java.io.File.createNewFile（）是否在网络文件系统中是原子的？

Question

EDIT : Well, I'm back a bunch of months later, the lock mechanism that I was trying to code doesn't work, because createNewFile isn't reliable on the NFS. Check the answer below.

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Here is my situation : I have only 1 application which may access the files, so I don't have any constraint about what other applications may do, but the application is running concurrently on several servers in the production environment for redundancy and performance purposes (a couple of machines are hosting each a couple of JVM with our apps).

Basically, what I need is to put some kind of flag in a folder to tell the other instances to leave this folder alone as another instance is already dealing with it.

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Many search results are telling to use FileLock to achieve this, but I checked the Javadoc, and from my understanding it will not help much, since it's using the hosting OS's locking possibilities. So I doubt that it will help much since there are different hosting machines.

This question covers a similar subject : Java file locking on a network , and the accepted answer is recommending to implement your own kind of cooperative locking process (using the File.createNewFile() as asked by the OP).

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The Javadoc of File.createNewFile() says that the process is atomically creating the file if it doesn't already exist. Does that work reliably in a network file system ?

I mean, how is it possible with the potential network lag to do both existence check and creation simultaneously ? :

The check for the existence of the file and the creation of the file if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the file.

Answer 1

No, createNewFile doesn't work properly on a network file system.

Even if the system call is atomic, it's only atomic regarding the OS, and not over the network. Over the time, I got a couple of collisions, like once every 2-3 months (approx. once every 600k files).

The thing that happens is my program is running in 6 separates instances over 2 separate servers, so let's call them A1,A2,A3 and B1,B2,B3.
When A1, A2, and A3 try to create the same file, the OS can properly ensure that only one file is created, since it is working with itself.
When A1 and B1 try to create the same file at the same exact moment, there is some form of network cache and/or network delays happening, and they both get a true return from File.createNewFile() .
My code then proceeds by renaming the parent folder to stop the other instances of the program from unnecessarily trying to process the folder and that's where it fails :

• On A1, the folder renaming operation is successful, but the lock file can't be removed, so A1 just lets it like that and keeps on processing new incoming folders.
• On B1, the folder renaming operation ( File.renameTo() , can't do much to fix it) gets stuck in a infinite loop because the folder was already renamed (also causing a huge I/O traffic according to my sysadmin), and B1 is unable to process any new file until the program is rebooted.

Answer 2

The check for the existence of the file and the creation of the file if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the file.

That can be implemented easily via the open() system call or its equivalents in any operating system I have ever used.

Answer 3

I mean, how is it possible with the potential network lag to do both existence check and creation simultaneously ?

There is a difference between simultaneously and atomically . Java doc is not saying anything about this function being a set of two simultaneous actions but two actions designed to work in atomic way. If this method is built to do two operations atomically than means file will never be created without checking file existence first and if file gets created by current call then it means there were no files present and if file doesn't get created that means there was already a file by that name.

I don't see a reason to doubt function being atomic or working reliably despite call being on network or local disk. Local call is equally unreliable - so many things can go wrong in an IO.

What you have to doubt is when trying to use empty file created by this function as a Lock as explained D-Mac's answer for this question and that is what explicitly mentioned in Java Doc for this function too.

You are looking for a directory lock and empty files working as a directory lock ( to signal other processes and threads to not touch it ) has worked quite well for me provided due care is taken to write logic to check for file existence,lock file clean up and orphaned locks.