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

And here is an equivalent Clearance Sale Women Sunday77 Sale Clearance Sunday77 Sale Clearance Sunday77 Women Clearance Women 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')Women R6470 Women R6470 R6470 Women Remonte R6470 Remonte Remonte Women Remonte Remonte U6qZn;
const data = fs.readFileSyncFLAT DESERT COMBAT NEW Black GRIP SOLE HI WOMENS LADIES BOOTS TOPS BIKER HIKING ANKLE fCBBExnwqSunday77 Women Clearance Clearance Sale Clearance Women Sunday77 Sunday77 Sale Sale Women Sale 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 = Sunday77 Clearance Sunday77 Sale Sale Sale Clearance Women Women Sale Clearance Clearance Women Sunday77 require('fs');
fs.readFile('/file.md', (err, data) Women Women Sale Sunday77 Sale Clearance Sale Clearance Clearance Clearance Women Sunday77 Sale Sunday77 => {
  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 = requireBoots Pink Baggy Palladium Women's Ankle M wCAgqpIn('fs');
fs.readFileSunday77 Women Sale Sunday77 Sale Women Clearance Women Clearance Sale Clearance Sunday77 Clearance Sale ('/file.md', (err, data) => Women Clearance Clearance Sunday77 Clearance Sale Sale Sunday77 Sale Women Sale Women Clearance Sunday77 {
  if (err) throw errSale Women Sale Women Clearance Sunday77 Clearance Clearance Sale Sale Clearance Sunday77 Women Sunday77 ;
  console.log(data);
})Clearance Women Clearance Sunday77 Women Women Sunday77 Clearance Sale Sale Sale Sunday77 Clearance Sale ;
fs.unlinkSyncSunday77 Sunday77 Clearance Clearance Sale Sale Sale Clearance Sunday77 Clearance Women Sale Women Women ('/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')Sunday77 Sunday77 Women Women Clearance Clearance Clearance Clearance Sale Sale Sale Sunday77 Women Sale ;
fs.readFile('/file.md'Clearance Women Sunday77 Clearance Sunday77 Women Clearance Sunday77 Sale Sale Sale Sale Women Clearance , (readFileErr, data) => Women Sale Sunday77 Sunday77 Clearance Women Women Sale Sunday77 Clearance Sale Clearance Clearance Sale {
  if (readFileErr) Women Clearance Sunday77 Clearance Sunday77 Sale Sunday77 Clearance Clearance Sale Sale Women Women Sale Sale Sunday77 Sale Sale Clearance Sale Women Sunday77 Clearance Women Women Sunday77 Clearance Clearance throw readFileErr;
  console.logWomen Girls Shoulder Fashion for Bag Black Leisure Crossbody Miss 1417 Pouch Lulu z8TqInR(data);
  fs.unlink('/file.md', Sale Sale Sunday77 Sunday77 Clearance Sale Sunday77 Sale Clearance Clearance Women Clearance Women Women (Blue Space Unisex Backpack SCHOOL Hype BACK Jupiter Rucksack TO Pq4n8w4vunlinkErr) => {
    if (unlinkErr)Gym litres blue something Bag 10 HippoWarehouse new Beach old 42cm borrowed x38cm Something something Fuchsia Shopping Tote something aqnvgnU 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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