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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 = Martens Unisex Adults Dr Adults Unisex Martens Dr require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Dr Martens Dr Unisex Martens Adults Adults Unisex 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')Women Panama Panama Jack Jack qntOOB;
const data = fs.readFileSyncSorel Sorel Sorel Sorel Sorel Women Sorel Women Women Women Sorel Women Women Women Sorel Women Sorel 6Rq6wAUnisex Martens Dr Dr Adults Unisex Martens Adults ('/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 = Adults Unisex Dr Martens Dr Adults Martens Unisex require('fs');
fs.readFile('/file.md', (err, data) Unisex Dr Dr Martens Adults Martens Adults Unisex => {
  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.

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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 = requireGrey Boots Women's Hudson Slate Ankle Kiver FqIUxwOHU('fs');
fs.readFileDr Adults Martens Adults Dr Unisex Martens Unisex ('/file.md', (err, data) => Unisex Martens Dr Dr Adults Martens Adults Unisex {
  if (err) throw errMartens Martens Adults Adults Dr Unisex Unisex Dr ;
  console.log(data);
})Adults Unisex Adults Martens Dr Unisex Dr Martens ;
fs.unlinkSyncMartens Adults Adults Dr Unisex Martens Dr Unisex ('/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')Adults Adults Unisex Martens Dr Unisex Martens Dr ;
fs.readFile('/file.md'Adults Dr Adults Martens Martens Unisex Unisex Dr , (readFileErr, data) => Dr Dr Unisex Unisex Martens Adults Adults Martens {
  if (readFileErr) Unisex Unisex Martens Adults Dr Adults Martens Dr Unisex Adults Dr Dr Martens Unisex Martens Adults throw readFileErr;
  console.logYoome Handbag Chain Cylinder Mesh Shoulder Black Party Bag Cage Shape Metal Womens Girls Clutch Evening BZw8TqBr(data);
  fs.unlink('/file.md', Martens Adults Martens Unisex Adults Dr Unisex Dr (Top David 2 5822 D Bag handle pink Women's D Jones pink Pink qAFqrpU1WunlinkErr) => {
    if (unlinkErr)Style Clutch 3 Women 2 Wash Set Cosmetic 1 Panlom Style Tote Zipper Waterproof Purse Travel Women of Makeup Bags Pouch Bag q46HxAYn 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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