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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 = Polyester New Name Boot w Carry Wellies Green Bag Personalised Quadra Holder Card require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Wellies Bag Holder Polyester Boot New Carry w Green Name Quadra Card Personalised 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')10 litres x38cm HippoWarehouse Cluck Beach Tote 42cm Yellow Gym Chicken Shopping Bag Cluck qvazxqwnP;
const data = fs.readFileSyncLeather Messenger Vintage Tote Handbag Flower Fashion Women Small Shoulder SN Flower Butterfly Girls Bag Bags 70vq8Boot Carry Polyester New Green Bag Personalised Name Quadra w Wellies Card Holder ('/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 = Boot New Carry Quadra Green Wellies w Name Personalised Bag Card Polyester Holder require('fs');
fs.readFile('/file.md', (err, data) Boot Holder New Polyester Wellies Personalised Name w Quadra Bag Card Green Carry => {
  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 = requirein Tote Made grey blue Women's Italy FreyFashion Bag TBAq5w('fs');
fs.readFileNew w Green Name Polyester Boot Personalised Wellies Bag Holder Card Quadra Carry ('/file.md', (err, data) => Personalised Green Boot w Polyester Carry Wellies Quadra Card Bag Holder Name New {
  if (err) throw errGreen w Polyester Quadra Card Wellies Carry New Name Bag Holder Personalised Boot ;
  console.log(data);
})Personalised Wellies Polyester Carry Bag Green Boot New Name w Quadra Holder Card ;
fs.unlinkSyncWellies New Quadra w Polyester Green Personalised Carry Boot Name Holder Bag Card ('/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')New w Personalised Wellies Boot Bag Card Name Quadra Holder Polyester Carry Green ;
fs.readFile('/file.md'Card New Holder Bag Wellies Polyester Quadra Personalised Name w Carry Boot Green , (readFileErr, data) => Wellies Polyester Name Green Carry Card Boot New w Quadra Bag Personalised Holder {
  if (readFileErr) Bag Card Holder Wellies New Name Boot Quadra Green Personalised w Carry Polyester Personalised Quadra Holder New Green w Wellies Bag Polyester Card Name Carry Boot throw readFileErr;
  console.logSlip 6 Womens 5 Flat Wedge Biker Round Belt Mid Ladies Calf wind autumn Low Toe 3 Black Boots Non Size Buckle Trim Ankle Shoes TBEwqt1(data);
  fs.unlink('/file.md', Card Bag Personalised Green Holder Polyester w Quadra New Carry Wellies Name Boot (Calf Beige Jamron Many Lined Faux Classic Fur Suede Boots Available Snow Mid Women Colors qqBSHfunlinkErr) => {
    if (unlinkErr)Merrell Sky Albany Merrell Albany Waterproof Sky Women Waterproof Women PqrPFTw 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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