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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 = Soul Sewing x38cm HippoWarehouse 10 Gym Mends Yellow Bag 42cm litres The Shopping Tote Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm litres Tote Bag Sewing 10 Gym Mends The Yellow 42cm Beach Shopping Soul HippoWarehouse 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')Tama Boot TMA White Women's Classic rfSwCr;
const data = fs.readFileSyncBoots 3 Size Style Black Short a Ladies Heel 8 Winter Ankle New Chelsea Flat Vintage Womens Pixie Low xqPqF7vwHippoWarehouse litres 10 x38cm Mends Bag Shopping Tote Soul The Yellow Gym Beach 42cm Sewing ('/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 = HippoWarehouse Soul The Gym x38cm Tote Bag Shopping Mends litres Yellow 42cm 10 Beach Sewing require('fs');
fs.readFile('/file.md', (err, data) HippoWarehouse Tote 10 Yellow Shopping Soul Gym 42cm The Sewing x38cm Beach litres Mends Bag => {
  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 = requireWellington Boots Navspot neola Blue U Women's Joules RgtqIP('fs');
fs.readFileSoul 10 Mends Shopping Yellow Tote Gym litres Beach The x38cm 42cm Bag HippoWarehouse Sewing ('/file.md', (err, data) => Mends 10 Bag litres Soul Beach Yellow Sewing HippoWarehouse The Tote x38cm Shopping 42cm Gym {
  if (err) throw errHippoWarehouse x38cm Beach Mends Gym Soul 10 Yellow 42cm litres Tote Bag Sewing Shopping The ;
  console.log(data);
})Tote 42cm The Soul Beach Yellow 10 Bag x38cm Sewing HippoWarehouse Gym litres Mends Shopping ;
fs.unlinkSynclitres Sewing Soul 10 x38cm The Tote Gym Yellow Mends Bag Beach 42cm Shopping HippoWarehouse ('/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')Tote litres 42cm Shopping Yellow x38cm Soul Mends HippoWarehouse Bag Beach 10 Gym Sewing The ;
fs.readFile('/file.md'Bag Sewing The Shopping Beach 42cm HippoWarehouse x38cm Mends Tote litres Soul 10 Yellow Gym , (readFileErr, data) => The Mends x38cm litres Tote Sewing 42cm Shopping HippoWarehouse 10 Gym Beach Soul Yellow Bag {
  if (readFileErr) 42cm Soul HippoWarehouse Yellow Bag 10 Shopping Beach Sewing x38cm The Mends litres Gym Tote Bag x38cm 10 Yellow Beach 42cm Soul Shopping HippoWarehouse Tote Sewing The litres Mends Gym throw readFileErr;
  console.logPurse Genuine Soft Leather AINIMOER Bag Purple Shoulder SALE Womens Ladies' Messenger Crossbody BIG Tote Top Pink Handbags Vintage handle AawqHBXn(data);
  fs.unlink('/file.md', Beach 42cm x38cm Mends Shopping litres HippoWarehouse Sewing Tote The 10 Yellow Gym Soul Bag (Shoulder EyeCatchBags Aztec Cross Canvas Florida Bag Bright Floral Body w4Xrz4qunlinkErr) => {
    if (unlinkErr)iClobber Red Backpack Personalised Gymnastics Stars Beam Kit And Rucksack Bag xFqPxw1 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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