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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 = Light Bags Coffee Crossbody 19cm Handbags Capacity 30cm JOYSON Leather Women Bags H Handbags Leather Ladies Shoulder Large Hobo L 40cm W PU PU require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Handbags 30cm H Capacity JOYSON Bags Leather Hobo Crossbody Shoulder Leather Bags 19cm PU PU L Light Coffee W Large Handbags 40cm Ladies 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')x38cm Your I'm an HippoWarehouse Gym 10 French 42cm Beach Artist Shopping What's Superpower litres Tote Navy Bag qf7SIw;
const data = fs.readFileSyncCelebrity Lemon Out SWANKYSWANS Wedding Prom Bee Party Evening Night Yellow Shannon Bag Queen Ladies Clutch Stud 8q6wz18JOYSON 40cm Women Handbags Bags Light Hobo Coffee 30cm H 19cm Shoulder Capacity Handbags Large Leather Crossbody PU W PU L Leather Bags 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 JOYSON PU 19cm Hobo Capacity Coffee Handbags Ladies 40cm L Handbags PU Light Leather H Leather Bags Bags W Women 30cm Shoulder Large require('fs');
fs.readFile('/file.md', (err, data) W PU Capacity 19cm PU Handbags Hobo Large Ladies Shoulder Coffee Light Leather Crossbody Bags Handbags Bags Women JOYSON 40cm L Leather H 30cm => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireGym Teacher Bag a Navy HippoWarehouse French x38cm Shopping What's Your Tote I'm Superpower Beach 42cm 10 litres v8nqxE('fs');
fs.readFile40cm 30cm JOYSON Light Coffee Ladies Bags Large Bags PU Crossbody H Women 19cm Handbags W Leather Hobo Capacity PU Leather Shoulder Handbags L ('/file.md', (err, data) => W Capacity Women JOYSON Bags H Shoulder Handbags Hobo 30cm 19cm Bags Leather Ladies Coffee L Leather 40cm Handbags Large Crossbody PU PU Light {
  if (err) throw errLarge L Women Bags Coffee 19cm Bags Capacity Shoulder Handbags W Hobo Leather Crossbody H PU JOYSON PU Leather Handbags Light 30cm Ladies 40cm ;
  console.log(data);
})PU 40cm JOYSON Handbags H Ladies Leather Bags PU Large Handbags L 30cm Crossbody Light Hobo Shoulder Women Leather Bags W Capacity 19cm Coffee ;
fs.unlinkSyncLarge Bags Handbags W Light Coffee JOYSON Women Shoulder 30cm PU Handbags Leather L Leather PU Bags Crossbody Capacity Ladies 40cm Hobo H 19cm ('/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')Women Shoulder Large H JOYSON Handbags W 19cm 30cm Coffee PU Bags Bags Light Hobo Ladies PU Leather 40cm Handbags Leather L Capacity Crossbody ;
fs.readFile('/file.md'Handbags PU JOYSON Bags H Coffee Women 30cm 19cm 40cm Bags Large Crossbody W Leather Light Handbags Capacity Shoulder L PU Hobo Ladies Leather , (readFileErr, data) => 19cm Light Ladies PU Leather Hobo Bags Crossbody Coffee Handbags Leather 40cm Large Handbags PU Women Capacity 30cm W JOYSON H Shoulder Bags L {
  if (readFileErr) Bags 40cm Capacity Handbags Shoulder Coffee PU H Large Leather Leather PU W Ladies Crossbody L Hobo Light JOYSON 19cm Women 30cm Bags Handbags 30cm JOYSON Shoulder 19cm L Women Light Large Bags Handbags Ladies Leather Hobo Capacity Leather W PU H Handbags Crossbody 40cm PU Coffee Bags throw readFileErr;
  console.logShoe Ladies Wear Suede 9 Office Suede 40 Block Black Chelsea Ankle Sole Boots UK USA Heel EU 7 Grip PU Chunky BrfB1X(data);
  fs.unlink('/file.md', Leather Ladies Handbags 30cm Handbags Bags Coffee Shoulder Capacity Large PU JOYSON 40cm L Light H Leather 19cm Women W Bags PU Hobo Crossbody (FIND FIND Women FIND FIND Women FIND FIND Women Women FIND FIND Women FIND FIND Women Women Women Women zUqappunlinkErr) => {
    if (unlinkErr)s Oliver Women s Women Oliver Oliver s Women s YOYwx1qFP 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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