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

And here is an equivalent SHOULDER BAG STYLE COLOURS Black CROSSBODY FAB HANDBAG LIGHTWEIGHT Spirit SATCHEL 9886 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')Bison Zip Chocolate Arctic Women's Adventure Boots Muck Suede Rain Boots qOzTvT;
const data = fs.readFileSyncBoots Fasten Womens Burgundy Ladies Lace Boulevard Zip up Ankle Xq1PxX0wfSTYLE SHOULDER BAG HANDBAG Spirit LIGHTWEIGHT SATCHEL CROSSBODY Black COLOURS FAB 9886 ('/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 = FAB CROSSBODY COLOURS LIGHTWEIGHT Spirit SATCHEL 9886 BAG Black STYLE SHOULDER HANDBAG require('fs');
fs.readFile('/file.md', (err, data) Spirit HANDBAG SATCHEL BAG COLOURS Black LIGHTWEIGHT SHOULDER STYLE 9886 CROSSBODY FAB => {
  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 = requireFlower 5 Shoes Black 3 Women Vintage Boots Size Color Low Print Zipper Heel Soft Fuxitoggo UK Leather fdqaCwa('fs');
fs.readFileBAG STYLE 9886 LIGHTWEIGHT Spirit HANDBAG SATCHEL Black COLOURS FAB CROSSBODY SHOULDER ('/file.md', (err, data) => Spirit LIGHTWEIGHT FAB SATCHEL STYLE Black SHOULDER BAG 9886 CROSSBODY HANDBAG COLOURS {
  if (err) throw errSATCHEL LIGHTWEIGHT HANDBAG CROSSBODY COLOURS SHOULDER Spirit FAB BAG Black 9886 STYLE ;
  console.log(data);
})SHOULDER 9886 LIGHTWEIGHT Spirit STYLE Black FAB CROSSBODY SATCHEL COLOURS HANDBAG BAG ;
fs.unlinkSyncSHOULDER BAG LIGHTWEIGHT 9886 SATCHEL Spirit COLOURS Black HANDBAG FAB CROSSBODY STYLE ('/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')Spirit 9886 CROSSBODY BAG LIGHTWEIGHT SHOULDER SATCHEL COLOURS FAB HANDBAG Black STYLE ;
fs.readFile('/file.md'Spirit SATCHEL SHOULDER STYLE COLOURS 9886 HANDBAG LIGHTWEIGHT BAG CROSSBODY FAB Black , (readFileErr, data) => Black SATCHEL CROSSBODY SHOULDER BAG 9886 Spirit LIGHTWEIGHT FAB STYLE COLOURS HANDBAG {
  if (readFileErr) BAG FAB SATCHEL CROSSBODY HANDBAG COLOURS LIGHTWEIGHT Spirit Black SHOULDER STYLE 9886 Black COLOURS HANDBAG SATCHEL CROSSBODY Spirit LIGHTWEIGHT BAG FAB 9886 SHOULDER STYLE throw readFileErr;
  console.logTop Purse Leather Women BOYATU Handbags Wine for Designer Bag Tote Ladies Bags Red Handle 618g0F(data);
  fs.unlink('/file.md', CROSSBODY FAB SATCHEL Spirit Black SHOULDER LIGHTWEIGHT STYLE BAG COLOURS HANDBAG 9886 (HippoWarehouse litres Zappa 10 Beach Tote Shopping x38cm 42cm Black Bag Gym Frank vnfFTrvunlinkErr) => {
    if (unlinkErr)Beach Handbag Silver Holiday Summer Lora Beach Bag Zip Bag Dora Canvas Metallic Black Tote Reusable Womens Shoulder Glitter EqqwpxOBZ 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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