java io read and write lock

Question

suppose I have a file that might gets written by one thread/process Writer and read by another thread/process Reader. Writer updates the file every x time interval, and Reader reads it every y time interval, if they happen to read and write to the file at the same time, will there be any issues? would the read block until writes finishes? or would the read fails? and vice versa? What's the best practice here?

Answer 1

You'll need to devise your own locking protocol to implement in the applications. Specifics depend on the underlying operating system, but in general, nothing will stop one process from reading a file even when another process is writing to it.

Java has a FileLock class that can be used to coordinate access to a file. However, you'll need to read the caveats carefully, especially those relating to the system-dependence of this feature. Testing the feature on the target operating system is extremely important.

A key concept of Java's FileLock is that it is only "advisory". Your process should be able to detect that another process holds a lock on a file, but your process can ignore it and do what it likes with the file, no restrictions.

The question is ambiguous whether multiple process will use the file, or merely separate threads within a single Java process. That's a big difference. If the problem requires only thread safety within a single process, a ReentrantReadWriteLock can provide a robust, high performance solution, without any platform-specific pitfalls.

Answer 2

Best practice is to not use a file for communication between processes. File are not designed for this purposes. Instead you should use messaging which IS designed for communication between processes. You can use files as well to audit what has been sent/received,

If you use files alone, you could come up with a solution which is good enough, but I don't believe you will have a solution which could be considered best practice.