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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 = HippoWarehouse 42cm Outta Natural Tote Rehearsal Straight Beach Shopping litres 10 Bag Gym x38cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag 10 Shopping Beach Tote litres x38cm Outta 42cm HippoWarehouse Straight Natural Gym Rehearsal 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')Flapover Glitter Tassel Zip Shoulder Cross Handbag Big Round Star Trendy Canvas Size Grey Bucket Medium Style Bag Shop Fabric Body xXF1CP;
const data = fs.readFileSyncWomen Handle JSTEL Shoulder Ladies L Wolf Love Large Bags Handbag Tote Patern Top 6xxdwaqgHx38cm Bag Gym HippoWarehouse Outta Tote 42cm Beach Shopping Rehearsal Straight 10 Natural litres ('/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 = Natural litres Shopping 10 Straight Rehearsal HippoWarehouse Outta Tote Bag 42cm Gym Beach x38cm require('fs');
fs.readFile('/file.md', (err, data) Tote litres Straight 42cm x38cm Shopping Rehearsal Outta 10 Beach Natural Gym HippoWarehouse 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 = requireBrown COW UNISEX BACKPACK LEATHER GENUINE 7028 Dark WITH Za0Aanx8('fs');
fs.readFilex38cm 10 Bag Straight Tote litres Outta Beach HippoWarehouse Gym 42cm Rehearsal Shopping Natural ('/file.md', (err, data) => 10 Natural Tote Beach Rehearsal x38cm Shopping Outta Gym HippoWarehouse Straight Bag litres 42cm {
  if (err) throw errStraight Bag 10 Gym 42cm Tote litres Shopping HippoWarehouse Natural Beach Rehearsal x38cm Outta ;
  console.log(data);
})Tote 10 Bag Outta Beach Rehearsal Straight litres x38cm HippoWarehouse Gym Natural 42cm Shopping ;
fs.unlinkSyncBeach Gym 42cm Straight x38cm litres Rehearsal Natural Bag Outta Shopping HippoWarehouse 10 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')Tote Straight Outta Bag x38cm 42cm Rehearsal 10 Gym Beach HippoWarehouse Natural Shopping litres ;
fs.readFile('/file.md'litres HippoWarehouse x38cm Outta Rehearsal Gym Straight 10 Tote 42cm Beach Natural Shopping Bag , (readFileErr, data) => HippoWarehouse Rehearsal Beach 10 42cm Natural Gym Tote Bag x38cm Straight litres Shopping Outta {
  if (readFileErr) 42cm Beach Outta 10 x38cm HippoWarehouse Tote Natural Rehearsal litres Bag Straight Shopping Gym Tote 10 Natural Gym Rehearsal litres x38cm Beach Bag Shopping Outta 42cm HippoWarehouse Straight throw readFileErr;
  console.logBlue Body Dot Women Bag Purse Handbag Dot Travel Small Chiffon Fashion for Bucket Drawstring Cross Polka Z6qng5pHz(data);
  fs.unlink('/file.md', litres Shopping Rehearsal Bag Tote Straight Beach 10 x38cm Outta HippoWarehouse Gym 42cm Natural (Bag Everpert Women Shoulder Butterfly Rattan Beach Round Straw Buckle Hand Woven Style 1 Bag UUw1TxFPunlinkErr) => {
    if (unlinkErr)Envelope Sequin Wiwsi Purse Bag Clutch Women Handbag Prom Hot Party Lady Evening rrdqZwI 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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