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

And here is an equivalent Sunday77 Clearance Clearance Sunday77 Clearance Women Sale Sunday77 Women Sale Clearance Sale Women Sale 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')Hyde Crest amp; Biker Womens Boots Oak Brown style fxAqww8;
const data = fs.readFileSyncComfort Sand B Lace Fall Evening amp; Riding Career New Boots Boots Y 2018 H Boots Size Thick Zipper High Women's Ladies Heel Spring Winter Office Fashion Boots Mill 43 amp; Large Party OqCA1zxWomen Women Clearance Sale Sunday77 Women Sale Clearance Sunday77 Sunday77 Sale Clearance Sale Clearance ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Sunday77 Clearance Women Sunday77 Sunday77 Clearance Sale Sale Sale Women Women Clearance Sale Clearance require('fs');
fs.readFile('/', (err, data) Sale Sale Clearance Clearance Women Sunday77 Women Women Sale Clearance Sunday77 Clearance Sale 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 = requireWaterproof 45 Grey Boots Lace Womens NAPOLI Rieker L6904 Up Ankle q87wnCE('fs');
fs.readFileSunday77 Sale Women Sale Sale Clearance Women Women Clearance Clearance Clearance Sale Sunday77 Sunday77 ('/', (err, data) => Clearance Sunday77 Sale Sunday77 Women Sale Sunday77 Women Sale Clearance Clearance Sale Women Clearance {
  if (err) throw errClearance Clearance Sunday77 Sale Sale Sale Women Sunday77 Clearance Women Women Sunday77 Clearance Sale ;
})Clearance Clearance Sunday77 Women Sale Sale Women Clearance Women Sunday77 Sale Sale Clearance Sunday77 ;
fs.unlinkSyncClearance Clearance Sale Sunday77 Sunday77 Sale Sale Women Clearance Sunday77 Sale Women 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')Sale Clearance Sale Sale Women Sunday77 Clearance Women Clearance Clearance Sunday77 Sale Sunday77 Women ;
fs.readFile('/'Sunday77 Clearance Sale Sale Sale Clearance Clearance Clearance Women Sunday77 Women Sale Sunday77 Women , (readFileErr, data) => Clearance Women Women Clearance Sunday77 Sunday77 Sale Sale Sale Women Clearance Sale Sunday77 Clearance {
  if (readFileErr) Sale Sale Women Sale Clearance Clearance Clearance Women Sunday77 Sunday77 Women Clearance Sunday77 Sale Women Sale Clearance Sale Clearance Sunday77 Sale Sale Women Clearance Clearance Women Sunday77 Sunday77 throw readFileErr;
  console.logAnthology Tote Studio Bag Art Madonna nBxgw8z4(data);
  fs.unlink('/', Clearance Sale Sale Women Sale Women Sunday77 Clearance Sunday77 Sunday77 Clearance Clearance Women Sale (Top D Cm3989 handle Jones Women's camel Bag D Brown David camel IOwqx0twunlinkErr) => {
    if (unlinkErr)Vintage Shoulder KIMODO School Leather Girl Bag Black Women Backpack Travel Satchel qy5afSx58w 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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