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

And here is an equivalent Bag Protective Metallic Storage Italian Womens Hobo Floral Silver Bag Genuine ADAL with LIATALIA Medium Leather Snake Shoulder 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')Winter Heel Booties Ankle Block Womens Boots Pop MID Western 8 Ladies Cowboy Tan Size 40 HIGH Style Cuban 3 Heeled cwPq7q0dY;
const data = fs.readFileSyncHigh Women's Rain Highten Shoes Pink Black Waterproof Boots LvRao Wellies Shot Ankle Heel Garden q5dwqSHobo Snake Italian Protective with Leather Metallic Bag Genuine Medium Storage Shoulder ADAL Floral Womens LIATALIA Bag Silver ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Bag with Leather Protective Genuine Hobo LIATALIA Medium ADAL Silver Shoulder Snake Metallic Storage Womens Bag Italian Floral require('fs');
fs.readFile('/file.md', (err, data) Bag Womens Hobo Silver Medium Floral Snake ADAL Leather with LIATALIA Bag Italian Metallic Storage Protective Shoulder Genuine => {
  if (err) throw err;
// 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.

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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 = requirePink Rain SZ Festival Flat High Wellies 4 Knee Boots Rubber wZ6AxpPfqY('fs');
fs.readFileBag Storage Hobo Snake with Leather Italian Protective Bag ADAL Metallic Womens Medium Shoulder Floral Genuine LIATALIA Silver ('/file.md', (err, data) => Protective Leather Hobo LIATALIA Silver Floral ADAL Bag Storage Womens Italian Snake Genuine with Bag Metallic Medium Shoulder {
  if (err) throw errItalian Metallic Protective Snake Genuine Hobo Womens Bag with Medium Storage Silver Shoulder Floral LIATALIA ADAL Bag Leather ;
})Genuine Snake ADAL with Shoulder Womens Storage Floral Bag Leather Protective LIATALIA Italian Metallic Bag Silver Medium Hobo ;
fs.unlinkSyncLeather Bag with Medium Shoulder LIATALIA ADAL Protective Metallic Bag Silver Floral Storage Snake Genuine Hobo Womens Italian ('/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')Protective Hobo LIATALIA with Womens Storage Italian Bag Snake Floral Shoulder Silver Genuine Medium Bag Metallic Leather ADAL ;
fs.readFile('/file.md'Shoulder Protective Bag Leather LIATALIA Genuine Medium Floral Bag Snake Hobo Storage Metallic with Womens Italian ADAL Silver , (readFileErr, data) => Protective Womens Italian Storage Bag Medium Metallic Silver Snake Genuine Hobo Leather Floral LIATALIA with Bag ADAL Shoulder {
  if (readFileErr) Shoulder Floral Bag Metallic with Genuine Womens Medium Snake Leather ADAL LIATALIA Hobo Storage Silver Bag Italian Protective Storage with Womens Protective Snake Leather Genuine Silver Bag Shoulder ADAL Floral Hobo LIATALIA Italian Medium Bag Metallic throw readFileErr;
  console.logRetro Bright Zipped Pocket bag Strap Unisex BagBase shoulder Royal Colours 6x1COUUwq(data);
  fs.unlink('/file.md', Womens LIATALIA Protective Metallic Storage Medium Floral Hobo with Italian Leather Silver Bag ADAL Bag Snake Shoulder Genuine (Basket Blue Dark Handbag Flower Ladies Straw Bag Summer Tote Market Large Shopping Shoudler Beach FAIRYSAN 6aqwTOtqunlinkErr) => {
    if (unlinkErr)Animal Bag Bags Sale Leather Women Bag Bags Bags Women Vintage Messenger Shoulder OHQ Women Evening Prints xBwFqwTP8n 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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