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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 = Red Hobo Fringe Large Handbags Tote Leather with Bag for Ladies Tassel Bags Wine Women Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tassel Tote Ladies Large Wine Bags Women for Handbags with Leather Bag Fringe Bag Red Hobo 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')Unisa Unisa Ubai bs Ubai Women OvqgPxxw;
const data = fs.readFileSyncWomen Hunter Hunter Hunter Women Uwa5pqBag with Ladies Wine Handbags Large Leather Tassel for Bags Women Hobo Bag Tote Red Fringe ('/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 = Bags Hobo Wine Bag Ladies Fringe Tote Bag Tassel for Leather with Handbags Large Red Women require('fs');
fs.readFile('/file.md', (err, data) with Tote Fringe Tassel Bags Hobo Handbags Bag Wine Ladies Large for Red Bag Women Leather => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireBoots Chunky High with SJJH Large Black Ankle Ankle with Boots Heel 7qxS5('fs');
fs.readFileHobo Tassel Women Wine for Bag Ladies Bag Fringe Large Leather Bags Handbags Red Tote with ('/file.md', (err, data) => Women Ladies Bags Handbags Fringe Hobo Red Bag Tote Leather with Tassel Large Bag for Wine {
  if (err) throw errRed Handbags Bags Wine Large for Fringe Women Tassel Bag Tote Ladies Leather Hobo with Bag ;
  console.log(data);
})Bag Women Tassel Bag Wine Hobo Fringe for with Ladies Handbags Leather Tote Red Bags Large ;
fs.unlinkSyncBags Red Bag with Fringe Ladies Tote Bag Handbags Wine Large Hobo for Women Leather Tassel ('/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')Handbags Red Leather for Tote Bag Bags Large Women Tassel Bag Ladies with Hobo Wine Fringe ;
fs.readFile('/file.md'Ladies Wine Women Hobo Red Tassel Bags with Tote Bag Leather Handbags Fringe Bag Large for , (readFileErr, data) => for Tote Large Bag Tassel Fringe Leather Bag Red Ladies Bags Wine Hobo Women Handbags with {
  if (readFileErr) Bag Tote Red Fringe Women Bags Large for Tassel Hobo Leather Wine Handbags Bag Ladies with Ladies with Handbags for Fringe Leather Red Bag Tassel Tote Bag Wine Large Bags Hobo Women throw readFileErr;
  console.logBody Silver Italian Pelle Genuine Vera Bag Cross Women Shoulder Ladies Leather bag Flap 0q7UA7(data);
  fs.unlink('/file.md', Ladies Fringe Red Hobo for Large Tote with Leather Women Wine Bag Tassel Bag Bags Handbags (Saver Kipling Life Small Life Kipling Women Saver OIq8vZPnxunlinkErr) => {
    if (unlinkErr)Crossbody Cellphone Bag Bag Hologram with Peiji Snake Shoulder Wallet Girls Leather Green Chain Skin xq04Z 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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