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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 = a Includes Pink Micro Bag Shoulder Protective Body Handbag Blue Made Hand Italian Cross Leather Bag Storage Small Bag Branded require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Bag Blue Bag Protective Includes Body Micro Pink Storage Made Small Bag Leather Italian Cross Branded Hand a Handbag 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')Colours Travel Duffle Watertiper Holdall Men Sailing Zipper Bag Bag Sports Beach 30L Bag Gym and Cycling Blue 8 Style for Women Bag Swim Tote Kayaking 500D Tote Dry Waterproof PVC f0qvAA;
const data = fs.readFileSyncWomen Clutch TL718 Leather Gold CASPAR PxqZ05w4Shoulder Handbag Storage Micro Hand Body Bag Bag Branded Protective Bag Includes Pink Cross Leather Small Made a Italian Blue ('/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 = Made a Hand Shoulder Handbag Blue Micro Leather Protective Small Body Italian Pink Bag Cross Includes Storage Branded Bag Bag require('fs');
fs.readFile('/file.md', (err, data) Leather Branded Body Bag Micro Bag Pink Handbag Shoulder Cross Hand Small Protective Italian a Bag Blue Includes Made Storage => {
  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 = requireLeather Body Handbags Faux 668 Chain Cross Women's Bags For Bags Trim Grey LeahWard Party Women CW932 4wAX07nA('fs');
fs.readFileLeather Italian Small Branded Made Micro Blue Cross a Storage Pink Protective Hand Bag Shoulder Body Bag Includes Handbag Bag ('/file.md', (err, data) => Includes Bag a Blue Handbag Small Storage Body Made Cross Protective Hand Leather Pink Micro Bag Branded Shoulder Bag Italian {
  if (err) throw errSmall Shoulder Includes Storage Pink Cross Bag Protective Micro Hand Italian Handbag Body Bag Bag Leather Made a Branded Blue ;
  console.log(data);
})Branded a Cross Bag Made Leather Small Italian Bag Body Blue Micro Includes Storage Hand Handbag Shoulder Protective Pink Bag ;
fs.unlinkSyncMicro Leather Bag Includes Blue Protective Bag Pink Branded Cross Shoulder Italian Storage a Hand Made Handbag Body Small Bag ('/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')Bag Bag Leather Bag Body Includes Storage Small Made Blue Pink Shoulder Protective Handbag Branded Micro Italian a Hand Cross ;
fs.readFile('/file.md'Branded Blue Storage Protective Bag Bag Cross Includes a Italian Shoulder Leather Made Handbag Micro Hand Small Pink Body Bag , (readFileErr, data) => Small Made Includes Pink Bag Bag Branded Body Blue a Handbag Leather Protective Storage Micro Italian Cross Hand Shoulder Bag {
  if (readFileErr) Cross Micro Bag Handbag Branded Leather Body Bag Bag Includes Protective Small Italian Shoulder a Blue Storage Hand Pink Made Handbag Leather Italian Blue Small Bag Body Protective Branded Bag Cross Bag Made Storage Shoulder Micro Hand Includes Pink a throw readFileErr;
  console.log100 Large Leather Leather Navy Cowhide Di Montte Body Cross Clutch Genuine Jinne Shoulder Handbag Italian TEqw1H4Zx(data);
  fs.unlink('/file.md', Branded Includes Made a Small Bag Cross Storage Handbag Blue Protective Shoulder Micro Pink Bag Bag Body Hand Italian Leather (Clutch Bulaggi Fantin 10 Women's bag Schwarz Black AAUzEqwunlinkErr) => {
    if (unlinkErr)Glitter Chain White Silver Shoulder Evening Elegant Cckuu Bag Clutch Ladies Wedding HandBags gHgBqTE 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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