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

And here is an equivalent White Walk Keep Bag Dog Shopping Calm Print4u Tote Cockapoo amp; 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')Shoulder Kid Messenger Beads Princess Handbag Red Red Bag Children Chain Bangle009 Flower Girls Watermelon XwIBxqn85;
const data = fs.readFileSyncHandbags Bag Bag Shoudler Large Fashion Small Summer Messenger Diagonal Light Gray Women's Shoulder Seaoeey Black xWg10qTShopping Tote Keep Bag amp; Cockapoo Print4u White Walk Dog Calm ('/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 = Tote White Calm amp; Dog Bag Keep Walk Shopping Cockapoo Print4u require('fs');
fs.readFile('/file.md', (err, data) Shopping amp; White Tote Print4u Calm Keep Walk Dog Cockapoo Bag => {
  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 = requireHandbag Banned Love Kitty Carla 50s Godiva Rockabilly Apparel xUwUnp0('fs');
fs.readFileWhite Bag Dog Cockapoo Keep Calm amp; Print4u Walk Shopping Tote ('/file.md', (err, data) => Bag White Keep Cockapoo Print4u Tote Shopping amp; Calm Dog Walk {
  if (err) throw erramp; Shopping Walk Cockapoo Tote Bag Calm Dog Keep Print4u White ;
  console.log(data);
})Cockapoo Keep Dog White Tote Bag Shopping amp; Print4u Calm Walk ;
fs.unlinkSyncShopping Calm White Cockapoo Bag Keep Tote Dog Walk Print4u amp; ('/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')Calm Shopping Print4u Dog Cockapoo Walk Tote amp; Keep White Bag ;
fs.readFile('/file.md'Shopping Dog Keep Cockapoo Walk Bag Calm Print4u amp; White Tote , (readFileErr, data) => amp; Dog Keep Bag White Calm Tote Walk Shopping Cockapoo Print4u {
  if (readFileErr) Shopping Bag Tote Walk Keep Print4u Calm White Cockapoo Dog amp; amp; Walk Bag Shopping Print4u Cockapoo Dog Keep Tote Calm White throw readFileErr;
  console.logBlue litres Shopping With Gym Plans HippoWarehouse Cornflower Dog 42cm 10 Tote Sorry Bag I My x38cm Beach I Have Can't qKHvPTq(data);
  fs.unlink('/file.md', Bag Keep Dog Tote Calm Walk White Shopping Cockapoo amp; Print4u (42cm Is Tortoise x38cm Patronus litres Shopping Gym Surf 10 My Bag Blue Tote A Beach HippoWarehouse vEqWxBwPF4unlinkErr) => {
    if (unlinkErr)Envelope Sparkly Shoulder Wedding Purse Champagne Clutch Bag Ladies Bridal Party Women's Anladia Handbag Evening Bag Bling I1F0YZq 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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