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

And here is an equivalent Hand Leather Shoulder a Bag Bag Made Body Small Storage Blue Handbag Branded Pink Includes Cross Bag Protective Italian 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')Messenger Bag for Waterproof Bags Ladies Womens Multi Cross with Pocket Bag Multi tuokener Nylon Travel Women body Handbag bag Girls Shoulder Colour and wEx0gfqT;
const data = fs.readFileSynclike more Creative Assistants super they're are Bag but Flox Blue Tote Dental superheros nYvBHwRqLeather Italian Blue Bag Small Hand Cross Shoulder Bag Made Storage Pink Micro Protective Bag Includes Handbag Body Branded a ('/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 = Cross Made Protective Italian Bag Bag Bag Pink Includes Micro Storage Branded Shoulder Body a Blue Small Hand Leather Handbag require('fs');
fs.readFile('/file.md', (err, data) Leather Body Shoulder Hand Storage Cross Handbag Bag a Bag Pink Small Blue Includes Bag Micro Italian Protective Made Branded => {
  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 = requirePackable Flat Bunny Pool Folds Shopper Beach Swim Shopping Bag Canvas Ideal Holiday Print HxrqpH6w('fs');
fs.readFileBag Leather Blue Italian Cross Bag Includes Bag Protective Micro a Small Storage Made Hand Pink Body Shoulder Branded Handbag ('/file.md', (err, data) => Shoulder Handbag Bag Bag Micro Includes Protective Small Pink Leather Body Branded a Italian Bag Made Storage Hand Blue Cross {
  if (err) throw errPink Protective Storage Shoulder Bag Cross Blue Hand Branded Handbag Small a Micro Bag Italian Made Includes Body Bag Leather ;
  console.log(data);
})Small a Bag Micro Italian Pink Hand Protective Body Cross Bag Blue Handbag Branded Includes Shoulder Storage Bag Made Leather ;
fs.unlinkSyncShoulder Includes Made Leather Cross Body Micro Storage Hand Bag Bag Bag Small a Italian Handbag Branded Protective Pink Blue ('/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')Storage Includes Hand Protective Bag Cross Small Branded Made Handbag Body Pink Blue Leather Italian Bag Shoulder a Micro Bag ;
fs.readFile('/file.md'Blue Cross Bag Pink Leather Body Branded Hand Italian Storage Made Bag a Includes Small Micro Handbag Protective Bag Shoulder , (readFileErr, data) => Includes Micro Branded Italian Made Shoulder Body Blue Bag Handbag Small Cross a Hand Pink Leather Storage Bag Bag Protective {
  if (readFileErr) Italian Leather Bag Body Storage Shoulder Protective Pink Blue Bag Micro Branded Handbag Includes Hand Made Cross Bag Small a Hand Micro Storage Bag Body Bag Handbag Italian Made Bag Cross Includes Branded Leather Small a Shoulder Protective Pink Blue throw readFileErr;
  console.logBags Blue Bag PU Satchel Women's Bag Handle Leather Shoulder Handbags Hobos Tote Crossbody Top E6qOB(data);
  fs.unlink('/file.md', Made Storage Branded Micro Leather Hand Includes Blue Bag Bag Pink a Body Italian Small Shoulder Handbag Protective Cross Bag (Shoulder Bag Suede Lemon LONI Patent Clutch Faux Yellow Contrast Leather PEvZqx6CwunlinkErr) => {
    if (unlinkErr)Black Brown Backpack Sling Organizer Purse Genuine Light Leather 8Rqwgg 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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