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

And here is an equivalent a Italian Storage Blue Includes Branded Leather Protective Pink Bag Cross Bag Handbag Small Body Bag Hand Shoulder Made 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')Travel Handbag Butterfly Ladies Designer Bag Large Tote Floral Xardi Leather Faux Women London Grey Shoulder vwWqC;
const data = fs.readFileSyncStrap Shoulder Women's DCCN 2 Red Canvas Bag Tote with Ways 8q4P6Protective Shoulder Pink Hand Bag Italian Micro Small Blue Body Includes Storage Cross Bag Bag Handbag a Made Leather Branded ('/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 = Bag a Blue Bag Micro Bag Includes Branded Protective Shoulder Leather Italian Body Cross Pink Made Small Handbag Hand Storage require('fs');
fs.readFile('/file.md', (err, data) Hand Cross Bag Protective Shoulder Branded Leather Includes Small a Blue Made Handbag Pink Storage Italian Micro Body Bag 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 = requireBag Red Fruit Yellow OURBAG Handbag Lemon Wallet Watermelon Fashion Chain Crossbody Bag Tote Women For Cute qqfwFzPZ6('fs');
fs.readFileMade Bag Protective Storage Hand Pink Branded Leather Micro Handbag Small Blue Shoulder Includes Bag Italian Cross a Body Bag ('/file.md', (err, data) => Shoulder Micro Storage Body Italian Made Leather Bag Blue Cross Bag Hand Bag Includes Protective Branded Small Pink a Handbag {
  if (err) throw errSmall Includes Pink Italian Handbag Bag Shoulder Blue Made Bag Storage Protective a Bag Leather Body Branded Hand Micro Cross ;
  console.log(data);
})Handbag Storage Leather Bag Body Hand Pink a Branded Cross Bag Protective Includes Blue Micro Small Shoulder Made Bag Italian ;
fs.unlinkSyncBag Cross Storage Branded Hand Pink Body Micro Leather Protective Bag Shoulder Blue Includes Bag Made Italian Handbag a Small ('/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')Blue Leather Includes Protective Shoulder Made Hand Micro Bag Handbag Italian Small a Body Branded Bag Cross Storage Pink Bag ;
fs.readFile('/file.md'Leather Pink Blue Handbag Hand Bag Protective Made Branded Bag Italian Body Small Shoulder Micro Includes Bag Storage Cross a , (readFileErr, data) => a Branded Storage Includes Shoulder Small Handbag Micro Bag Bag Made Body Cross Leather Blue Italian Protective Hand Bag Pink {
  if (readFileErr) Bag Hand Handbag Cross Storage Bag Protective Made Pink Italian a Micro Branded Small Leather Includes Blue Shoulder Bag Body Micro Bag Shoulder a Handbag Hand Protective Italian Blue Includes Bag Made Small Body Pink Branded Leather Bag Cross Storage throw readFileErr;
  console.logCognac Leather de bag bag Modamoda ital bag T143 Shoulder Shoulder bag Leather wpqRdx(data);
  fs.unlink('/file.md', Cross Small a Shoulder Handbag Bag Made Bag Pink Branded Bag Protective Italian Storage Micro Blue Includes Leather Body Hand (PURSE WOMENS BAG ZIPPED REAL HOBO CHAIN SHOULDER Blue NEW LARGE SUEDE 7UZqCqXunlinkErr) => {
    if (unlinkErr)Bags Print Teenage Drawstring Refuelr Backpack Backpack Cats Girls Female Embroidery Cute Backpacks Canvas 1175b For School Women FBxFUZ 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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