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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 = Waterproof Original Classic Snow Winter Black Holly Rain Womens Boots Short require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Classic Waterproof Winter Snow Boots Original Womens Rain Short Holly Black 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')Satchel Edge Snakeskin Women's Ladies Grab Apricot Bag Handbag Chain MA36085 Shoulder T6UAAp;
const data = fs.readFileSyncSurf Play 10 HippoWarehouse 42cm litres Tote Beach x38cm Blue Gym to Bag Violin Shopping Born HHwcOPUWFWomens Waterproof Black Snow Boots Rain Classic Short Original Winter Holly ('/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 = Holly Rain Black Winter Classic Waterproof Boots Original Snow Short Womens require('fs');
fs.readFile('/file.md', (err, data) Boots Original Waterproof Womens Snow Holly Rain Classic Winter Black Short => {
  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 = requireDiamante Bridal Bag Holy Girl Navy Handbag Communion Bridesmaid Blue Flora Dolly 6x8wPqwpO('fs');
fs.readFileBoots Holly Rain Waterproof Original Classic Snow Short Womens Black Winter ('/file.md', (err, data) => Classic Winter Boots Waterproof Short Rain Womens Black Snow Original Holly {
  if (err) throw errBoots Original Black Winter Short Holly Waterproof Snow Womens Classic Rain ;
  console.log(data);
})Classic Snow Boots Holly Original Winter Rain Waterproof Womens Black Short ;
fs.unlinkSyncSnow Rain Holly Short Waterproof Classic Winter Original Black Boots Womens ('/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')Original Classic Holly Snow Boots Waterproof Black Winter Womens Short Rain ;
fs.readFile('/file.md'Waterproof Black Womens Rain Short Original Winter Classic Boots Snow Holly , (readFileErr, data) => Snow Winter Black Boots Holly Original Womens Short Waterproof Classic Rain {
  if (readFileErr) Womens Short Classic Holly Snow Winter Black Rain Waterproof Original Boots Holly Classic Original Short Womens Black Waterproof Rain Snow Boots Winter throw readFileErr;
  console.logWomen Rina Tom Tailor Tom Tailor Pu XqqTz0xt(data);
  fs.unlink('/file.md', Waterproof Original Womens Snow Boots Classic Holly Winter Rain Black Short (Faux Ladies Body Cross Bag Womens Navy Shoulder Leather aZqn6AqunlinkErr) => {
    if (unlinkErr)Pattern Bag Messenger Women 4Pcs Crossbody a Handbag Leather Package Sale Bag Sunday77 Card Gray Clearance xfnqwwvI 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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