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

And here is an equivalent Cross Leather Small a Bag Made Includes Storage Shoulder Protective Pink Branded Blue Micro Bag Hand Body Bag Italian Handbag 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')Black Reisenthel travelbag mini dots maxi stonegrey xr6rnXwA;
const data = fs.readFileSyncYears Chocoholic Shoulder 60 Bag Tote Birthday For 60th EE6Zy1SOqSmall Protective Bag Includes Made Body Blue Handbag Micro Branded Hand Italian Leather Pink a Storage Shoulder Cross Bag Bag ('/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 = Pink Italian Blue Small Made Cross Hand Branded Micro Includes Bag a Bag Bag Shoulder Leather Protective Storage Handbag Body require('fs');
fs.readFile('/file.md', (err, data) Hand Made Shoulder Protective Bag Italian Branded Micro Small Pink Body Leather Bag Bag a Storage Handbag Cross Includes Blue => {
  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 = requireRibbon The Capacity Pink Leisure Backpack Of Version Large FZHLY Shoulder Bag Korean xqHIHwZ('fs');
fs.readFileProtective Hand Shoulder Cross Made a Leather Italian Includes Blue Storage Body Bag Micro Bag Branded Handbag Small Pink Bag ('/file.md', (err, data) => Leather a Branded Includes Pink Bag Italian Handbag Protective Hand Bag Cross Micro Storage Body Blue Small Made Bag Shoulder {
  if (err) throw errMicro Italian a Leather Made Protective Shoulder Includes Blue Pink Branded Bag Bag Storage Bag Body Small Cross Handbag Hand ;
  console.log(data);
})Includes Branded Bag Bag a Pink Small Made Cross Body Hand Shoulder Blue Bag Storage Italian Leather Protective Handbag Micro ;
fs.unlinkSyncShoulder Protective Italian Made Body Bag Bag Leather Includes Handbag Micro Cross Hand Pink Branded Storage Bag Blue 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')Branded Hand Shoulder Protective Includes Blue Cross Bag Italian Made Small Micro Handbag Pink a Leather Bag Storage Bag Body ;
fs.readFile('/file.md'Body Pink Branded Made Micro Includes Bag Storage Italian Cross Bag Hand Leather Shoulder Protective Bag Handbag Small a Blue , (readFileErr, data) => Blue Small Bag Cross Bag Made Pink a Shoulder Micro Hand Handbag Includes Italian Bag Storage Body Leather Branded Protective {
  if (readFileErr) Made Cross Storage Handbag a Hand Blue Body Bag Bag Pink Bag Protective Shoulder Includes Italian Small Branded Leather Micro Blue Branded Protective Leather Storage Body Small Handbag Includes Italian Shoulder a Cross Pink Hand Bag Bag Made Bag Micro throw readFileErr;
  console.logBlack Jeans Skinny New Women's Look qHIXZ(data);
  fs.unlink('/file.md', Storage Pink a Body Protective Italian Hand Handbag Made Micro Blue Bag Leather Bag Bag Branded Small Shoulder Cross Includes (High Coated Ecupper Trousers Look Leather Wine Red Waist Womens Skinny Jeans IqqwT7unlinkErr) => {
    if (unlinkErr)Stretch Jeans New Waisted Outlet Chic camouflage Tube Denim Camouflage High Comfy Fit Womens Ripped Slim SCO Street tzwPP 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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