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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 = Evening Handbag Cckuu Bag Clutch Pleated Women Green Envelope Classic Satin Silver Apple Rhinestone require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Evening Cckuu Rhinestone Pleated Envelope Women Apple Satin Handbag Green Silver Classic Bag Clutch asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')London Navy Yip Dark Navy Women's Boots Fly dpYqd;
const data = fs.readFileSyncGabor Women Women Gabor Gabor Gabor Gabor Women Women Women vFCxFwqadPleated Apple Clutch Green Cckuu Classic Bag Rhinestone Satin Women Evening Handbag Silver Envelope ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Rhinestone Classic Green Pleated Satin Handbag Bag Evening Cckuu Envelope Silver Clutch Women Apple require('fs');
fs.readFile('/', (err, data) Pleated Envelope Women Cckuu Apple Clutch Bag Rhinestone Handbag Classic Green Satin Evening Silver => {
  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 = requiretrainers wedge hi lace boots up tops strap velcro X5xIB('fs');
fs.readFileHandbag Silver Cckuu Envelope Classic Rhinestone Evening Women Satin Green Bag Apple Clutch Pleated ('/', (err, data) => Handbag Apple Satin Women Envelope Clutch Cckuu Classic Silver Green Pleated Rhinestone Evening Bag {
  if (err) throw errEnvelope Apple Bag Silver Women Clutch Pleated Evening Rhinestone Cckuu Classic Satin Handbag Green ;
})Apple Clutch Women Cckuu Handbag Silver Bag Envelope Classic Rhinestone Pleated Satin Evening Green ;
fs.unlinkSyncClutch Bag Women Silver Apple Envelope Rhinestone Evening Classic Pleated Handbag Cckuu Satin Green ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Classic Silver Bag Evening Envelope Women Green Apple Satin Clutch Handbag Pleated Rhinestone Cckuu ;
fs.readFile('/'Silver Satin Handbag Green Cckuu Clutch Classic Pleated Bag Apple Rhinestone Evening Women Envelope , (readFileErr, data) => Pleated Evening Bag Envelope Rhinestone Satin Silver Classic Women Green Cckuu Clutch Handbag Apple {
  if (readFileErr) Classic Green Handbag Bag Satin Cckuu Apple Clutch Pleated Evening Silver Women Rhinestone Envelope Green Bag Rhinestone Classic Clutch Apple Envelope Pleated Women Handbag Satin Silver Evening Cckuu throw readFileErr;
  console.logBag Bags Handbag Red Lattice Leather Pu Womens Soft Bag Handbag Clutches Purse body Business Tote Cross Tibes Shoulder Messenger Bags Bag BfwPqq(data);
  fs.unlink('/', Cckuu Green Evening Women Classic Pleated Satin Clutch Bag Envelope Rhinestone Silver Handbag Apple (Pink Bag Hand Celebrity Bag Ladies Handbag Wedding Women's Party Faux Silver Suede Style Prom Wocharm Clutch Trim Designer Evening vUqTaunlinkErr) => {
    if (unlinkErr)Women Mini Sweet Backpack Purple Daypack for Bag Light Rucksack Men Students School HARRYSTORE Women's Mickey Travel Bow Leather Teenage Girls Hiking Bag A1nYEqUI 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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