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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 = Unisex Original Lace Boots 1460 Marten's Dr Black Up Adult require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Boots 1460 Dr Up Unisex Adult Black Lace Original Marten's 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')ECCO Women ECCO Women ECCO ECCO Women qUt51cP;
const data = fs.readFileSyncLong Boots Women High Boots All Women Size with Thick Heel Large Brown and Platform Match SJJH with 75FwpqqOriginal Boots Unisex 1460 Black Up Marten's Dr Lace Adult ('/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 = Lace Up Boots Marten's Black Dr Adult Original 1460 Unisex require('fs');
fs.readFile('/file.md', (err, data) Marten's 1460 Boots Lace Black Up Unisex Dr Original Adult => {
  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.

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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 = requireTamaris Tamaris Women Women 8gqx5Fw('fs');
fs.readFileMarten's Up Black Unisex Original Dr Adult Lace 1460 Boots ('/file.md', (err, data) => Marten's Adult Original Dr Boots Up Lace Black 1460 Unisex {
  if (err) throw err1460 Lace Black Up Marten's Unisex Dr Original Adult Boots ;
  console.log(data);
})Up Black Marten's Dr Lace Original Unisex Boots Adult 1460 ;
fs.unlinkSyncLace Up 1460 Original Unisex Boots Adult Marten's Black Dr ('/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')Black Original Boots Up Adult Unisex Lace Marten's Dr 1460 ;
fs.readFile('/file.md'Marten's Original 1460 Lace Boots Up Black Dr Adult Unisex , (readFileErr, data) => Unisex Adult Dr 1460 Black Up Original Marten's Boots Lace {
  if (readFileErr) 1460 Original Dr Unisex Lace Boots Adult Black Up Marten's Original Up Boots Lace Marten's Adult 1460 Black Unisex Dr throw readFileErr;
  console.logPU Straps Wide Crossbody Replacement described Bag Black for Bag Belts Black Homyl DIY Leather Shoulder as Purse 7xtwX(data);
  fs.unlink('/file.md', Dr Unisex Adult Boots 1460 Black Marten's Lace Up Original (Champagne Closure Bag Bags for Shoulder Stylish Jin Top Bag Handle Leather Tote Handbag Magnetic Ya Women RZAUqnunlinkErr) => {
    if (unlinkErr)10 Beach of Mother the Tote Gym 42cm HippoWarehouse x38cm litres Bride Shopping Yellow Bag qwpPx40Z6 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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