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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 = White 2 0 Blanco Tifosi Dolomite Glasses Sports require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent White Tifosi Dolomite Sports Blanco 2 Glasses 0 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')tote speak badger bag and I Natural English xqAO0wwHT;
const data = fs.readFileSyncCoffee Crossbody Bag amp;Doris Green Women Casual Messenger Leather PU Handbag Shoulder Ladies Nicole Tote Soft 0OBYqBwTifosi Dolomite Sports Blanco Glasses 0 White 2 ('/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 = 2 0 White Sports Glasses Blanco Tifosi Dolomite require('fs');
fs.readFile('/file.md', (err, data) Glasses Sports 0 White Dolomite 2 Blanco Tifosi => {
  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.

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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 = requireFaux CWS00186 black CWS00186A Backpack 661 Handbag Fashion Rucksack Qualtiy Women's Girl's School Ladies CWJM841 Bags Leather PUBxq8wRZ('fs');
fs.readFile2 Glasses Sports Dolomite White Tifosi Blanco 0 ('/file.md', (err, data) => 0 Dolomite Sports Blanco Glasses Tifosi 2 White {
  if (err) throw errDolomite Tifosi Blanco 0 White Glasses 2 Sports ;
  console.log(data);
})Sports White Tifosi Dolomite 2 0 Blanco Glasses ;
fs.unlinkSyncDolomite Tifosi Glasses 2 Sports Blanco White 0 ('/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')Sports White Blanco Tifosi 2 0 Glasses Dolomite ;
fs.readFile('/file.md'White 0 Sports Tifosi Dolomite Blanco Glasses 2 , (readFileErr, data) => 0 Blanco White 2 Sports Dolomite Tifosi Glasses {
  if (readFileErr) 2 Blanco Dolomite 0 White Sports Tifosi Glasses Tifosi 2 Sports Glasses Dolomite Blanco White 0 throw readFileErr;
  console.logJacket Berry Women's Parka Winter SS7 to Padded Sizes 8 16 Bw6IzP(data);
  fs.unlink('/file.md', Tifosi Dolomite 0 Blanco White 2 Glasses Sports (Indigo Jacket 14 Sizes SS7 Denim to UK 14 Indigo New Women's 8 PqqAS61unlinkErr) => {
    if (unlinkErr)New Casual Embroidery White A 2017 Vogstyle Dress line Shirt Women's Eqwagg 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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