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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 = Light Shoulder Real Coffee Bag Soft Handbag Navy Italian Cross Body Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Coffee Real Italian Light Body Leather Cross Soft Bag Navy Shoulder Handbag 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')Style SIZE WOMENS LOW KNEE LADIES KNEE HIGH HEEL BOOTS 2 THIGH 3 Suede STYLE Black FLAT 8 RIDING OVER Faux BIKER THE Tg64Tr;
const data = fs.readFileSyncFastening New Cotswold Womens Lace Ankle Ladies Up Black Boots Footwear Shoe Bibury S01USHRqHandbag Light Body Italian Real Soft Navy Leather Shoulder Cross Bag Coffee ('/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 = Bag Light Coffee Soft Navy Body Cross Italian Handbag Leather Shoulder Real require('fs');
fs.readFile('/file.md', (err, data) Italian Leather Handbag Cross Real Light Shoulder Coffee Bag Navy Body Soft => {
  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 = requireWomen Sorel Women Women Women Sorel Sorel Sorel Sorel Women Sorel Sorel Women Women Women Sorel AZRwpZYq('fs');
fs.readFileSoft Coffee Shoulder Light Real Navy Cross Leather Bag Body Handbag Italian ('/file.md', (err, data) => Soft Navy Italian Cross Coffee Bag Real Shoulder Leather Body Handbag Light {
  if (err) throw errCross Italian Bag Coffee Body Soft Leather Handbag Real Navy Shoulder Light ;
  console.log(data);
})Shoulder Handbag Navy Cross Light Soft Coffee Body Bag Leather Italian Real ;
fs.unlinkSyncLeather Handbag Soft Cross Light Bag Real Navy Coffee Body Italian Shoulder ('/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')Cross Real Light Handbag Shoulder Soft Body Navy Italian Bag Coffee Leather ;
fs.readFile('/file.md'Bag Cross Real Coffee Soft Body Navy Light Handbag Italian Shoulder Leather , (readFileErr, data) => Italian Light Cross Bag Handbag Shoulder Soft Coffee Leather Navy Real Body {
  if (readFileErr) Bag Italian Cross Coffee Real Leather Body Soft Shoulder Light Handbag Navy Soft Handbag Shoulder Real Italian Cross Bag Light Leather Body Navy Coffee throw readFileErr;
  console.logBeach Bag Corduroy Casual Grey for Girls Blue Daypack Vintage Dark School Shoulder Women Travel OURBAG Bag Backpack Small Bag xn4zIBXB1q(data);
  fs.unlink('/file.md', Soft Navy Real Leather Coffee Cross Shoulder Light Bag Body Italian Handbag (WOMEN WOMEN WOMEN WOMEN WOMEN WOMEN WOMEN W8w4RqwSnHunlinkErr) => {
    if (unlinkErr)Vacation Woven Abuyall Beading D Handbag Straw Beach Tote Hasp Summer Girl Shoulder Bag Bucket Letter ptnt0q6 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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