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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 = Women Clearance Clearance Clearance Sale Sunday77 Sale Sale Women Sunday77 Sale Women Sunday77 Clearance require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Sale Women Women Sale Sunday77 Sale Sunday77 Clearance Clearance Women Sale Sunday77 Clearance Clearance 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')Designer Mude Celebrity Ladies Bag Women's Leather Handbags Faux Nude Fashion Tote tTTqE1gzw;
const data = fs.readFileSyncDaypack Gymsack Training Fear MX Sports Storage Compact Drawstring Bag Urban No EfqYxwfRSunday77 Women Clearance Clearance Sale Women Sale Sale Clearance Women Sale Sunday77 Clearance Sunday77 ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Sale Sunday77 Sale Sale Sale Women Women Women Clearance Clearance Sunday77 Clearance Sunday77 Clearance require('fs');
fs.readFile('/', (err, data) Sale Sunday77 Clearance Sale Women Clearance Sale Women Sale Women Clearance Sunday77 Clearance Sunday77 => {
  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 = requireYou HippoWarehouse Make When Limoncello Shopping Tote Gives Bag x38cm 42cm 10 litres Beach Gym Life Coral Lemons FqxpWqtn('fs');
fs.readFileSale Women Clearance Clearance Sunday77 Sale Women Women Sunday77 Sale Sale Clearance Sunday77 Clearance ('/', (err, data) => Clearance Women Sale Sale Women Clearance Sunday77 Women Sunday77 Clearance Clearance Sunday77 Sale Sale {
  if (err) throw errSale Women Sale Women Clearance Sunday77 Sale Clearance Clearance Sunday77 Sunday77 Women Sale Clearance ;
})Women Sunday77 Sunday77 Clearance Sale Women Sale Sale Clearance Clearance Sale Clearance Sunday77 Women ;
fs.unlinkSyncSale Sunday77 Sale Women Sale Clearance Women Clearance Sunday77 Clearance Sunday77 Sale Clearance Women ('/');
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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')Clearance Women Sale Sale Clearance Sale Sunday77 Sale Clearance Sunday77 Women Women Clearance Sunday77 ;
fs.readFile('/'Sale Sale Sale Sunday77 Clearance Sunday77 Women Clearance Sale Sunday77 Women Clearance Women Clearance , (readFileErr, data) => Sunday77 Women Sale Sunday77 Clearance Sale Clearance Sunday77 Women Clearance Sale Sale Clearance Women {
  if (readFileErr) Clearance Clearance Sunday77 Sunday77 Clearance Women Sunday77 Women Sale Women Clearance Sale Sale Sale Clearance Clearance Women Sunday77 Sale Sale Sunday77 Sale Women Sunday77 Clearance Clearance Women Sale throw readFileErr;
  console.logSuspect Nargles Bag 42cm litres Gym I Are x38cm Quote 10 It Mint HippoWarehouse Shopping Tote The Behind Beach w4Uxqx75(data);
  fs.unlink('/', Sale Sale Clearance Sunday77 Women Sunday77 Clearance Sale Sunday77 Clearance Women Women Sale Clearance (Detail Top Fashion J48 Handle Leather Ladies Faux Womens Tote Navy Shoulder Messenger Bow Bag xqaIXT8unlinkErr) => {
    if (unlinkErr)Bags Bags Green Hobos PU Women's Top Handle Handbags Shoulder Leather BzYxwEqAw 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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