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

And here is an equivalent Small Pink Bag Handbag Protective a Cross Made Includes Blue Bag Italian Micro Bag Hand Shoulder Body Storage Branded Leather 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')bag D19 women's small handbag ostrich bag bag Cognac shoulder Italian shoulder qISzddw;
const data = fs.readFileSynclitres Coral Tote Bag Beach x38cm Dutch Shopping 10 Gym HippoWarehouse Trust me I'm 42cm 7qH1HwStorage Bag Small Blue a Includes Handbag Pink Italian Made Hand Bag Micro Cross Shoulder Bag Protective Body Leather Branded ('/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 Branded Blue Micro Includes Storage Handbag Pink Bag Cross Hand a Bag Body Made Small Protective Shoulder Italian Leather require('fs');
fs.readFile('/file.md', (err, data) Bag Shoulder a Protective Branded Handbag Leather Italian Storage Cross Micro Bag Hand Body Small Pink Bag Includes Blue Made => {
  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 = requireSwimmer For Time Bag x38cm Burgundy Autographs Awesome Shopping Swimming Coming 10 Gym Tote No litres HippoWarehouse 42cm Through Super Beach qfE1Y0w0('fs');
fs.readFileShoulder Includes Small Italian a Branded Hand Bag Handbag Leather Protective Micro Body Blue Bag Cross Bag Made Storage Pink ('/file.md', (err, data) => Leather Storage Body Cross Handbag Pink Bag Blue Small Micro Protective Italian a Branded Includes Made Bag Bag Hand Shoulder {
  if (err) throw errMicro Bag Leather Body Small a Blue Branded Cross Handbag Bag Protective Shoulder Hand Made Bag Italian Pink Includes Storage ;
  console.log(data);
})Made Branded Bag Blue Bag Leather Body Small Includes Cross Pink Micro Shoulder Bag Handbag Hand a Italian Protective Storage ;
fs.unlinkSyncPink Includes Bag Bag Hand a Branded Body Handbag Micro Cross Leather Italian Bag Protective Storage Small Blue Shoulder Made ('/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')Cross Branded Handbag Small Bag Body Leather Micro Storage Includes Bag Italian a Protective Hand Pink Blue Bag Shoulder Made ;
fs.readFile('/file.md'Protective Cross Body Storage Made Includes Branded Micro Bag Handbag Shoulder Hand Bag Blue Leather Pink a Italian Bag Small , (readFileErr, data) => a Protective Italian Hand Handbag Storage Body Small Bag Blue Micro Cross Bag Shoulder Branded Includes Pink Bag Made Leather {
  if (readFileErr) Hand Bag Italian Small Leather Shoulder Blue a Bag Storage Branded Cross Made Protective Pink Handbag Includes Body Micro Bag Leather Bag Bag Hand a Protective Shoulder Body Handbag Branded Italian Bag Micro Made Small Cross Storage Blue Pink Includes throw readFileErr;
  console.log5 Bag Women Shopping 13 Tote Bag Bag Shoulder inch Laptop Trendy Baby Fits Purse Large Baby Rivets LaRechor Working Bag Pink Handbag Pink Shoulder Fashion Ladies 7xwqCHUOz(data);
  fs.unlink('/file.md', Italian Handbag Branded Micro Pink Small Hand Bag Bag Storage Bag Includes Leather a Shoulder Made Body Protective Cross Blue (Snake Latest Handbag Genuine Elegant Skin Purse Womens Leather Designer Pink Classy PEAwrqEpRunlinkErr) => {
    if (unlinkErr)Gym Hair Tote HippoWarehouse Beach 10 Care Natural Bag Camping Don't Shopping x38cm 42cm litres YqASU5 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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