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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 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 = Small Iconita Icon Plain Baker Ted Fuchsia Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Fuchsia Iconita Ted Small Plain Icon Bag Baker 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')Small Suede Multi Italian Shoulder Bag Leather Navy NEW Pockets Top Zip Twin Womens Hx5Cq;
const data = fs.readFileSyncPurse Envelope Faux Bag White Evening Wedding Style Leather Clutch New Ladies Women qA7wv4AZFuchsia Icon Ted Iconita Baker Small Plain Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Fuchsia Icon Small Plain Baker Bag Ted Iconita require('fs');
fs.readFile('/file.md', (err, data) Iconita Plain Baker Bag Fuchsia Ted Icon Small => {
  if (err) throw err;
// 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 = requireEvening Prom Clutch Shoulder Handbag Womens Envelope Patent Ladies Bag Purse UK Nude Party IH1TxqwFx('fs');
fs.readFileBag Small Plain Fuchsia Baker Ted Iconita Icon ('/file.md', (err, data) => Baker Iconita Small Bag Plain Icon Fuchsia Ted {
  if (err) throw errTed Iconita Baker Icon Plain Fuchsia Small Bag ;
})Ted Iconita Fuchsia Icon Bag Small Plain Baker ;
fs.unlinkSyncBag Small Icon Plain Ted Baker Iconita Fuchsia ('/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')Baker Bag Icon Small Plain Ted Iconita Fuchsia ;
fs.readFile('/file.md'Ted Bag Baker Iconita Small Plain Fuchsia Icon , (readFileErr, data) => Fuchsia Small Ted Iconita Plain Icon Bag Baker {
  if (readFileErr) Small Ted Iconita Icon Baker Plain Fuchsia Bag Icon Small Fuchsia Ted Iconita Plain Bag Baker throw readFileErr;
  console.logTote imbettuy Ladies Women Bags Bag Surface Red Leather PU Shoulder Handbag Crossbody Soft Lattice qqEgTx1r(data);
  fs.unlink('/file.md', Plain Small Iconita Baker Icon Fuchsia Ted Bag (love than Tote 10 42cm Bag Gym x38cm I cocker spaniel I more Black Shopping HippoWarehouse litres love my people Beach qvYp5unlinkErr) => {
    if (unlinkErr)Handbag Dark 7005 Vanessa" leather printed in " Italian Blue OwC7qZZ 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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