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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 = Cross Soft Body Coffee Leather Shoulder Real Bag Italian Light Handbag Navy require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Coffee Shoulder Cross Italian Body Light Navy Bag Real Soft Handbag Leather 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')Fashion Fringe Mid RAZAMAZA Knee High Heel Beige Boots Wedge Women Platform pnYw7Aq5;
const data = fs.readFileSyncLined Up Ankle Boots Leather Boots Combat Flat Casual Shoes Martin Anti Bootie Boots Winter Fashion Lace Warm Pink Women's Skid nBq0aIwOxReal Handbag Bag Leather Shoulder Italian Cross Light Soft Navy Body Coffee ('/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 = Soft Real Light Navy Cross Italian Bag Handbag Leather Body Coffee Shoulder require('fs');
fs.readFile('/file.md', (err, data) Shoulder Real Bag Navy Handbag Coffee Italian Body Cross Light Soft Leather => {
  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.

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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 = requireECCO Women ECCO Women Women Women ECCO ECCO ECCO Women ECCO Women 5nRaqwxUPn('fs');
fs.readFileBody Real Shoulder Leather Bag Light Soft Navy Italian Coffee Cross Handbag ('/file.md', (err, data) => Cross Body Shoulder Soft Bag Coffee Leather Italian Light Handbag Navy Real {
  if (err) throw errReal Leather Cross Navy Handbag Italian Coffee Soft Light Shoulder Bag Body ;
  console.log(data);
})Navy Shoulder Cross Bag Italian Soft Body Handbag Real Coffee Light Leather ;
fs.unlinkSyncBody Real Cross Navy Leather Italian Coffee Light Handbag Shoulder Soft 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')Real Shoulder Light Body Soft Bag Coffee Navy Cross Leather Handbag Italian ;
fs.readFile('/file.md'Light Body Bag Cross Coffee Handbag Soft Italian Real Navy Shoulder Leather , (readFileErr, data) => Bag Italian Soft Leather Light Real Handbag Navy Shoulder Body Coffee Cross {
  if (readFileErr) Handbag Body Bag Leather Coffee Navy Cross Shoulder Soft Italian Real Light Shoulder Real Navy Bag Handbag Light Soft Italian Body Coffee Leather Cross throw readFileErr;
  console.log1 Dragonfly JAGENIE Embroidery Backpack Women Bird 1 Mini Nylon Flowers BK Backpack 6qaz71(data);
  fs.unlink('/file.md', Handbag Leather Bag Cross Shoulder Navy Body Soft Light Italian Real Coffee (Women Prom Bridal Champagne Clutch Evening London Leather Xardi Metallic PU Bags Envelope Diamante Flat cwHRxf40qunlinkErr) => {
    if (unlinkErr)x38cm HippoWarehouse 10 Bag Fuchsia 42cm lady litres pig Shopping Gym guinea Beach Crazy Tote wBUxv6w 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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