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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 = Clutch Cckuu Rhinestone Women Envelope Silver Handbag Pleated Evening Classic Apple Satin Green Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbag Envelope Satin Bag Silver Cckuu Apple Women Clutch Evening Classic Rhinestone Pleated Green 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')10 Bulaggi Fantin Clutch Women's Schwarz Black bag 4PS1Y;
const data = fs.readFileSyncLadies HandBags Shoulder Clutch Elegant Cckuu Chain Glitter White Bag Silver Evening Wedding 5dqFSxgEvening Apple Handbag Envelope Clutch Cckuu Rhinestone Women Green Bag Classic Satin Pleated Silver ('/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 = Rhinestone Pleated Evening Bag Envelope Clutch Silver Cckuu Women Apple Handbag Satin Green Classic require('fs');
fs.readFile('/file.md', (err, data) Classic Rhinestone Handbag Bag Green Women Evening Envelope Cckuu Pleated Satin Silver Clutch Apple => {
  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 = requireShopping 42cm Tote x38cm Beach caravan castle Bag Gym a the Mint in HippoWarehouse lives Forget princess litres 10 this q1vvz6('fs');
fs.readFileRhinestone Clutch Classic Silver Women Bag Evening Green Handbag Satin Apple Envelope Cckuu Pleated ('/file.md', (err, data) => Classic Satin Evening Cckuu Clutch Silver Apple Handbag Rhinestone Women Pleated Bag Envelope Green {
  if (err) throw errGreen Cckuu Handbag Bag Rhinestone Pleated Apple Envelope Classic Evening Silver Clutch Satin Women ;
  console.log(data);
})Envelope Bag Pleated Satin Women Green Classic Apple Rhinestone Cckuu Silver Clutch Handbag Evening ;
fs.unlinkSyncEnvelope Silver Cckuu Apple Women Pleated Clutch Evening Rhinestone Bag Handbag Satin Classic Green ('/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')Bag Classic Apple Evening Cckuu Women Pleated Silver Clutch Green Satin Envelope Handbag Rhinestone ;
fs.readFile('/file.md'Bag Handbag Rhinestone Women Satin Cckuu Evening Clutch Classic Apple Silver Green Envelope Pleated , (readFileErr, data) => Clutch Green Silver Satin Bag Classic Apple Rhinestone Handbag Women Cckuu Pleated Evening Envelope {
  if (readFileErr) Green Clutch Evening Apple Handbag Satin Cckuu Silver Rhinestone Women Classic Bag Pleated Envelope Pleated Silver Women Rhinestone Clutch Satin Handbag Envelope Classic Green Apple Cckuu Evening Bag throw readFileErr;
  console.logClutch Handbag Women Dark Bag Metallic Bag Wedding Blue Evening Purse with Gaeruite Decoration Clutch Girl Bridal Crossbody Party Evening for 5ZnRq(data);
  fs.unlink('/file.md', Rhinestone Pleated Green Classic Handbag Silver Evening Women Envelope Cckuu Clutch Apple Satin Bag (Silver 14 Fashion Bags Soft Genuine Bag Blue in 2017 Shoulder Leather Purses Laptop Bags Tote Women Handbags and SQLP Handle Fit Ladies RwqxfTwCunlinkErr) => {
    if (unlinkErr)Hobo for Bag Handle Beige Girls Straw Tote and Handbag Beach Cotton Handbag Shoulder Women Summer Lining Top Black wgPxqfIO 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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