In my Java app, on Linux, I need to periodically read some text files that change often. (these text files are updated by a separate app). Do I need to be concerned about the rare case when attempting to read the file at the exact moment it is being updated? If so, how can I guarantee that my reads always return without failing? Does the OS handle this for me, or could I potentially read 1/2 a file?
thanks.
The OS can help you achieve consistent reads, but it requires that both apps are written with this in mind.
In a nutshell, you open the file in your java app with exclusive read/write permission - this ensures that no one else, including your other app is modifying the file while you are reading it. The FileLock class can help you ensure you have exclusive access to a file.
Your other app will then periodically try to write to the file. If it does this at the same time you are reading the file, then access will be denied, and the other app should retry. This is the critical part, since if the app doesn't expect the file to be unavailable and treats this as a fatal error condition, the write will fail, and app doesn't save the data and may fail/exit etc.
If the other app must always be able to write to the file, then you have to avoid using exclusive reads. Instead, you have to try to detect an inconsistent read, such as by checking the last modified timestamp when you start reading, and when you finish reading. If the timestamps are the same, then you are good to go and have a consistent read.
Yes, you need to worry about this.
No, your reads shouldn't "fail" AFAIK, unless the file is momentarily being locked, in which you can catch the exception and try again after a brief pause. You might certainly, though, get more or less data than you expected.
(If you post code we might be able to comment more accurately on what'll happen.)
int saying how many bytes were read, and if it doesn't match what you expected then there was an update.