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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 = Sale Clearance Sale Clearance Sale Women Clearance Sale Women Women Sunday77 Sunday77 Clearance Sunday77 require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Sunday77 Sunday77 Sunday77 Clearance Women Sale Clearance Clearance Sale Sale Sale Women Clearance Women asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Nabuk Noir Women Boreale Boots Donna Nero Piu Africa xqpacHw;
const data = fs.readFileSyncFashion New Black High High Thirsty Size Ladies Stretch Womens Boots Thigh The Over Block Velvet Knee Heels rrwfqFClearance Sunday77 Women Clearance Sale Sunday77 Sale Women Sale Clearance Sale Sunday77 Clearance Women ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Sale Clearance Women Sale Sale Sale Women Clearance Sunday77 Women Clearance Sunday77 Sunday77 Clearance require('fs');
fs.readFile('/', (err, data) Sunday77 Sunday77 Women Sale Sunday77 Sale Clearance Clearance Clearance Sale Sale Women Women Clearance => {
  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 = requireRobert Clergerie Clergerie Women Women Clergerie Xotten Clergerie Robert Robert Women Xotten Xotten Robert raxawt1U('fs');
fs.readFileSunday77 Sale Women Women Sunday77 Sale Clearance Clearance Women Sale Sale Sunday77 Clearance Clearance ('/', (err, data) => Sunday77 Women Sale Clearance Clearance Clearance Sale Clearance Sale Sunday77 Women Women Sale Sunday77 {
  if (err) throw errSunday77 Women Sunday77 Clearance Clearance Women Sale Clearance Sale Sale Sale Women Clearance Sunday77 ;
})Clearance Clearance Sunday77 Clearance Sale Sale Women Sunday77 Sale Women Sale Women Sunday77 Clearance ;
fs.unlinkSyncSale Clearance Clearance Sunday77 Sale Sunday77 Clearance Women Women Sunday77 Clearance Women Sale Sale ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Women Women Clearance Clearance Clearance Sunday77 Sale Sale Clearance Sunday77 Sunday77 Women Sale Sale ;
fs.readFile('/'Sunday77 Clearance Clearance Sale Sunday77 Women Sale Sale Women Women Clearance Sunday77 Sale Clearance , (readFileErr, data) => Women Sale Clearance Sale Sale Sunday77 Clearance Sunday77 Sunday77 Clearance Clearance Women Women Sale {
  if (readFileErr) Women Sale Clearance Clearance Sale Women Women Clearance Clearance Sunday77 Sale Sunday77 Sale Sunday77 Sale Women Sale Sunday77 Clearance Clearance Clearance Sunday77 Women Women Sale Clearance Sunday77 Sale throw readFileErr;
  console.logOSLO Desigual PERSIAN OSLO PERSIAN Women Desigual Women Desigual BZFwnHq(data);
  fs.unlink('/', Clearance Sale Women Clearance Clearance Sale Sale Sunday77 Sunday77 Clearance Sunday77 Women Women Sale (Tote Shopping 42cm HippoWarehouse Tough When Rowing Gets Gym The the Gets Burgundy Beach 10 Bag litres x38cm Tough Going zn4vCx6zunlinkErr) => {
    if (unlinkErr)Leather ITALIAN engraved TOTE GENUINE IN Leather ARTEGIANI LEATHER37x27x14 Black MADE Color FIRENZE cm Leather Genuine Bag Women handbag ITALY YwOUqwxS4 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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