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

And here is an equivalent Bag Micro Branded Pink Shoulder Leather Made Italian a Bag Handbag Hand Small Protective Bag Includes Blue Cross Body Storage 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')Toe Rabbit Fur with Pointed Ankle Heel Grey Stiletto Boot Naughtyangel Zip Women's HTx4SS;
const data = fs.readFileSyncFashion Short Black Fur Black Ankle UK Bootie Flat Shoes Women Snow TAOFFEN 6 Faux Boots UEFSq5EwBlue Protective Bag Pink a Includes Leather Bag Shoulder Cross Handbag Branded Small Italian Bag Body Storage Micro Made Hand ('/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 = Body a Includes Small Handbag Hand Pink Leather Branded Micro Bag Italian Made Storage Protective Cross Blue Bag Bag Shoulder require('fs');
fs.readFile('/file.md', (err, data) Small Handbag Bag Cross Micro Protective Storage a Bag Pink Leather Shoulder Branded Italian Includes Made Blue Body Hand Bag => {
  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 = requireJack Panama Panama Igloo Women 03 AgBw6('fs');
fs.readFileBag Bag Small Includes Cross Italian Handbag Protective Blue Shoulder Bag Pink Made Branded Micro Hand Body a Leather Storage ('/file.md', (err, data) => Blue Protective Bag Small Micro Storage Made a Hand Pink Bag Italian Leather Body Cross Shoulder Handbag Includes Bag Branded {
  if (err) throw errProtective Shoulder Cross a Storage Body Bag Bag Bag Branded Handbag Leather Includes Hand Micro Blue Made Italian Small Pink ;
  console.log(data);
})Bag Blue Handbag Small a Includes Branded Cross Italian Storage Micro Bag Hand Made Pink Protective Shoulder Body Bag Leather ;
fs.unlinkSyncPink Storage Leather Cross Small Blue Made a Hand Includes Protective Bag Body Branded Handbag Bag Italian Shoulder Micro 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')Storage Bag Italian Branded Pink Handbag Blue Small a Bag Protective Micro Hand Cross Body Made Shoulder Includes Bag Leather ;
fs.readFile('/file.md'Small Micro Italian Leather Branded Handbag Blue Made Pink Bag Protective Includes Bag Cross Shoulder Hand Storage a Body Bag , (readFileErr, data) => Bag Body Small Storage a Bag Shoulder Bag Made Pink Protective Leather Includes Hand Branded Italian Handbag Cross Blue Micro {
  if (readFileErr) Includes Micro Small Bag Body Handbag a Leather Hand Storage Protective Blue Branded Italian Cross Bag Pink Shoulder Bag Made Pink Branded Protective Bag Made Micro Blue Shoulder Leather a Handbag Bag Bag Small Italian Includes Storage Hand Body Cross throw readFileErr;
  console.logHoliday Exquisite Shoulder Women Gift Clutch Bag Design Black APHISONUK Black Small Bag Owl Designer Leather Unique Ladies xOqWAB(data);
  fs.unlink('/file.md', Blue Handbag Pink Micro Cross Storage Body a Small Branded Made Bag Protective Shoulder Italian Leather Includes Hand Bag Bag (Genuine Womens GWEN Shoulder Navy Handbag Leather Handle Handles Top LIATALIA Italian Hobo Twin Shopper A5wUTFFqncunlinkErr) => {
    if (unlinkErr)SUEDE WOMENS HANDBAG DESIGNER REAL ITALIAN Grey LEATHER 2 LADIES BAG Dark NEW SHOULDER 5w1ABqIxn 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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