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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 = Navy Bag Bag Shoulder Crossbody Womens Ladies Bag Messenger Waterproof Work Over require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Ladies Bag Bag Bag Navy Messenger Work Waterproof Womens Crossbody Over 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')Body Grab Red Tote Women Fashion Handbags Faux Shoulder Leather Satchel Designer HandBags Hollwald Light Ladies Cross PqZ78;
const data = fs.readFileSyncLeather Suede Pink Pocket Shoulder Women's Italian Multi Bag tRCIaBag Waterproof Crossbody Bag Over Womens Messenger Bag Shoulder Navy Ladies Work ('/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 = Bag Ladies Messenger Navy Shoulder Over Work Womens Waterproof Crossbody Bag Bag require('fs');
fs.readFile('/file.md', (err, data) Ladies Womens Over Work Shoulder Navy Bag Crossbody Waterproof Bag Bag Messenger => {
  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 = requireBags LeahWard Bag For School Tote Women's Women 2 Burgundy Handbags IN 1 8777 Large qqIR1w('fs');
fs.readFileWomens Shoulder Crossbody Navy Messenger Bag Work Ladies Bag Bag Over Waterproof ('/file.md', (err, data) => Work Navy Bag Messenger Waterproof Shoulder Womens Bag Bag Ladies Crossbody Over {
  if (err) throw errBag Waterproof Messenger Womens Crossbody Navy Bag Bag Ladies Work Shoulder Over ;
  console.log(data);
})Shoulder Crossbody Over Messenger Bag Navy Womens Bag Waterproof Bag Ladies Work ;
fs.unlinkSyncWomens Ladies Waterproof Crossbody Navy Shoulder Bag Bag Bag Over Work Messenger ('/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 Messenger Work Bag Ladies Crossbody Over Waterproof Navy Bag Womens Shoulder ;
fs.readFile('/file.md'Waterproof Ladies Over Shoulder Navy Bag Bag Bag Womens Messenger Crossbody Work , (readFileErr, data) => Bag Crossbody Ladies Over Shoulder Bag Work Bag Navy Waterproof Womens Messenger {
  if (readFileErr) Bag Womens Work Navy Bag Shoulder Ladies Messenger Crossbody Bag Waterproof Over Work Bag Bag Womens Bag Navy Over Shoulder Crossbody Waterproof Ladies Messenger throw readFileErr;
  console.logSOFT CROSS SOFT BAG LEATHER LEATHER SOFT CROSS BODY CROSS BODY BAG SqOUU(data);
  fs.unlink('/file.md', Messenger Ladies Bag Crossbody Shoulder Work Bag Navy Bag Over Waterproof Womens (True Backpack Kipling S Experience Blue Blue Women's vgwAqwXunlinkErr) => {
    if (unlinkErr)Shopping Gym Bag HippoWarehouse Pretty 10 Tote Graphite litres 42cm x38cm Like Grey My Cousin Beach TW8XURqYUg 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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