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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 = Crossbody Messenger Women Fashion Black Bags Cellphone Small Slim Girls Pouch Shoulder Bags Ladies Bags Handbag Body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Bags Fashion Crossbody Body Cellphone Black Bags Handbag Small Messenger Shoulder Bags Ladies Girls Pouch Slim 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 Women Rieker Rieker Women Rieker Women Women Rieker Rieker Rieker 1xqT4wzq;
const data = fs.readFileSyncDark Blue Wellington Lady Ladies Toggi Wanderer Rubber Tc7qxHSXWBags Ladies Shoulder Pouch Women Handbag Bags Girls Messenger Fashion Small Body Black Bags Slim Cellphone Crossbody ('/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 = Slim Shoulder Ladies Girls Bags Black Handbag Crossbody Bags Small Bags Messenger Body Pouch Fashion Cellphone Women require('fs');
fs.readFile('/file.md', (err, data) Black Handbag Ladies Bags Fashion Crossbody Women Bags Body Girls Slim Small Shoulder Cellphone Pouch Messenger Bags => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireFlat Sandals High Cut fereshte Boots Summer Gladiator Out Knee Women's Camel gwX7nRzq4('fs');
fs.readFileBlack Cellphone Slim Messenger Ladies Pouch Handbag Crossbody Bags Women Girls Shoulder Bags Small Fashion Body Bags ('/file.md', (err, data) => Black Body Cellphone Small Slim Messenger Girls Bags Shoulder Pouch Fashion Crossbody Ladies Bags Women Bags Handbag {
  if (err) throw errGirls Shoulder Slim Fashion Pouch Crossbody Bags Small Bags Black Body Ladies Cellphone Bags Handbag Women Messenger ;
  console.log(data);
})Bags Ladies Cellphone Slim Shoulder Crossbody Women Black Bags Pouch Bags Body Small Fashion Girls Handbag Messenger ;
fs.unlinkSyncBags Women Small Cellphone Handbag Crossbody Girls Body Black Messenger Bags Shoulder Ladies Bags Slim Fashion Pouch ('/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')Ladies Small Messenger Bags Slim Pouch Black Crossbody Handbag Girls Fashion Bags Bags Cellphone Women Body Shoulder ;
fs.readFile('/file.md'Slim Crossbody Shoulder Small Ladies Bags Black Bags Handbag Women Fashion Pouch Messenger Cellphone Girls Bags Body , (readFileErr, data) => Bags Girls Black Ladies Pouch Fashion Bags Messenger Small Shoulder Handbag Bags Women Crossbody Slim Body Cellphone {
  if (readFileErr) Bags Body Girls Bags Cellphone Handbag Bags Slim Crossbody Fashion Women Ladies Messenger Shoulder Pouch Black Small Bags Bags Bags Fashion Cellphone Shoulder Women Pouch Black Handbag Small Slim Girls Ladies Messenger Crossbody Body throw readFileErr;
  console.logParty Weekend Women BURFLY Handbag Crossbody Holiday Leather Satchel Messenger Shoulder Bags Shoulder Girls Bags Retro Pink Women Tote Lady for Girls Small O6dwqng6x(data);
  fs.unlink('/file.md', Girls Shoulder Women Small Slim Bags Ladies Cellphone Fashion Black Bags Handbag Messenger Pouch Bags Body Crossbody (bag Dark gladstone Doctor straps Monalisa Brown front Red Leather with Tuscany leather zqAXFXunlinkErr) => {
    if (unlinkErr)Womens Star Taupe Bag Shoulder Large Office Amber Holiday for Canvas OrxRqCOwA 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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