Question

Here’s the scenario: I have a multi threaded java web application which is running inside a servlet container. The application is deployed multiple times inside the servlet container. There are multiple servlet containers running on different servers.

Perhaps this graph makes it clear:

server1 +- servlet container    +- application1    |  +- thread1    |  +- thread2    +- application2       +- thread1       +- thread2 server2 +- servlet container    +- application1    |  +- thread1    |  +- thread2    +- application2       +- thread1       +- thread2 

There is a file inside a network shared directory which all those threads can access. And they do access the file frequently. Most of the time the file is only read by those threads. But sometimes it is written.

I need a fail safe solution to synchronize all those threads so data consistency is guaranteed.

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Solutions which do not work (properly):

1. Using java.nio.channels.FileLock
   I am able to synchronize threads from different servers using the FileLock class. But this does not work for threads inside the same process (servlet container) since file locks are available process wide.

2. Using a separate file for synchronization
   I could create a separate file which indicates that a process is reading from or wrinting to the file. This solution works for all threads but has several drawbacks:

   • Performance. Creating, deleting and checking files are rather slow operations. The low weight implementations with one synchronization file will prevent parallel reading of the file.
   • The synchronization file will remain after a JVM crash making a manual clean up necessary.
   • We have had already strange problems deleting files on network file systems.

3. Using messaging
   We could implement a messaging system which the threads would use to coordinate the file access. But this seems too complex for this problem. And again: performance will be poor.

Any thoughts?

Answer 1

If you only need to write the file rarely, how about writing the file under a temporary name and then using rename to make it ‘visible’ to the readers?

This only works reliably with Unix file systems, though. On Windows, you will need to handle the case that some process has the file open (for reading). In this case, the rename will fail. Just try again until the rename succeeds.

I suggest to test this thoroughly because you might run into congestion: There are so many read requests that the writer task can’t replace the file for a long time.

If that is the case, make the readers check for the temporary file and wait a few moments with the next read until the file vanishes.