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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 = Micro Blue Italian Cross Bag Branded Storage Handbag Bag Made Protective Body Hand Pink Bag a Leather Small Shoulder Includes require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Made Includes Small Pink Blue Italian Handbag Storage Bag Cross Micro Protective Branded Hand Shoulder Body Bag Bag Leather a 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 Seibel Josef Women Seibel Seibel Josef Seibel Josef Josef Women Women Women Seibel Josef Josef UIHFqTU;
const data = fs.readFileSyncWomen Women Skechers Skechers Skechers Skechers Women Skechers Skechers Women Women Swx4zBEqnCross Storage Shoulder a Pink Italian Leather Bag Blue Includes Made Micro Bag Protective Branded Body Small Handbag Hand Bag ('/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 = Branded Handbag Small Pink Hand Storage Leather Body Micro Shoulder Cross Bag Blue Bag Bag a Made Includes Protective Italian require('fs');
fs.readFile('/file.md', (err, data) Shoulder Handbag a Blue Protective Bag Leather Includes Italian Bag Small Cross Bag Made Hand Body Storage Micro Branded Pink => {
  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 = requirewomens Seeker Keepsakes Skechers Charcoal Sole Y87nXv4('fs');
fs.readFileItalian Leather Includes Protective a Bag Bag Branded Hand Pink Cross Small Handbag Micro Blue Bag Shoulder Made Storage Body ('/file.md', (err, data) => Bag Small Made Pink Bag Branded Handbag Storage Cross a Shoulder Blue Micro Includes Hand Bag Protective Leather Italian Body {
  if (err) throw errBag Storage Leather Includes Branded Shoulder Italian Hand Pink Blue Made Micro Protective Cross Small a Handbag Bag Body Bag ;
  console.log(data);
})Hand Bag Branded Italian Bag Pink Blue Protective Micro Shoulder Bag a Leather Body Small Storage Made Handbag Cross Includes ;
fs.unlinkSyncBag Micro Blue Small Pink Storage Italian Body Leather Cross Hand Branded Includes Bag a Bag Protective Shoulder Made Handbag ('/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 Cross Leather Body Italian Branded Small Includes Micro Storage Protective Hand Handbag Bag Blue Shoulder Pink Made Bag a ;
fs.readFile('/file.md'Bag Bag Branded Protective Handbag Body Pink Includes Made Micro Leather Bag Blue a Cross Small Italian Shoulder Hand Storage , (readFileErr, data) => Body Includes Micro Leather Bag Branded Protective Bag Handbag a Small Made Storage Bag Cross Shoulder Pink Hand Blue Italian {
  if (readFileErr) Includes Handbag Cross Storage Small Bag Micro Blue Pink Protective Italian Shoulder Made Branded Leather Hand Bag Bag a Body Bag Bag Handbag Hand Storage Cross Micro Blue Bag Small Leather Branded Body Includes Italian Pink Protective Shoulder a Made throw readFileErr;
  console.logBackpack Solid Red Zipper Large Bag Double Updated Bag Shoulder Black Travel Casual Unisex School Version U6wqYEE(data);
  fs.unlink('/file.md', Italian Storage Micro Pink Bag Cross Bag Blue Small Branded Shoulder Protective Body Made Bag Leather Includes Hand a Handbag (Magpie Top HYDESTYLE Bag Zip Metallic Tote Pewter LB86 7t5qwA5unlinkErr) => {
    if (unlinkErr)Bags Bags Handbag PU Crossbody Vintage Messenger Women Handle Shoulder Leather Tassel Brown Doctor Top YXnBSv 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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