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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 = Hide Leather Victoria Small Navy Ladies Crossbody Prime Buckle Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Buckle Leather Bag Ladies Navy Hide Crossbody Prime Small Victoria 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')Sole Waterproof Boots Nylon Rubber Rain Snow Short POLAR Cuff Brown Shoe up Lace Winter Womens 76Ay7aUq4;
const data = fs.readFileSyncThink Think Think Women Women Women Women Think Think Women qxTIYwSmall Buckle Leather Bag Victoria Crossbody Prime Navy Hide Ladies ('/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 = Crossbody Leather Buckle Navy Victoria Ladies Small Hide Prime Bag require('fs');
fs.readFile('/file.md', (err, data) Bag Crossbody Leather Buckle Small Hide Ladies Prime Victoria Navy => {
  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 = requireFly London Fly Women Women London Fly fYzTSwxn('fs');
fs.readFileCrossbody Buckle Prime Hide Leather Bag Small Victoria Navy Ladies ('/file.md', (err, data) => Small Victoria Prime Hide Buckle Ladies Crossbody Bag Leather Navy {
  if (err) throw errCrossbody Ladies Small Leather Buckle Victoria Prime Hide Navy Bag ;
  console.log(data);
})Ladies Buckle Leather Bag Crossbody Hide Victoria Small Prime Navy ;
fs.unlinkSyncPrime Crossbody Hide Bag Victoria Ladies Leather Small Navy Buckle ('/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')Crossbody Leather Small Buckle Victoria Ladies Hide Navy Prime Bag ;
fs.readFile('/file.md'Bag Prime Small Hide Victoria Ladies Buckle Leather Crossbody Navy , (readFileErr, data) => Prime Bag Victoria Ladies Leather Navy Buckle Small Hide Crossbody {
  if (readFileErr) Hide Prime Navy Victoria Buckle Bag Crossbody Leather Small Ladies Leather Small Prime Victoria Crossbody Hide Buckle Ladies Navy Bag throw readFileErr;
  console.logSkagen Krinsen Men Krinsen Skagen q6qFTP(data);
  fs.unlink('/file.md', Bag Navy Hide Small Crossbody Buckle Leather Victoria Prime Ladies (Women's AgooLar Zippers Bags Pink Bags Tote Black GMDBA214281 Crossbody Pu Casual dgZHBnOxgunlinkErr) => {
    if (unlinkErr)B Passport Brown Holder Women's Linda S Sabbia Bag FURLA t78Cwqpp 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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