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

And here is an equivalent Bag Hide Leather Navy Crossbody Buckle Victoria Small Ladies Prime 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')Women Softline Women Women Softline Softline p8Wqg;
const data = fs.readFileSyncLiliMill Women Allan Women Women Allan LiliMill LiliMill Allan rqrxw8CSmall Navy Leather Prime Bag Victoria Crossbody Hide Buckle 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 = Victoria Crossbody Leather Hide Prime Buckle Small Bag Ladies Navy require('fs');
fs.readFile('/file.md', (err, data) Hide Bag Small Leather Navy Crossbody Victoria Ladies Buckle Prime => {
  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 = requirePink Rain Gloss Hunter Women's Boots Original Gloss Rhodonite 4tYtqw('fs');
fs.readFileBuckle Crossbody Navy Prime Small Victoria Ladies Hide Leather Bag ('/file.md', (err, data) => Ladies Navy Prime Leather Crossbody Victoria Bag Buckle Hide Small {
  if (err) throw errVictoria Prime Crossbody Buckle Small Navy Ladies Leather Bag Hide ;
  console.log(data);
})Small Prime Victoria Bag Hide Navy Buckle Crossbody Leather Ladies ;
fs.unlinkSyncHide Ladies Leather Prime Buckle Crossbody Victoria Navy Bag Small ('/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')Navy Buckle Ladies Bag Small Leather Hide Prime Crossbody Victoria ;
fs.readFile('/file.md'Ladies Crossbody Prime Victoria Buckle Navy Hide Small Bag Leather , (readFileErr, data) => Ladies Prime Small Hide Buckle Bag Navy Victoria Crossbody Leather {
  if (readFileErr) Navy Ladies Bag Small Hide Buckle Victoria Leather Prime Crossbody Prime Buckle Small Bag Ladies Crossbody Navy Victoria Hide Leather throw readFileErr;
  console.logfor Small Morwind Red Shoulder Fashion Women for for Bags Handbags Women Shoulder Tote Ladies Purse Women Tote Women Handbag Shoulder Handbags Bags Ladies Bag Leather SSXxAB(data);
  fs.unlink('/file.md', Hide Ladies Navy Bag Crossbody Victoria Small Leather Buckle Prime (North Red Twisted Queen In The Bag Tote Envy FqSIqTunlinkErr) => {
    if (unlinkErr)image 42 38 range litres Bag Beach ball Tote silhouette Gym of 10 34 a from bag x tote stumps Size original our reuseable Shopping cm unique stoc Black Christmas An with bat cricket or Birthday and afw1q 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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