NFS client side locking for a file currently being written to in the remote NFS server

Question

I have mounted a NFS mount in my local machine (So, I suppose my machine is the NFS client) and a file is being written in the NFS mount by the remote machine hosting the NFS mount (NFS server). Now, how will I be able to detect using JAVA that the file being written to in the remote machine has been closed before me reading from my local machine?

Or, are there any alternative ways to achieve what I want?

Edit: I would also like to mention that I have no control over the remote NFS server. I can only mount in read only mode.

Answer 1

As you stated that you have no control over the NFS server nor the program updating the file on the NFS server, there is not a lot you can do. The only thing you can really make use of is the fact that when a file is updated its modification time is updated. This should be available to your java client using File.lastModified(). If your application remembers the last modification date, you can then at least know if there is data to process since you last read it. It's definitely not perfect. There are race conditions etc, but depending on what exactly your application needs/does, it may be enough. You can't tell if the file is closed or open on the server.

If the file modifications are slow to appear, you could also mount using the noac mount option. While this hurts NFS performance on a regular NFS client, it may work for you if your machine is not using a lot of other NFS files on that drive. See "man nfs".

As a side note, I can't see that file locking will help as you gave no indication that the application on the NFS server uses locks anyway.