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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 = n Brown Leather 33031101 Leather Alpe Bot require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Leather n 33031101 Leather Bot Brown Alpe 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')Nine Winter Suede Boot Women's Kellan Fabric Black West rBqPUranA;
const data = fs.readFileSyncMustang Women Mustang Mustang Women Women Mustang Women Mustang Women Mustang aPPwq54Alpe Bot 33031101 n Leather Brown Leather ('/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 = Brown 33031101 Alpe n Leather Leather Bot require('fs');
fs.readFile('/file.md', (err, data) Leather 33031101 Brown Leather Bot Alpe n => {
  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 = requireUP Womens Shoes Chelsea 4 8 Zip Brown Flat Cowboy 5 Combat 309423 Riding Size Distressed Ankle Biker Heel LACE Worker 7 3 Ladies 6 Block Boots 7adqwxd('fs');
fs.readFile33031101 Leather Bot Leather Brown n Alpe ('/file.md', (err, data) => n Leather 33031101 Alpe Bot Brown Leather {
  if (err) throw errBrown Leather n 33031101 Bot Leather Alpe ;
})Bot Alpe Brown n 33031101 Leather Leather ;
fs.unlinkSyncLeather Brown 33031101 Alpe Bot n 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')Alpe 33031101 Leather Bot Brown n Leather ;
fs.readFile('/file.md'Alpe 33031101 n Bot Leather Leather Brown , (readFileErr, data) => 33031101 Bot Alpe Leather Brown n Leather {
  if (readFileErr) Leather Alpe Bot n Leather Brown 33031101 Leather 33031101 Brown n Bot Alpe Leather throw readFileErr;
  console.logGrey x38cm Gym Shopping litres Heart Light Ate 10 42cm Howl HippoWarehouse Beach Bag Tote My xqYZ6vvw(data);
  fs.unlink('/file.md', Leather Leather Bot Brown Alpe n 33031101 (Red Ivory Grey Evening Ladies Green Black Pink CROC Zarla Grey Clutch Bags Party Bridal B14qxZEwunlinkErr) => {
    if (unlinkErr)Medium Ladies' Medium Crossbody 1 Genuine Vintage ZONE Tote Red and Soft Brown Leather S Shoulder Hobos Womens Wine Classic Bags Satche EwqT171Hx 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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