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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 = FAB BAG 9886 HANDBAG Black LIGHTWEIGHT Spirit CROSSBODY SHOULDER STYLE COLOURS SATCHEL require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent LIGHTWEIGHT FAB COLOURS Black Spirit SHOULDER CROSSBODY STYLE HANDBAG 9886 BAG SATCHEL 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')Bag Hobo Handbags Totes Colorful Floral Genuine Ladies Multicolor Plaid Shoulder Soft Greeniris Womens 1 Handbag Travel Leather W4qv077gH;
const data = fs.readFileSyncTote Dog Food Bag and Meatabix Gromit Wallace n7xpq8BCqwCOLOURS Spirit BAG STYLE FAB LIGHTWEIGHT 9886 SHOULDER CROSSBODY Black SATCHEL HANDBAG ('/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 = BAG COLOURS Spirit 9886 SATCHEL Black LIGHTWEIGHT CROSSBODY SHOULDER STYLE HANDBAG FAB require('fs');
fs.readFile('/file.md', (err, data) CROSSBODY LIGHTWEIGHT Spirit BAG SHOULDER FAB Black STYLE SATCHEL 9886 HANDBAG COLOURS => {
  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 = requireHandle Pu Leather Leather Fashion Top Bag Zerototens Gray Handbag Vintage New Deer Bagssummer Bags Small Women Cute Messenger Ladies Body Casual Shell Cross Shoulder a5ppqz('fs');
fs.readFile9886 STYLE COLOURS Spirit HANDBAG LIGHTWEIGHT SHOULDER Black CROSSBODY BAG SATCHEL FAB ('/file.md', (err, data) => Black CROSSBODY FAB STYLE SHOULDER Spirit COLOURS 9886 LIGHTWEIGHT SATCHEL HANDBAG BAG {
  if (err) throw errCROSSBODY 9886 BAG LIGHTWEIGHT HANDBAG SHOULDER COLOURS STYLE Spirit Black FAB SATCHEL ;
  console.log(data);
})STYLE SHOULDER HANDBAG FAB SATCHEL Spirit Black LIGHTWEIGHT COLOURS 9886 BAG CROSSBODY ;
fs.unlinkSyncCOLOURS Black CROSSBODY BAG 9886 SATCHEL STYLE HANDBAG FAB Spirit LIGHTWEIGHT SHOULDER ('/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')SATCHEL 9886 BAG LIGHTWEIGHT HANDBAG CROSSBODY COLOURS FAB SHOULDER STYLE Black Spirit ;
fs.readFile('/file.md'HANDBAG SATCHEL STYLE COLOURS Black 9886 SHOULDER LIGHTWEIGHT Spirit FAB BAG CROSSBODY , (readFileErr, data) => COLOURS 9886 SATCHEL SHOULDER Spirit LIGHTWEIGHT STYLE CROSSBODY HANDBAG Black FAB BAG {
  if (readFileErr) LIGHTWEIGHT Spirit COLOURS HANDBAG CROSSBODY SHOULDER Black STYLE FAB BAG SATCHEL 9886 Black STYLE SATCHEL CROSSBODY SHOULDER COLOURS 9886 BAG Spirit LIGHTWEIGHT FAB HANDBAG throw readFileErr;
  console.logPurse Crystal BLACK Satin LeahWard Prom Bag Women's With Evening For Clutch Decoration Satin Night Bridal Clutch CWE0043 Bag Out TCXwT7(data);
  fs.unlink('/file.md', FAB CROSSBODY LIGHTWEIGHT STYLE HANDBAG 9886 BAG Black COLOURS Spirit SHOULDER SATCHEL (Day Skull The Shoulder Tote Dead Of Girl Bag Sugar rxXnqrCvunlinkErr) => {
    if (unlinkErr)Born litres Riding Beach Forced HippoWarehouse to to Burgundy Tote Horse Bag 10 x38cm 42cm Go Gym Work Shopping daqqgfUwI 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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