Nginx NJS fs.writeFileSync is atomic writing and race condition prevention ?

[Valentin V. Bartenev]

On Friday, 19 October 2018 06:33:57 MSK HENG wrote:
> Hello:
> 
> I am new to Nginx NJS, and I want to write a website with NJS.
> 
> I want to write a simple JSON database with NJS fs.writeFileSync ,just like
> Node.js LowDB.
> 
> but I have no idea . Does NJS fs.writeFileSync is atomic writing and race
> condition prevention ?
> 
> If NJS fs.writeFileSync is NOT atomic writing, it can NOT be treate as a
> normal database file write.
> 
> Thank you !
> 

The fs.writeFileSync function calls a simple write(), which is atomic with
regular files.

But at the first glance to work with file like a database using multiple
nginx worker processes it's not enough to have atomic writing function.

You need to have atomic the whole sequence of operations:
read file -> update data -> write file.

Since currently there's no locking functionality in njs, the only way to
achieve this is to use only one worker process.

  wbr, Valentin V. Bartenev