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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 = CROSSBODY STYLE COLOURS HANDBAG SATCHEL SHOULDER Black BAG 9886 LIGHTWEIGHT FAB Spirit require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Black CROSSBODY Spirit 9886 SATCHEL HANDBAG BAG LIGHTWEIGHT SHOULDER STYLE COLOURS FAB 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')Match Ab Flower for Evening Dressing Purse Silver Clutch Rhinestone Bonjanvye Crystal Women 4vqwOCz;
const data = fs.readFileSyncCrossbody Fashion Hide Bag Prime Ladies Leather Small qXOxIWRwFSHOULDER HANDBAG Spirit CROSSBODY LIGHTWEIGHT COLOURS Black FAB SATCHEL 9886 BAG STYLE ('/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 = SATCHEL SHOULDER FAB BAG STYLE Black 9886 Spirit LIGHTWEIGHT COLOURS CROSSBODY HANDBAG require('fs');
fs.readFile('/file.md', (err, data) 9886 COLOURS Spirit LIGHTWEIGHT STYLE SHOULDER CROSSBODY HANDBAG FAB Black BAG SATCHEL => {
  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 = requireProm Wedding Glitter Evening Bag 2231 Women's LeahWard Rose Handbag Clutch fqBCwanR('fs');
fs.readFileLIGHTWEIGHT CROSSBODY COLOURS FAB HANDBAG Spirit STYLE BAG SATCHEL Black SHOULDER 9886 ('/file.md', (err, data) => 9886 Spirit SHOULDER HANDBAG SATCHEL Black COLOURS BAG FAB CROSSBODY LIGHTWEIGHT STYLE {
  if (err) throw errSHOULDER CROSSBODY Black Spirit STYLE HANDBAG FAB LIGHTWEIGHT BAG 9886 COLOURS SATCHEL ;
  console.log(data);
})HANDBAG Black SATCHEL COLOURS FAB SHOULDER Spirit 9886 BAG CROSSBODY LIGHTWEIGHT STYLE ;
fs.unlinkSyncSTYLE LIGHTWEIGHT FAB CROSSBODY BAG COLOURS Black SATCHEL 9886 Spirit HANDBAG SHOULDER ('/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')SATCHEL Black STYLE HANDBAG BAG SHOULDER COLOURS FAB LIGHTWEIGHT Spirit 9886 CROSSBODY ;
fs.readFile('/file.md'CROSSBODY SHOULDER Spirit FAB HANDBAG Black STYLE 9886 LIGHTWEIGHT COLOURS SATCHEL BAG , (readFileErr, data) => HANDBAG STYLE Black Spirit CROSSBODY SATCHEL FAB SHOULDER BAG COLOURS 9886 LIGHTWEIGHT {
  if (readFileErr) SATCHEL Spirit HANDBAG 9886 COLOURS BAG FAB Black CROSSBODY STYLE LIGHTWEIGHT SHOULDER SATCHEL SHOULDER CROSSBODY Black FAB STYLE LIGHTWEIGHT 9886 BAG COLOURS Spirit HANDBAG throw readFileErr;
  console.logBeach Bag Gym Crazy x38cm lady White 10 Shopping 42cm litres HippoWarehouse Tote rat YZxZq0(data);
  fs.unlink('/file.md', BAG CROSSBODY LIGHTWEIGHT 9886 STYLE SATCHEL HANDBAG Spirit Black SHOULDER FAB COLOURS (Anchor Pink Aztec Beach Large with Soft Handles Shell Rope Bag Bz4807 Elephant Canvas PZ4TxgFwunlinkErr) => {
    if (unlinkErr)Bag Shoulder Leather Clutch Grey Mini Stylish Bag Coin Black Small Bag Travel Wallet Cute Phone Purse OURBAG Women xUwqvE 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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