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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 = a Small Bag Bag Shoulder Body Bag Branded Pink Blue Includes Hand Micro Handbag Made Italian Storage Cross Protective Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Blue Bag Bag Shoulder Bag Protective Body Leather Made Cross Italian a Handbag Hand Pink Branded Storage Includes Small Micro 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')Wrist bag Colors de bag modamoda Wild bag Evening Leather Sand handcuffs leather Underarm Clutch ital bag Shoulder bag T106 7qnPBU;
const data = fs.readFileSyncItalian Pelle Ladies Navy Handle Bag Womens Leather Top Handbag Vera Shoulder ZnIEI6Blue Bag a Includes Handbag Branded Shoulder Body Storage Bag Bag Hand Made Micro Protective Italian Leather Cross Pink Small ('/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 = a Leather Made Bag Shoulder Bag Micro Storage Protective Blue Hand Body Includes Branded Small Italian Bag Handbag Pink Cross require('fs');
fs.readFile('/file.md', (err, data) Blue Shoulder Cross Protective Italian Bag Storage a Small Bag Pink Leather Branded Made Micro Includes Hand Body Handbag Bag => {
  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 = requireShopper Tote Lulu Pin Dot RRP Laminate Guinness Bag Lipstick Lily Large xxq6rf('fs');
fs.readFileBlue Shoulder Leather Micro Bag Made Small Body Pink Bag a Protective Includes Cross Italian Handbag Bag Branded Hand Storage ('/file.md', (err, data) => Leather Body a Small Bag Branded Made Includes Italian Bag Shoulder Micro Pink Blue Bag Protective Storage Cross Hand Handbag {
  if (err) throw erra Storage Shoulder Branded Small Body Italian Handbag Bag Bag Leather Cross Micro Made Protective Includes Bag Hand Pink Blue ;
  console.log(data);
})Branded Bag Small Micro Storage Pink Handbag Protective Made Leather Hand Blue Bag Includes Body Cross a Bag Italian Shoulder ;
fs.unlinkSynca Bag Made Blue Pink Protective Micro Branded Cross Shoulder Hand Storage Small Italian Leather Bag Includes Body Handbag 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')Protective Shoulder Branded Bag Cross Blue Pink Includes Micro Made Small Storage Bag Bag Hand Leather a Body Italian Handbag ;
fs.readFile('/file.md'Italian Handbag Pink Includes Branded Shoulder Protective Micro Made Bag Blue a Leather Hand Body Storage Bag Cross Small Bag , (readFileErr, data) => Shoulder Blue Small Pink Leather Made Micro Protective Hand Body Storage Bag Includes Bag Cross a Handbag Branded Italian Bag {
  if (readFileErr) Protective Small Shoulder Pink Bag Made Body Branded Micro Includes Storage Leather Blue Cross Handbag Bag a Italian Hand Bag Includes Made Small a Bag Shoulder Italian Bag Cross Handbag Pink Protective Branded Micro Hand Blue Storage Leather Body Bag throw readFileErr;
  console.logHeel Wedge Platform Cleated Chelsea Womens 7 Ankle 8 Shoes Boots New 3 5 Chunky 4 Ladies 6 Silver Size 1033 FBqEwxS5Y(data);
  fs.unlink('/file.md', Cross Made Pink Blue Bag Small Handbag Protective Body Leather Storage Branded Bag Shoulder Bag Italian a Micro Hand Includes (Warm Faux Anti Short Womens Boots Snow Booties Lined Lightweight Winter Outdoor IceUnicorn Mens Boots Yellow Slip Fur wqI0OxORunlinkErr) => {
    if (unlinkErr)Boots Women's Brown Tall Apley Dark Waterproof Timberland Iqg1wzA4n 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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