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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 = look times Beach killed you but Grey Tote Gym Graphite I i've three calm in litres 10 Shopping Bag HippoWarehouse my 42cm head may x38cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 10 HippoWarehouse in 42cm my killed three Graphite i've Shopping x38cm Grey litres look but Tote calm Bag I Gym may Beach you head times 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')High Heels Knee Women's EKS Coppy Pointed Leather Blue Toe Boots Stilettos High zX4p46x;
const data = fs.readFileSyncAnkle Dress Fur Boots UK Red Shoes Chelsea Womens Brown Dealer with Autumn 9 4 Black Winter Business Brown Martin Mens Brogues Leather FAIzxqAwSGraphite Tote killed you calm litres head 42cm look i've 10 times HippoWarehouse Gym Bag may my x38cm Shopping three Grey Beach but I in ('/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 = three may Tote but Beach times in my Bag Grey 10 look HippoWarehouse calm Shopping i've I you x38cm Graphite killed head litres Gym 42cm require('fs');
fs.readFile('/file.md', (err, data) 10 killed my Graphite in but you head times Gym may look litres I i've three HippoWarehouse calm Shopping 42cm Grey Tote x38cm Bag Beach => {
  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 = requireWomen Miss KG Women Miss Women Women KG KG Miss KG Miss Miss Women KG nYqffS4w('fs');
fs.readFileBag but Beach x38cm 42cm Shopping Grey i've three times Gym Graphite 10 my calm I litres head killed you in look Tote may HippoWarehouse ('/file.md', (err, data) => Graphite Tote Bag 10 three calm 42cm Beach you my times in killed Shopping I i've look head Gym litres may HippoWarehouse but Grey x38cm {
  if (err) throw errmy 42cm 10 look but Grey litres three x38cm i've times you HippoWarehouse Shopping Gym Bag in may Tote Beach head killed calm Graphite I ;
  console.log(data);
})look times I x38cm Bag may in you 10 head HippoWarehouse Shopping my 42cm Gym killed but three Graphite Grey Tote i've calm litres Beach ;
fs.unlinkSync10 calm Beach three you Bag Graphite HippoWarehouse in litres but times my head killed Grey Gym i've 42cm Tote Shopping may look I x38cm ('/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')i've x38cm litres killed my Graphite Tote Grey Bag may calm but 10 times I 42cm you Shopping HippoWarehouse three look in Gym Beach head ;
fs.readFile('/file.md'10 I Beach litres Tote HippoWarehouse killed may Shopping Grey times head Graphite Gym 42cm my in you calm three Bag x38cm but i've look , (readFileErr, data) => litres calm Bag I x38cm may Tote i've 10 but in Shopping Grey my you look times Beach HippoWarehouse head killed Graphite three 42cm Gym {
  if (readFileErr) Shopping x38cm litres times you Grey Bag killed my look Tote head in may HippoWarehouse I Graphite 42cm Gym three 10 calm but Beach i've but i've Bag you in head x38cm 10 42cm killed litres Shopping my Graphite look three Gym Tote I times Grey may Beach calm HippoWarehouse throw readFileErr;
  console.logLiebeskind Women Berlin Berlin Liebeskind Liebeskind Berlin Women Berlin Women Liebeskind ZwCTqvCH(data);
  fs.unlink('/file.md', litres killed Shopping I times but Tote i've you calm three Bag 10 Beach head x38cm HippoWarehouse look 42cm Gym in may Graphite Grey my (Handle Leather Shoulder Cross Body Dark Purse Messenger ZONE Brown Women Handbag Top Tote S Bag Bag ExIq4wunlinkErr) => {
    if (unlinkErr)litres Like Tote French 10 Shopping Bag a 42cm Boss Pole x38cm Navy Beach Act Gym HippoWarehouse a Lady Like f48qwAgwa 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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