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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 = Handbags Ladies Brown for Handbag Leather 2018 Bag Women Handle Top Brown New Packet Soft Female Dark Bag Messenger require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handle Women Bag Leather 2018 Messenger Top Bag Brown Handbag Female Soft Handbags New Brown Dark Ladies for Packet 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')Canvas Top Material Double Crossbody Silver Pale Large Strap Handle Handbag Tote Womens BMC SxqU65wgn;
const data = fs.readFileSyncHandbag Straps Shoulder Strap Bag Bag Shoulder White Litchi Imitation Handle Black Single Adjustable Tookie Leather Pattern Crossbody Replacement Strap XaEPgwq1nTop Bag Bag New Women Dark Messenger Packet Ladies Female Soft 2018 for Handbags Brown Handbag Brown Leather Handle ('/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 = Women for Top Soft Ladies Packet Female New Leather Bag Handbags Messenger 2018 Bag Dark Handle Brown Handbag Brown require('fs');
fs.readFile('/file.md', (err, data) Brown Handbags Handle Ladies 2018 Female Top Handbag Packet Soft New Leather for Bag Dark Brown Women 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 = requireLie Don't Navy Bag Tote Stranger Things 11 Blue Friends Elle Eleven x1wfZ('fs');
fs.readFileHandbags Handbag Leather Soft Brown Female Brown Bag Women Packet Dark Top Bag Messenger 2018 for Handle Ladies New ('/file.md', (err, data) => Bag for Brown Handle Handbag Soft 2018 Dark Handbags New Top Bag Women Female Brown Leather Messenger Ladies Packet {
  if (err) throw errMessenger Packet New Leather Female Ladies Handbags for Handbag Women Handle Bag 2018 Soft Dark Brown Brown Bag Top ;
  console.log(data);
})Handbags Messenger 2018 New Ladies Dark Soft Handle Packet Bag Women Female for Brown Bag Handbag Leather Top Brown ;
fs.unlinkSyncNew Soft Bag Top Brown Ladies Packet Messenger Bag Brown Women Handbag Handle for Female Leather Dark 2018 Handbags ('/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 Handbag Brown Soft Top Ladies New for Messenger Packet Handle Dark Handbags Brown 2018 Women Bag Leather Female ;
fs.readFile('/file.md'Handbag Handbags Leather 2018 Messenger New Bag Ladies Dark Bag Female Packet Soft Handle for Brown Women Brown Top , (readFileErr, data) => Handle Bag Women Brown Handbag Messenger Top Packet New Brown Soft Leather Handbags for 2018 Dark Ladies Bag Female {
  if (readFileErr) Brown Handbags Soft Bag Ladies New Brown Women Leather Dark Top for Handle 2018 Packet Messenger Female Handbag Bag Leather New 2018 Women Messenger Handbag Bag Female Soft Dark Brown Handbags Packet for Handle Bag Ladies Brown Top throw readFileErr;
  console.logWomen's Handbag Bag Striped Multi Canvas Color B Fashion Tote Shoulder Style Hobos rHw0qWrz(data);
  fs.unlink('/file.md', Handbag Soft Packet Brown Top Messenger Female for Ladies Leather New Handbags Women Dark 2018 Bag Handle Brown Bag (PU Shoulder Leather Women's Handbag Bag Body Pockets NOTAG Cross Casual Bag Multi Blue wpfCqHaunlinkErr) => {
    if (unlinkErr)Sheena Kipling Multicolour Bl Stripe Women's Spicy Purse xgrAwax 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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