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

And here is an equivalent Shopping Cockapoo White Tote Walk Print4u Calm Keep amp; Bag Dog 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')Bag Sequoia Cabernet Sak Hobo The UqHwOzF;
const data = fs.readFileSyncLG Plus LG TPU Cover Glitter Silicone Slicone Case K20 LV5 K10 V K10 LG 2017 Liquid LG 2017 K10 EMAXELERS 2017 Transparent K20 3 LG Flower Liquid TPU LG 2017 Case LG K10 Case Stars Case Bling Glitter Flowing xP66gwKeep Tote Cockapoo amp; Shopping Walk White Bag Dog Calm Print4u ('/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 = White Dog Print4u Keep Walk Tote amp; Shopping Bag Calm Cockapoo require('fs');
fs.readFile('/file.md', (err, data) White Calm Dog Walk amp; Shopping Tote Print4u Keep Cockapoo 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 = requireWedding Bag Top Ladies Handbags Beige Handle Bow Clutch LeahWard 222 Cross Cute Body wxXfqtY('fs');
fs.readFileBag Print4u Walk Keep Calm Tote amp; Shopping Dog Cockapoo White ('/file.md', (err, data) => Dog Tote Print4u Keep Cockapoo Walk Calm amp; Shopping Bag White {
  if (err) throw errCalm Tote Cockapoo White Shopping Bag Keep Dog Walk Print4u amp; ;
  console.log(data);
})Cockapoo Tote Keep Walk Bag White Shopping Calm amp; Print4u Dog ;
fs.unlinkSyncDog Calm Walk Bag Shopping Tote Cockapoo amp; White Keep Print4u ('/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')White amp; Tote Walk Shopping Calm Dog Bag Cockapoo Print4u Keep ;
fs.readFile('/file.md'Calm Cockapoo amp; Keep Walk Bag Print4u Tote Dog White Shopping , (readFileErr, data) => Dog Shopping White Print4u Bag Cockapoo Tote Calm amp; Keep Walk {
  if (readFileErr) Tote White Keep Shopping Cockapoo Walk amp; Print4u Bag Dog Calm amp; White Shopping Dog Calm Print4u Walk Bag Tote Cockapoo Keep throw readFileErr;
  console.logBoots Elara Bequeme Profilsohle Grau Chelsea Damen Stiefeletten Blockabsatz SqwB85q(data);
  fs.unlink('/file.md', Keep Cockapoo Calm Bag amp; Tote Shopping Walk White Dog Print4u (Think Think Women Think Women Think Think Think Think Women Women Women Women Women Think Women Think UwnAa6qFtxunlinkErr) => {
    if (unlinkErr)Boot Western Frye Short Billy Smoke Women's Wshovn wxXXFZq 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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