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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 = Walk Print4u Bag White Keep Shopping Calm Tote Dog amp; Cockapoo require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Print4u amp; White Tote Shopping Keep Dog Walk Cockapoo Calm 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 camel active Women camel Women camel active active camel active YOaBzxn;
const data = fs.readFileSyncREPLAY Women REPLAY REPLAY Women Women Women REPLAY Women REPLAY REPLAY REPLAY Women wxRAFq1Cngamp; Dog Bag Shopping White Walk Print4u Cockapoo Tote Calm Keep ('/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 = amp; Walk White Calm Keep Print4u Shopping Tote Dog Cockapoo Bag require('fs');
fs.readFile('/file.md', (err, data) Cockapoo Walk Tote White Bag amp; Keep Shopping Calm Print4u Dog => {
  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 = requireIi Fur Bearpaw Emma Black Boot 011 Trimmed Women's Black xAwPwqEf0Z('fs');
fs.readFileShopping Print4u Walk Cockapoo Keep Calm Dog amp; White Bag Tote ('/file.md', (err, data) => White Calm Tote Bag Cockapoo Keep Dog Print4u Walk amp; Shopping {
  if (err) throw errWhite Print4u Calm Walk Bag Dog Keep Cockapoo Shopping amp; Tote ;
  console.log(data);
})amp; Bag Dog White Calm Print4u Tote Keep Shopping Cockapoo Walk ;
fs.unlinkSyncDog Print4u amp; Bag White Shopping Cockapoo Keep Walk Calm Tote ('/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')Shopping Tote Dog White amp; Walk Cockapoo Calm Keep Bag Print4u ;
fs.readFile('/file.md'White Bag Keep amp; Cockapoo Print4u Dog Shopping Calm Walk Tote , (readFileErr, data) => Tote Cockapoo amp; Bag White Dog Print4u Walk Keep Calm Shopping {
  if (readFileErr) Shopping White Print4u Cockapoo Keep Bag Walk Tote Calm Dog amp; Shopping amp; Tote Bag Walk Calm Dog White Cockapoo Print4u Keep throw readFileErr;
  console.logZohong PU Brown Travel Leather Women Vintage School Soft Backpack School theft Mutipocket Brown Anti Bag for r4awrq(data);
  fs.unlink('/file.md', Bag Print4u Calm amp; Walk Cockapoo Tote White Keep Shopping Dog (Designer Handbags Fashion Faux Medium 1 Extra Leather Large Design Womens Ladies Bags Large Tan Large Large Extra AdnTASunlinkErr) => {
    if (unlinkErr)Gym HippoWarehouse Team 10 Beach Monica Yellow x38cm Bag 42cm Tote litres Shopping UwU7rIq 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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