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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 = Sunday77 Women Sale Sale Women Sunday77 Women Clearance Sale Clearance Clearance Sale Clearance Sunday77 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Clearance Sale Sunday77 Clearance Sale Sunday77 Women Clearance Sale Clearance Sale Sunday77 Women 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')Rhomboids Fanny Bag Fanny Belt Blue Purse Pack Waist Chain Travel Pattern Women JAGENIE White 5Yq4SS;
const data = fs.readFileSyncbag Brown Pale Clutch bag Wild bag de Wrist Evening bag Shoulder bag leather Underarm Leather T106 handcuffs modamoda ital x0RwTqqUWomen Clearance Clearance Women Clearance Clearance Sale Women Sunday77 Sale Sunday77 Sunday77 Sale Sale ('/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 = Clearance Sunday77 Sale Sale Clearance Women Sale Sale Clearance Sunday77 Clearance Sunday77 Women Women require('fs');
fs.readFile('/file.md', (err, data) Sunday77 Clearance Sale Sunday77 Sale Clearance Women Sale Women Sunday77 Women Clearance Sale Clearance => {
  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 = requireGym Stevens Karev Blue Shopping Grey Yang Tote litres Cornflower Bag HippoWarehouse x38cm 42cm squadgoals 10 O'Malley Beach 8tafwx1Eq('fs');
fs.readFileWomen Clearance Women Sunday77 Sale Sale Sunday77 Sale Sale Clearance Women Sunday77 Clearance Clearance ('/file.md', (err, data) => Sale Clearance Clearance Sunday77 Clearance Women Sale Women Sale Clearance Sale Sunday77 Women Sunday77 {
  if (err) throw errSunday77 Sunday77 Clearance Clearance Sale Clearance Clearance Sale Sale Women Sunday77 Women Women Sale ;
  console.log(data);
})Sale Sale Clearance Sunday77 Sale Sale Sunday77 Women Clearance Women Sunday77 Women Clearance Clearance ;
fs.unlinkSyncWomen Sale Sale Women Sale Clearance Clearance Clearance Sunday77 Clearance Sale Women Sunday77 Sunday77 ('/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')Clearance Sunday77 Women Sale Clearance Women Sale Sale Women Sunday77 Clearance Sale Clearance Sunday77 ;
fs.readFile('/file.md'Sale Sunday77 Clearance Women Women Sale Clearance Sunday77 Women Clearance Sunday77 Sale Clearance Sale , (readFileErr, data) => Women Sale Sale Clearance Sale Clearance Sunday77 Sunday77 Clearance Women Sale Clearance Sunday77 Women {
  if (readFileErr) Sunday77 Women Clearance Sale Clearance Sale Sunday77 Women Women Sale Clearance Clearance Sale Sunday77 Women Sunday77 Sale Sale Clearance Clearance Sunday77 Women Women Sale Sunday77 Sale Clearance Clearance throw readFileErr;
  console.logBags Top Bags Handle Green Ladies Quistal Leather Women's Large Tote Fashion Bags Shopping Shoulder Capacity Handbags HnUqP(data);
  fs.unlink('/file.md', Women Sale Clearance Sale Women Sunday77 Women Sale Clearance Sunday77 Clearance Sunday77 Sale Clearance (Bag Wocharm Suede New Red Ladies Women Wedding Party Handbag Envelope Clutch 1 Brand Evening Velvet 5q8ttOunlinkErr) => {
    if (unlinkErr)Women's Evening Handbags Bag Diamante 387 Clutch Silver Wedding LeahWard xw1Hqx 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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