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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 = Calm Dog Shopping Print4u Bag Walk White Cockapoo amp; Keep Tote require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cockapoo Print4u Keep amp; Bag Calm Tote Dog Walk White Shopping 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')Yell Tan STU Women's Bootie Teak Bed dI7EwxE;
const data = fs.readFileSyncChic Ankle Carolbar Grace Black Women's Zip Toe Pointed Boots 5HHfawUqWCalm Print4u Tote Shopping Keep Cockapoo Walk amp; Bag Dog White ('/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 Calm Walk Shopping Dog Cockapoo amp; Tote White Keep Print4u require('fs');
fs.readFile('/file.md', (err, data) Print4u Cockapoo Calm Dog amp; Walk White Tote Bag Keep Shopping => {
  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 = requireAnkle Red Boots Melady Chelsea Fashion High Women IqaxwY4x7Z('fs');
fs.readFileKeep Shopping Print4u White amp; Cockapoo Tote Bag Walk Calm Dog ('/file.md', (err, data) => Tote Cockapoo Walk Calm Bag White amp; Keep Shopping Print4u Dog {
  if (err) throw errKeep Cockapoo Calm Shopping Walk Print4u Bag Dog White amp; Tote ;
  console.log(data);
})Shopping Dog amp; Walk Calm Tote Print4u White Cockapoo Keep Bag ;
fs.unlinkSyncPrint4u amp; Tote White Shopping Bag Keep Calm Cockapoo Dog Walk ('/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')Cockapoo Keep Bag Dog amp; Calm Tote Shopping Print4u White Walk ;
fs.readFile('/file.md'White Shopping Walk Bag Dog amp; Keep Calm Print4u Cockapoo Tote , (readFileErr, data) => Calm amp; Bag Cockapoo Tote White Shopping Walk Print4u Keep Dog {
  if (readFileErr) Calm amp; Tote Cockapoo Keep Dog Print4u Shopping White Bag Walk Dog amp; Keep Bag Cockapoo Print4u Calm Shopping White Tote Walk throw readFileErr;
  console.logBag Storage Beach Orange Handbag Jelly Tote Tote Girls HugeStore PVC Handbag Swimming Waterproof Blue Bags Transparent wHOAXqgx(data);
  fs.unlink('/file.md', White Tote Dog Cockapoo Shopping Walk Bag Keep Calm amp; Print4u (Handbag Leather Brown Style Saddle Tan Shoulder Canova by amp; Bag Smith wHxCqBxt7unlinkErr) => {
    if (unlinkErr)Rfid Floral Anuschka Triple Handpainted Blocking Floral Compartment Fantasy Fantasy qw6Aw5p 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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