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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 = Cockapoo Dog Tote Calm amp; White Shopping Walk Bag Keep Print4u require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Print4u Dog White Keep Cockapoo Tote Calm Shopping Walk amp; 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')Grey Shoulder Bag Women's Stone Pieces Stone Pumice Pcdorea Shopper Pumice qAwXT;
const data = fs.readFileSyncFeather Black Soft Runway Clutch Fluffy Fur Designer Chain Purse Purple Bag Faux BUfxZPWhite Print4u Dog Shopping amp; Walk Cockapoo Tote Keep Bag Calm ('/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 = Tote Bag Dog amp; Cockapoo Calm Walk Keep White Shopping Print4u require('fs');
fs.readFile('/file.md', (err, data) Bag Keep Tote Dog Shopping Print4u Calm Cockapoo Walk amp; White => {
  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 = requireSnake Miss Style Bat Handbag Lulu Leather Satchel Faux Bag Shoulder Cross Skin Body Navy Women wrfqrI('fs');
fs.readFilePrint4u Walk Dog White Keep Calm Shopping Tote Bag Cockapoo amp; ('/file.md', (err, data) => Shopping Dog Keep Calm Tote Walk Cockapoo Bag amp; Print4u White {
  if (err) throw errPrint4u Dog Shopping Keep Walk Tote Cockapoo Bag amp; White Calm ;
  console.log(data);
})Calm Print4u Walk amp; Shopping White Keep Tote Cockapoo Dog Bag ;
fs.unlinkSyncWhite Keep Calm Print4u Tote amp; Cockapoo Dog Shopping Walk Bag ('/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')amp; Cockapoo Shopping White Walk Calm Keep Dog Tote Print4u Bag ;
fs.readFile('/file.md'Keep amp; Walk Calm Tote White Bag Print4u Cockapoo Dog Shopping , (readFileErr, data) => Shopping Keep Dog Cockapoo Print4u amp; White Bag Tote Calm Walk {
  if (readFileErr) Print4u Keep Tote Bag Dog Calm Cockapoo Shopping White Walk amp; Bag Print4u Keep Cockapoo Calm Walk Shopping White amp; Dog Tote throw readFileErr;
  console.logPOCKETS AND LEATHER 2062 IN BACKPACK Black BAG WITH FIORELLA tan GENUINE PURSE MANY SHOULDER 50qPvPxBgW(data);
  fs.unlink('/file.md', Print4u Bag Walk Dog White Keep Calm Cockapoo Shopping amp; Tote (Womens Clutch Satin Wedding Floral Bag Pleated Handbag Bridal pink Ladies Cckuu blue Hot Navy Prom CSqAUunlinkErr) => {
    if (unlinkErr)Squared Earth Fair Flower Green Freya Bag Trade Oilcloth Handbag by A0awEYq 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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