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Test for understanding how blocking works
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| const fs = require('fs'); | |
| const log = (text) => { | |
| console.log(new Date() + ' ' + text); | |
| } | |
| const util = require('util'); | |
| const sleep = async ms => { | |
| return new Promise(resolve => setTimeout(resolve, ms)); | |
| }; | |
| const asyncFunc1 = async () => { | |
| log('f1 Start'); | |
| await sleep(2000); | |
| log('f1 End'); | |
| }; | |
| const asyncFunc2 = async () => { | |
| log('f2 Start'); | |
| await sleep(1000); | |
| let fd = await util.promisify(fs.open)('/tmp/example.txt', 'w'); | |
| var longString = (new Array(10*1024*1024)).join("x"); | |
| syncFunc(fd, longString); | |
| log('f2 End'); | |
| }; | |
| const syncFunc = async (fd, content) => { | |
| console.log('### do something sync...'); | |
| for (let i = 0; i < 1000; i++) { | |
| fs.writeSync(fd, content); | |
| } | |
| }; | |
| log('### Startin...'); | |
| asyncFunc1(); | |
| log('### do another job....'); | |
| asyncFunc2(); | |
| log('### Stoppin...'); |
Now try changing line 26...
from
const syncFunc = (fd, content)
to
const syncFunc = async (fd, content)
Updated, re-run:
$ node blocking-test.js
Thu Nov 14 2019 10:03:20 GMT+0000 (Greenwich Mean Time) ### Startin...
Thu Nov 14 2019 10:03:20 GMT+0000 (Greenwich Mean Time) f1 Start
Thu Nov 14 2019 10:03:20 GMT+0000 (Greenwich Mean Time) ### do another job....
Thu Nov 14 2019 10:03:20 GMT+0000 (Greenwich Mean Time) f2 Start
Thu Nov 14 2019 10:03:20 GMT+0000 (Greenwich Mean Time) ### Stoppin...
### do something sync...
Thu Nov 14 2019 10:03:48 GMT+0000 (Greenwich Mean Time) f2 End
Thu Nov 14 2019 10:03:48 GMT+0000 (Greenwich Mean Time) f1 End
Would need to put syncFunc inside a setTimeout but, since it uses synchronous IO, to my understanding it will always run to completion without being interrupted by other events.
This is mine:
$ node --version
v10.14.1
$ node main.js
Thu Nov 14 2019 09:50:27 GMT+0000 (Greenwich Mean Time) ### Startin...
Thu Nov 14 2019 09:50:27 GMT+0000 (Greenwich Mean Time) f1 Start
Thu Nov 14 2019 09:50:27 GMT+0000 (Greenwich Mean Time) ### do another job....
Thu Nov 14 2019 09:50:27 GMT+0000 (Greenwich Mean Time) f2 Start
Thu Nov 14 2019 09:50:27 GMT+0000 (Greenwich Mean Time) ### Stoppin...
Thu Nov 14 2019 09:50:29 GMT+0000 (Greenwich Mean Time) f1 End
### do something sync...
Thu Nov 14 2019 09:50:50 GMT+0000 (Greenwich Mean Time) f2 End
What version of node are you running?
$ node -v
v10.17.0
Did the semantics of sync IO functions change between Node versions? Sounds like a large backward compatibility break.
Upgraded, no change:
[10:21:58][giorgio@Newton:~]$ node -v
v12.13.0
[10:22:21][giorgio@Newton:~]$ node blocking-test.js
Thu Nov 14 2019 10:22:25 GMT+0000 (Greenwich Mean Time) ### Startin...
Thu Nov 14 2019 10:22:25 GMT+0000 (Greenwich Mean Time) f1 Start
Thu Nov 14 2019 10:22:25 GMT+0000 (Greenwich Mean Time) ### do another job....
Thu Nov 14 2019 10:22:25 GMT+0000 (Greenwich Mean Time) f2 Start
Thu Nov 14 2019 10:22:25 GMT+0000 (Greenwich Mean Time) ### Stoppin...
### do something sync...
Thu Nov 14 2019 10:22:58 GMT+0000 (Greenwich Mean Time) f2 End
Thu Nov 14 2019 10:22:58 GMT+0000 (Greenwich Mean Time) f1 End
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Sample output (writes a huge file on disk):