File Access Strategy in a Multi-Threaded Environment (Web App)

Question

I have a file which is an XML representation of some data that is taken from a Web service and cached locally within a Web Application. The idea being is that this data is very static, but just might change. So I have set it up to cache to a file, and stuck a monitor against it to check if it has been deleted. Once deleted, the file will be refreshed from its source and rebuilt.

I am now running in to problems though, because obviously in a multi-threaded environment it falls over as it is trying to access the data when it is still reading/writing the file.

This is confusing me, because I added a object to lock against, and this is always locked during read/write. It was my understanding that attempted access from other threads would be told to "wait" until the lock was released?

Just to let you know, I am real new to multi-threaded development, so I am totally willing to accept this is a screw up on my part :)

• Am I missing something?
• What is the best file access strategy in a multi-threaded environment?

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Edit

Sorry - I should have said this is using ASP.NET 2.0 :)

Answer 1

Here is the code that I use to make sure a file is not locked by another process. It's not 100% foolproof, but it gets the job done most of the time:

    /// <summary>
    /// Blocks until the file is not locked any more.
    /// </summary>
    /// <param name="fullPath"></param>
    bool WaitForFile(string fullPath)
    {
        int numTries = 0;
        while (true)
        {
            ++numTries;
            try
            {
                // Attempt to open the file exclusively.
                using (FileStream fs = new FileStream(fullPath,
                    FileMode.Open, FileAccess.ReadWrite, 
                    FileShare.None, 100))
                {
                    fs.ReadByte();

                    // If we got this far the file is ready
                    break;
                }
            }
            catch (Exception ex)
            {
                Log.LogWarning(
                   "WaitForFile {0} failed to get an exclusive lock: {1}", 
                    fullPath, ex.ToString());

                if (numTries > 10)
                {
                    Log.LogWarning(
                        "WaitForFile {0} giving up after 10 tries", 
                        fullPath);
                    return false;
                }

                // Wait for the lock to be released
                System.Threading.Thread.Sleep(500);
            }
        }

        Log.LogTrace("WaitForFile {0} returning true after {1} tries",
            fullPath, numTries);
        return true;
    }

Obviously you can tweak the timeouts and retries to suit your application. I use this to process huge FTP files that take a while to be written.

Answer 2

If you're locking on a object stored as a static then the lock should work for all threads in the same Application Domain, but perhaps you need to upload a code sample so we can have a look at the offending lines.

That said, one thought would be to check if IIS is configured to run in Web Garden mode (i.e. more than 1 process executing your application) which would break your locking logic. While you could fix such a situation with a mutex it'd be easier to reconfigure your application to execute in a single process, although you'd be wise to check the performance before and after messing with the web garden settings as it can potentially affect performance.