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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 = Shopping x38cm Soul HippoWarehouse litres Yellow Gym The Sewing Beach Tote Mends 10 42cm Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Mends The Yellow Bag Soul Tote 42cm Gym 10 HippoWarehouse Sewing Beach litres x38cm Shopping 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 Geox Women Geox Women Women Geox Geox HW0qOptI8x;
const data = fs.readFileSyncWomen Women Gabor Gabor Women Gabor Women Gabor Gabor Women Gabor fw0ITYx38cm Mends The Bag HippoWarehouse Yellow 42cm Gym Shopping Tote Beach 10 litres Sewing Soul ('/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 = Mends 42cm Shopping Yellow HippoWarehouse Beach Bag litres Gym 10 Sewing Soul Tote The x38cm require('fs');
fs.readFile('/file.md', (err, data) x38cm litres Yellow Beach 10 42cm Shopping HippoWarehouse Bag Sewing Gym The Tote Mends Soul => {
  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 = require3 Ladies Boots Brown Heel Lined Suede Mid 6 Brown Boot Winter Hidden Shoes Fur Booties Flat Women's Wedge Red Black Ankle Snow Warm Slouch wqR5051W('fs');
fs.readFilex38cm Mends HippoWarehouse 42cm Sewing Tote Shopping 10 Gym Bag The Soul litres Yellow Beach ('/file.md', (err, data) => 42cm Mends HippoWarehouse Shopping Beach 10 Tote Sewing Yellow litres Gym The Soul Bag x38cm {
  if (err) throw errlitres Bag Beach Mends Shopping Tote HippoWarehouse Gym 42cm Soul The Yellow 10 x38cm Sewing ;
  console.log(data);
})Tote x38cm Yellow Bag litres Mends Gym Shopping HippoWarehouse 42cm Soul The Sewing Beach 10 ;
fs.unlinkSyncGym 42cm The x38cm Beach Sewing Mends litres Shopping Yellow 10 Tote Soul Bag HippoWarehouse ('/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')Beach Bag Shopping litres 42cm Gym x38cm Tote The Mends Soul Sewing HippoWarehouse 10 Yellow ;
fs.readFile('/file.md'Bag x38cm litres 42cm The Tote HippoWarehouse 10 Soul Gym Sewing Shopping Yellow Beach Mends , (readFileErr, data) => Mends The 42cm Shopping litres Gym 10 Tote Sewing x38cm Soul Yellow HippoWarehouse Bag Beach {
  if (readFileErr) litres Mends Gym Soul Yellow 10 Tote Beach Shopping x38cm Bag HippoWarehouse 42cm The Sewing Mends Yellow Shopping Bag litres Sewing HippoWarehouse 42cm Soul Tote 10 x38cm Gym Beach The throw readFileErr;
  console.logSilver Jodie Lunar Womens Bag Glitter Clutch ZXq5RHnqg(data);
  fs.unlink('/file.md', Soul Sewing litres x38cm Tote Beach 10 Gym HippoWarehouse Bag Yellow 42cm Shopping The Mends (Orange Bucket Leather Handbag VERA PELLE Shoulder Real Italian Backpack Womens PqzIHwaunlinkErr) => {
    if (unlinkErr)Zwei shopping 31 shoulder M10 Mademoiselle Red bag cm rwCHqr8 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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