File locking supported on all platforms?

[Peter Levart]

Hi Stefan,

You can always use the "create file exclusively" feature that should 
work on any POSIX compliant filesystem (NFS included):

Files.createFile(path)

...you could implement an "exclusive lock" mechanism with this by 
creating and then updating a lock file periodically so that its "last 
modified" time is kept recent. Unlock would just remove the file. 
Observing the file with expired timestamp will give you permission to 
remove it even if you are not the lock owner.

Regards, Peter

On 7/11/20 12:13 PM, Stefan Reich wrote:
> Hmm that's what I feared. So to be really sure I should do an actual 
> test. Maybe I can get away with doing two locks on the same file 
> within one JVM for testing the feature? I should try that.
>
> Thanks
>
> On Sat, 11 Jul 2020 at 12:10, Jonas Konrad <me at yawk.at 
> <mailto:me at yawk.at>> wrote:
>
>     I think it should just fail silently / without error, that's what
>     fcntl
>     does at least. I'm not sure if there's a good way to determine
>     whether
>     locking will actually work.
>
>     - Jonas
>
>     On 7/11/20 11:47 AM, Stefan Reich wrote:
>     > Hi Jonas,
>     >
>     > what would happen if I try to lock a file on these platforms? Can I
>     > reliably detect this case? That would already help a lot.
>     >
>     > The use case is ensuring a database's consistency which I now do
>     over a
>     > socket protocol, but file locks would be faster and more reliable.
>     >
>     > Stefan
>     >
>     > On Sat, 11 Jul 2020 at 11:45, Jonas Konrad <me at yawk.at
>     <mailto:me at yawk.at>
>     > <mailto:me at yawk.at <mailto:me at yawk.at>>> wrote:
>     >
>     >     Hey,
>     >
>     >     Some file systems on Linux, eg old NFS, do not support
>     locking at all.
>     >
>     >     - Jonas
>     >
>     >     On 7/11/20 11:36 AM, Stefan Reich wrote:
>     >      > Hi,
>     >      >
>     >      > a quick question regarding file locks in Java. I seem to
>     remember
>     >      > reading somewhere that this feature isn't guaranteed to
>     work on all
>     >      > platforms. However, I can now find no mention of that
>     e.g. here
>     >      >
>     >   
>      <https://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileLock.html>.
>     >      >
>     >      > The page does list that numerous special assumptions may
>     fail, for
>     >      > example that programs which /don't/ lock a file are
>     prevented from
>     >      > changing it when it is locked by another program.
>     >      >
>     >      > However, I only care about the basic feature, namely that one
>     >     local file
>     >      > cannot be locked by two Java programs at once and will
>     cause one
>     >     of them
>     >      > to throw an exception.
>     >      >
>     >      > Is this guaranteed to work everywhere?
>     >      >
>     >      > Many greetings
>     >      > Stefan
>     >      >
>     >      > --
>     >      > Stefan Reich
>     >      > BotCompany.de // Java-based operating systems
>     >
>     >
>     >
>     > --
>     > Stefan Reich
>     > BotCompany.de // Java-based operating systems
>
>
>
> -- 
> Stefan Reich
> BotCompany.de // Java-based operating systems

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