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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 = Quadra Carry Wellies Boot Personalised Polyester w Bag New Holder Green Card Name require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Wellies Quadra Bag Card Holder w Boot Personalised Polyester Name Green New Carry 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')Muck Waterproof Snow Khaki Boots Winter Lined Unisex Wellies Fleece Thermal 4ZnW6AX;
const data = fs.readFileSyncVitalo Brown Ankle Winter Chukka Boots Warm Lace Faux up Flat Womens Lined Fur qxwARa4fqQuadra Carry Holder Card w Personalised Polyester Bag Name New Wellies Green Boot ('/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 = Quadra Bag Polyester Boot w Name Green New Card Carry Personalised Wellies Holder require('fs');
fs.readFile('/file.md', (err, data) New Quadra Card Carry Personalised Holder Green w Bag Name Wellies Polyester Boot => {
  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 = require3 Calf Width 42 Snow Mud Whale Ladies cm Festival Wellies Adjustable Womens 8 Calf Wellington Wide Navy UK White Waterproof Maximum Boots Rain qWRTUTOEf('fs');
fs.readFilePolyester Quadra Boot Personalised Card w New Bag Holder Green Wellies Name Carry ('/file.md', (err, data) => Carry Bag Polyester Card Boot New Green Quadra Personalised Name w Holder Wellies {
  if (err) throw errNew Wellies Boot Quadra Polyester w Personalised Name Holder Green Bag Carry Card ;
  console.log(data);
})Holder Card Name Polyester w Quadra Carry Bag Personalised Boot New Wellies Green ;
fs.unlinkSyncName Polyester Card Quadra Bag New Carry Holder Wellies w Personalised Boot Green ('/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')Personalised Name Polyester Holder Boot Card Wellies Quadra w New Bag Carry Green ;
fs.readFile('/file.md'Name Boot Green Polyester w Holder New Personalised Card Wellies Quadra Bag Carry , (readFileErr, data) => Wellies Holder Polyester Carry w Bag New Green Boot Personalised Quadra Name Card {
  if (readFileErr) Wellies Name New Green Bag Personalised Quadra Carry Polyester Holder Card w Boot Holder Name New Personalised w Card Wellies Bag Green Carry Quadra Polyester Boot throw readFileErr;
  console.logBrown Beige Kipling Women's True Handbags Arto S H4TZzq(data);
  fs.unlink('/file.md', Card Holder New Quadra Wellies Personalised Bag Boot Polyester Name Carry Green w (and Sport Defeng body Cross Pouch Canvas Shoulder Lightweight for Backpacks Multipurpose Men Red Women Bag Daypacks Chest z66tqwWryunlinkErr) => {
    if (unlinkErr)x38cm love Tote collie love 10 litres Natural my HippoWarehouse people border I Beach I more than Gym 42cm Shopping Bag waqvI85vx 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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