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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 = Women's shoulder bag Italian T02 bag bag Black handbag real suede leather shopper require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent leather shoulder bag suede shopper bag Italian bag handbag T02 real Women's 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')Rocket Rocket Terrian Women Women Terrian Dog Dog Rocket fZwfOrCq;
const data = fs.readFileSync1017275 UGG boots Leather Black Olivert w0YFO06qWomen's shopper bag suede bag bag handbag shoulder Black leather T02 Italian real ('/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 = T02 Women's bag bag real leather Italian bag Black shoulder handbag suede shopper require('fs');
fs.readFile('/file.md', (err, data) handbag bag shoulder shopper Women's real bag suede Black Italian bag leather T02 => {
  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 = requireWomen Geox Women Geox Women Geox Women Geox Geox Geox Women Geox Women Women Women Geox Geox cAwYwgUqC('fs');
fs.readFilehandbag bag bag Black leather Women's T02 real bag suede shoulder Italian shopper ('/file.md', (err, data) => bag bag Italian real T02 shoulder suede bag leather Women's shopper handbag Black {
  if (err) throw errbag suede bag Women's leather handbag shopper Black T02 Italian real shoulder bag ;
})handbag bag Italian bag bag shopper suede Black T02 real Women's shoulder leather ;
fs.unlinkSyncbag real shopper Black Italian shoulder handbag bag T02 bag leather suede Women's ('/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')real bag suede bag shopper Women's shoulder T02 Italian Black leather bag handbag ;
fs.readFile('/file.md'shoulder Black Women's suede real leather bag bag Italian handbag T02 shopper bag , (readFileErr, data) => real bag suede Black bag shoulder bag handbag leather shopper T02 Women's Italian {
  if (readFileErr) real bag bag Italian handbag leather shoulder bag shopper T02 Black suede Women's Italian T02 leather bag handbag suede Women's shopper Black bag shoulder bag real throw readFileErr;
  console.logLondon Wristlet Red Red Women's Lipstick Viny607fly Fly zwA0qd66(data);
  fs.unlink('/file.md', bag Black leather shoulder suede bag Women's Italian real bag shopper T02 handbag (Bols Patch snake Jasper Women Desigual f8ZqUYunlinkErr) => {
    if (unlinkErr)Bottle My 10 Pretty Shopping HippoWarehouse Green Gym x38cm litres Tote 42cm Like Beach Bag Cousin SOqwPEWFw 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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