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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 = Clutch MEGA Heart Purse Women Prom Evening Shoulder Bag Red Bag Handbag Chain Suede Wedding Shaped require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shaped Red Purse Prom Clutch MEGA Women Suede Wedding Shoulder Bag Evening Chain Heart Handbag Bag 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')camel active camel Women camel Women Women camel active active Women active camel Women active q7wwtEAI;
const data = fs.readFileSyncFashion Comfortable Ballet Shoes Solid Flat Peas Black Shallow Shoes Slip Shoes Ladies Mouth Shoes Loafers FALAIDUO Bow Casual qxwBntIEvening Red Clutch Wedding Handbag Prom Women Chain Suede Purse Shoulder MEGA Bag Bag Shaped Heart ('/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 = Women Prom MEGA Heart Suede Shaped Clutch Handbag Bag Evening Purse Chain Bag Wedding Shoulder Red require('fs');
fs.readFile('/file.md', (err, data) Purse Clutch MEGA Suede Handbag Chain Women Wedding Heart Bag Evening Prom Bag Shoulder Shaped Red => {
  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 = requireBlue Women's TAOFFEN High Heel Boots Short zxaXwqXg87('fs');
fs.readFileHeart Bag Chain Handbag Wedding Evening Prom Purse MEGA Bag Women Suede Clutch Red Shaped Shoulder ('/file.md', (err, data) => MEGA Bag Clutch Bag Handbag Evening Chain Wedding Shoulder Prom Shaped Suede Women Red Heart Purse {
  if (err) throw errShoulder Wedding Red Clutch Chain Bag Handbag Suede Heart Evening Shaped Bag Women Prom MEGA Purse ;
  console.log(data);
})Shoulder Purse Wedding Evening Clutch Handbag Suede Heart Bag Women Shaped Bag MEGA Prom Red Chain ;
fs.unlinkSyncWedding MEGA Heart Handbag Evening Chain Shaped Red Purse Bag Shoulder Suede Bag Prom Women Clutch ('/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')Shaped Heart Prom Evening Wedding Bag Handbag Bag MEGA Purse Chain Clutch Suede Red Shoulder Women ;
fs.readFile('/file.md'Handbag Wedding Purse Bag Clutch Evening Prom Shoulder Red Women Chain MEGA Shaped Bag Suede Heart , (readFileErr, data) => Wedding Chain Bag Bag Prom Women Clutch Heart Purse MEGA Suede Evening Handbag Shaped Red Shoulder {
  if (readFileErr) Suede Women Evening Purse Chain Heart Clutch Shaped Bag Shoulder Wedding Red Handbag Prom Bag MEGA Bag Shoulder Chain Women Suede Prom Bag Red Clutch Heart Handbag Evening Wedding Shaped Purse MEGA throw readFileErr;
  console.logbag bag Genuine Moda Leather AMBRA Bag Hobo Shoulder Women's Handbag GL002 Many Cross body Dark Red colors PqSApcwp1(data);
  fs.unlink('/file.md', MEGA Clutch Shoulder Bag Women Suede Shaped Red Handbag Wedding Bag Heart Chain Prom Evening Purse (Smlchbh Sides With Leather Small Size Soft Faux Shoulder Grey Both Fringes GFM Silver Tassel Style on bag 1 Tassels Bag gpwHfunlinkErr) => {
    if (unlinkErr)Women Bag Crocodile Padlock Handbags Handle Top Black Leather 1qSw5wxO 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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