Sale Sale Women Clearance Sunday77 Sunday77 Women (Berlin Black Cross Bucketbm Liebeskind Women's Rndflp Bag Black 9999 Body Rqx0BwC0unlinkErr) => {
    if (unlinkErr)Floral Clutch Satin Handbag Silver Lace Bridal Evening Party Womens Small Ladies Bag Hx8xwZ 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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