FileSystemWatcher vs polling to watch for file changes

I have seen the file system watcher fail in production and test environments. I now consider it a convenience, but I do not consider it reliable. My pattern has been to watch for changes with the files system watcher, but poll occasionally to catch missing file changes.

Edit: If you have a UI, you can also give your user the ability to "refresh" for changes instead of polling. I would combine this with a file system watcher.

The biggest problem I have had is missing files when the buffer gets full. Easy as pie to fix--just increase the buffer. Remember that it contains the file names and events, so increase it to the expected amount of files (trial and error). It does use memory that cannot be paged out, so it could force other processes to page if memory gets low.

Here is the MSDN article on buffer : FileSystemWatcher..::.InternalBufferSize Property

Per MSDN:

Increasing buffer size is expensive, as it comes from non paged memory that cannot be swapped out to disk, so keep the buffer as small as possible. To avoid a buffer overflow, use the NotifyFilter and IncludeSubdirectories properties to filter out unwanted change notifications.

We use 16MB due to a large batch expected at one time. Works fine and never misses a file.

We also read all the files before beginning to process even one...get the file names safely cached away (in our case, into a database table) then process them.

For file locking issues I spawn a process which waits around for the file to be unlocked waiting one second, then two, then four, et cetera. We never poll. This has been in production without error for about two years.

The FileSystemWatcher may also miss changes during busy times, if the number of queued changes overflows the buffer provided. This is not a limitation of the .NET class per se, but of the underlying Win32 infrastructure. In our experience, the best way to minimize this problem is to dequeue the notifications as quickly as possible and deal with them on another thread.

As mentioned by @ChillTemp above, the watcher may not work on non-Windows shares. For example, it will not work at all on mounted Novell drives.

I agree that a good compromise is to do an occasional poll to pick up any missed changes.

Also note that file system watcher is not reliable on file shares. Particularly if the file share is hosted on a non-windows server. FSW should not be used for anything critical. Or should be used with an occasional poll to verify that it hasn't missed anything.

Personally, I've used the FileSystemWatcher on a production system, and it has worked fine. In the past 6 months, it hasn't had a single hiccup running 24x7. It is monitoring a single local folder (which is shared). We have a relatively small number of file operations that it has to handle (10 events fired per day). It's not something I've ever had to worry about. I'd use it again if I had to remake the decision.

I currently use the FileSystemWatcher on an XML file being updated on average every 100 milliseconds.

I have found that as long as the FileSystemWatcher is properly configured you should never have problems with local files.

I have no experience on remote file watching and non-Windows shares.

I would consider polling the file to be redundant and not worth the overhead unless you inherently distrust the FileSystemWatcher or have directly experienced the limitations everyone else here has listed (non-Windows shares, and remote file watching).

I have run into trouble using FileSystemWatcher on network shares. If you're in a pure Windows environment, it might not be an issue, but I was watching an NFS share and since NFS is stateless, there was never a notification when the file I was watching changed.

I'd go with polling.

Network issues cause the FileSystemWatcher to be unreliable (even when overloading the error event).

I had some big problems with FSW on network drives: Deleting a file always threw the error event, never the deleted event. I did not find a solution, so I now avoid the FSW and use polling.

Creation events on the other hand worked fine, so if you only need to watch for file creation, you can go for the FSW.

Also, I had no problems at all on local folders, no matter if shared or not.

Returning from the event method as quickly as possible, using another thread, solved the problem for me:

private void Watcher_Created(object sender, FileSystemEventArgs e)
{
    Task.Run(() => MySubmit(e.FullPath));
}