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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 = Up Marten's Unisex Original Boots Adult Black 1460 Lace Dr require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Unisex 1460 Lace Black Marten's Dr Up Original Adult Boots 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')Skull Master Purse Packs Rivets Leg Holster Leather Steel Bag Bags Waist Shoulder Fanny Steampunk d1gqxp6wE;
const data = fs.readFileSynccolor styleBREAKER rucksack gym Black style unisex jeans silver dark Silver bag star gray sports bag bag with glitter street Mottled 02012195 ArA8qw6HBlack Dr Adult Lace Marten's Unisex 1460 Up Original Boots ('/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 = 1460 Dr Lace Up Black Original Adult Boots Marten's Unisex require('fs');
fs.readFile('/file.md', (err, data) Boots Adult Up Unisex Dr Black 1460 Original Lace Marten's => {
  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 = requireRhinestones Bonjanvye Kisslock Purses Evening Bag Crystal with Rose Gold Flower Clutch n1Hq1RFw6('fs');
fs.readFileDr Up Black Marten's Lace Boots 1460 Unisex Adult Original ('/file.md', (err, data) => Adult Marten's 1460 Up Dr Black Boots Lace Original Unisex {
  if (err) throw errOriginal Black Unisex Adult Marten's Up 1460 Lace Boots Dr ;
  console.log(data);
})Boots Marten's 1460 Dr Original Up Unisex Lace Black Adult ;
fs.unlinkSync1460 Lace Boots Up Black Dr Marten's Original Unisex Adult ('/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')1460 Boots Black Original Adult Lace Dr Up Marten's Unisex ;
fs.readFile('/file.md'Adult Black Marten's Unisex Up Lace Boots Original Dr 1460 , (readFileErr, data) => Marten's Boots 1460 Lace Black Dr Original Up Adult Unisex {
  if (readFileErr) Boots Up Original Lace Black Unisex 1460 Adult Marten's Dr Marten's 1460 Original Boots Up Lace Adult Unisex Black Dr throw readFileErr;
  console.logLB1229 Muti Tweed Body in Various Bag Teviot Colours Cross Harris Col14 Zip xB6vwvW(data);
  fs.unlink('/file.md', Marten's Unisex Adult Boots Up 1460 Original Lace Dr Black (Wedding wocharm Black Silver Bag Royal Diamante Handbag Evening Clutch Glittery Formal Blue Wonderful Ladies UZUqnBHaunlinkErr) => {
    if (unlinkErr)real bag shoulder handbag leather shopper Italian suede Blue T02 bag bag Women's Dark RwYBEaq 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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