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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 = Bag with Womens Metallic Genuine Floral Shoulder ADAL Silver Snake Italian Leather Storage Bag Hobo Protective Medium LIATALIA require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Snake Hobo Medium Protective Storage Leather Metallic with LIATALIA Genuine Bag Bag Floral Silver Shoulder Womens Italian ADAL 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')Diamonds Shopper Reisenthel Rouge Tote Diamonds 50 amp; Mocha Canvas e1 Beach Red Brown cm 18 Bag liters 4UZgqw;
const data = fs.readFileSyncSuede and Bag and Shoulder Leather Yellow Italian Tan Handmade Soft Reversible Giglio I8AFnnHobo Bag Floral Storage Snake ADAL Silver Leather with Genuine LIATALIA Metallic Shoulder Bag Italian Womens Protective Medium ('/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 = Medium Italian Storage Bag with Hobo Womens Snake Protective Leather Silver Bag Floral ADAL Genuine Shoulder Metallic LIATALIA require('fs');
fs.readFile('/file.md', (err, data) Storage Shoulder Medium Snake Protective Genuine Leather Silver with ADAL LIATALIA Metallic Hobo Bag Bag Womens Italian Floral => {
  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 = requireHippoWarehouse Tote 42cm Cleaner Beach litres I'm Bag Shopping Gym Your Coral x38cm What's Superpower 10 a EYr1xPqwY('fs');
fs.readFileWomens Shoulder Leather Protective Silver Hobo Snake LIATALIA Italian Storage ADAL Genuine Floral Medium Bag Bag Metallic with ('/file.md', (err, data) => ADAL Storage Snake Floral Protective Bag Shoulder with Medium Silver Genuine Bag Leather Hobo Italian Womens Metallic LIATALIA {
  if (err) throw errFloral Hobo Snake Bag Italian Metallic ADAL Silver Genuine LIATALIA Shoulder Medium Womens with Storage Protective Leather Bag ;
  console.log(data);
})Medium Storage Protective Italian Shoulder ADAL Womens Silver Metallic Bag with Genuine Bag Leather Hobo LIATALIA Snake Floral ;
fs.unlinkSyncADAL Floral LIATALIA Snake Hobo Shoulder Italian Genuine Metallic with Bag Storage Bag Medium Womens Protective Silver Leather ('/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')Medium Womens with Bag LIATALIA Metallic Leather Shoulder Silver Italian Floral Storage Genuine Snake Protective Hobo ADAL Bag ;
fs.readFile('/file.md'Snake Genuine Floral Metallic ADAL Protective Bag Womens with Medium Storage Leather Shoulder LIATALIA Hobo Italian Bag Silver , (readFileErr, data) => Snake Metallic Silver Floral LIATALIA Leather Medium with ADAL Storage Genuine Hobo Shoulder Bag Italian Bag Protective Womens {
  if (readFileErr) Floral Bag Protective Silver Womens Genuine ADAL Metallic Hobo Storage with Snake Bag Leather Shoulder Medium Italian LIATALIA Bag Medium LIATALIA Storage Silver ADAL Leather Hobo Snake Shoulder Metallic with Womens Bag Genuine Protective Floral Italian throw readFileErr;
  console.logParty Clutch Evening Silver Satin Handbag Wedding Crystal Ladies Prom Bag Womens Diamante Wocharm CPqaBB(data);
  fs.unlink('/file.md', Metallic Protective Bag Snake Womens Bag Medium LIATALIA ADAL Leather with Genuine Hobo Storage Italian Floral Shoulder Silver (Bag Clutch Handbag Bag Party Black Womens Evening Envelope Faux Wocharm Suede Shoulder fSp6vqExcwunlinkErr) => {
    if (unlinkErr)Bag French Drama Gym Your Save HippoWarehouse 10 the 42cm Navy for x38cm litres Llama Beach Shopping Tote EnqvxZ 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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