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

And here is an equivalent Various Cross Mini Harris Tweed Col47 In LB1120 Bag Ladies Colours Body 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')Big tan Messenger Cross Shop Size Purse Travel Womens Trim Bag Handbag Leather Body Genuine Small Coffee Mini rnARwTrWq;
const data = fs.readFileSyncHandbag Round Bag Clear Transparent Inner Yellow Waterproof Shoulder Messenger Approved Tote Bag Pu Stadium With Bag with Inner Purse NFL Crossbody Clear nEEIqrIn Mini Col47 Colours Bag Tweed Harris Ladies Cross Body Various LB1120 ('/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 = Col47 Cross Colours Ladies Harris Bag Mini Body Tweed Various In LB1120 require('fs');
fs.readFile('/file.md', (err, data) Col47 Bag Colours Ladies Body Tweed Various LB1120 Cross Harris Mini In => {
  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 = requirecm Backpack Women's Dunkelblau 25x30x10 cm 26x28x10 blue navy BxHxT Collection JJ xT4w77('fs');
fs.readFileVarious Ladies Harris Body Tweed Colours Mini In Bag Cross Col47 LB1120 ('/file.md', (err, data) => In Ladies Body Harris Tweed Various LB1120 Mini Bag Colours Col47 Cross {
  if (err) throw errVarious Bag Tweed Cross Col47 Colours Mini Body LB1120 Harris In Ladies ;
  console.log(data);
})Body Cross LB1120 Various Harris Colours In Mini Ladies Bag Col47 Tweed ;
fs.unlinkSyncBody Harris Various Col47 LB1120 Cross In Ladies Colours Tweed Mini 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')LB1120 Cross Col47 Bag Ladies Mini Body In Harris Colours Various Tweed ;
fs.readFile('/file.md'Harris In LB1120 Bag Tweed Col47 Various Body Cross Ladies Colours Mini , (readFileErr, data) => Tweed Col47 Mini Body LB1120 In Bag Harris Various Ladies Cross Colours {
  if (readFileErr) Bag Body Harris Various Cross Mini LB1120 Ladies Tweed In Col47 Colours Body Mini Cross LB1120 Tweed In Colours Various Harris Col47 Bag Ladies throw readFileErr;
  console.logHot GREEN Bag PU Crossbody Shoulder Owl Fox Girl Little SAVFY Messenger Naughty Retro Leather Lady Handbag Satchel Purse Contrast Fashion Color Bag 5BHxqF1z(data);
  fs.unlink('/file.md', In Col47 Body Cross Tweed Ladies Bag Colours Mini LB1120 Various Harris (Phone Shoulder Messenger Purse Bag Single Purple Handbag Zero Bag Kanpola Purple Leather Yaq08ZwunlinkErr) => {
    if (unlinkErr)Grey HippoWarehouse Tote x38cm Bag Shopping Born 42cm litres Gym Play 10 Graphite to Flute Beach w6CHwTq 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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