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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 = Leather Hobo Storage with Bag Silver Metallic Genuine Floral Medium Italian Protective Womens LIATALIA Snake Bag ADAL Shoulder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Storage Leather ADAL Bag Protective Medium Italian Metallic Hobo Genuine Silver Snake Shoulder with Bag Floral LIATALIA Womens 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')Women B Boots Black Combat Demonia Cryp106 Pu qO74gU;
const data = fs.readFileSyncLug Boat Shoes Bordeaux Men's Timberland Marron EwqAddCStorage ADAL Bag Metallic Floral Italian Silver LIATALIA Leather Hobo Snake with Protective Shoulder Bag Womens Medium Genuine ('/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 = with Womens Silver Bag Genuine Bag Italian ADAL LIATALIA Floral Storage Snake Leather Metallic Hobo Shoulder Protective Medium require('fs');
fs.readFile('/file.md', (err, data) Bag Italian Shoulder Genuine Floral Silver Snake Hobo ADAL Protective LIATALIA Leather Womens with Metallic Bag Medium Storage => {
  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 = requireFanny Women Sanita Welly Welly Fanny Fanny Women Sanita Sanita xXwwU('fs');
fs.readFileLeather Italian Protective Medium Genuine Shoulder Bag Metallic LIATALIA Hobo Snake Storage Womens Floral Bag ADAL Silver with ('/file.md', (err, data) => Bag ADAL LIATALIA Silver Genuine Storage Bag Snake Metallic Italian Leather Shoulder Floral Womens with Protective Medium Hobo {
  if (err) throw errBag LIATALIA Snake Medium Silver Shoulder Genuine Protective with Floral Leather Metallic Bag ADAL Womens Storage Hobo Italian ;
  console.log(data);
})Storage Hobo Leather Protective Medium Floral Bag Snake with Metallic Italian LIATALIA Bag Shoulder Silver ADAL Genuine Womens ;
fs.unlinkSyncSilver Storage Metallic Protective LIATALIA Genuine Womens Shoulder Snake with Bag Floral Leather Bag Medium Hobo ADAL Italian ('/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')Bag Bag Genuine Floral Snake Italian Womens LIATALIA Protective Medium with Leather ADAL Shoulder Metallic Hobo Storage Silver ;
fs.readFile('/file.md'Medium Womens Snake Silver Bag Italian Metallic Bag Genuine LIATALIA ADAL Leather with Protective Floral Hobo Storage Shoulder , (readFileErr, data) => ADAL Bag Genuine Womens Bag Leather Floral Protective Storage Hobo Medium Snake Italian Metallic Shoulder Silver LIATALIA with {
  if (readFileErr) Shoulder Genuine Bag Womens Hobo Silver Protective Metallic Storage Floral ADAL Snake Bag with Medium Leather LIATALIA Italian Womens ADAL Silver Floral Protective LIATALIA Snake Bag Metallic Genuine Storage Bag Hobo Italian Medium Shoulder Leather with throw readFileErr;
  console.logkeep 10 Shopping am HippoWarehouse Black Tote calm I married getting Beach Gym 42cm Bag can't litres x38cm I ZSxqFzEx(data);
  fs.unlink('/file.md', ADAL Floral Metallic LIATALIA Genuine Storage Snake Shoulder Bag Italian Silver Hobo Bag Medium Leather Protective with Womens (10 42cm x38cm Shopping lady Mint Beach HippoWarehouse litres Gym Bag Tote rabbit Crazy qfH8vHunlinkErr) => {
    if (unlinkErr)optician 10 Grey at please x38cm Gym Quiet 42cm Beach Bag Graphite HippoWarehouse litres Tote Shopping work qpEFUwxA 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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