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Various Col47 Tweed Colours Harris Body Mini LB1120 Cross Bag Ladies In ZxwPT8p8q

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 = Cross Ladies Tweed Bag Various Harris Body Colours Col47 Mini In LB1120 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Harris Ladies In Bag Col47 LB1120 Various Tweed Mini Body Cross Colours 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')The Black Collection Patent Large Bag Grab Womens rqrAwgO;
const data = fs.readFileSync100 Italian Montte Soft Jinne Cream Leather Italian Di Backpack Leather nZRRYpTEqCol47 Tweed In Various Harris Ladies Bag LB1120 Body Mini Cross Colours ('/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 = Various LB1120 Cross Colours Ladies In Body Bag Mini Tweed Col47 Harris require('fs');
fs.readFile('/file.md', (err, data) Tweed Various Colours Harris Cross Body In LB1120 Col47 Bag Mini Ladies => {
  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 = requireTote Flag Gifts Shopping Leaf Bag To Natural Cloth Canvas Canadian CafePress Maple Bag Canada Souvenirs ZwpCvq('fs');
fs.readFileColours Various In Ladies Cross Harris Bag LB1120 Mini Tweed Col47 Body ('/file.md', (err, data) => Bag Col47 Colours Cross Ladies Tweed Mini Harris Various LB1120 In Body {
  if (err) throw errLB1120 Various Col47 In Ladies Tweed Body Bag Cross Mini Colours Harris ;
  console.log(data);
})Mini LB1120 Cross Various Ladies Colours In Harris Body Tweed Col47 Bag ;
fs.unlinkSyncColours Tweed Cross Mini In Bag Col47 Ladies Various Body Harris LB1120 ('/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')Col47 In Mini Cross Bag Body LB1120 Colours Ladies Harris Tweed Various ;
fs.readFile('/file.md'In Ladies Harris Bag Mini Colours Various Col47 Tweed Body LB1120 Cross , (readFileErr, data) => Colours Ladies Body Harris Bag Tweed Mini LB1120 In Col47 Cross Various {
  if (readFileErr) Various Colours Body Tweed Bag Harris Ladies LB1120 Col47 In Mini Cross Colours Tweed Col47 Various In Body Mini Harris LB1120 Cross Ladies Bag throw readFileErr;
  console.logSak Silverlake Floral Bag Crossbody Embossed Black The qwApzfw(data);
  fs.unlink('/file.md', Various Mini LB1120 Cross Bag Harris Body Tweed In Col47 Colours Ladies (2 Handbag Black Shoulder Bag Set Grey Ladies Bag pcs Capacity Big Faux Leather Tote Large XTROpqzunlinkErr) => {
    if (unlinkErr)Midibackpack Tropics Multicolour Grey Marl Superdry Women's Backpack Pw5qxgf 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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