Multiple instances of application using Lucene.Net - lucene

Question

I'm developing a WPF application that uses Lucene.Net to index data from files being generated by a third-party process. It's low volume with new files being created no more than once a minute.
My application uses a singleton IndexWriter instance that is created at startup. Similarly an IndexSearcher is also created at startup, but is recreated whenever an IndexWriter.Commit() occurs, ensuring that the newly added documents will appear in search results.
Anyway, some users need to run two instances of the application, but the problem is that newly added documents don't show up when searching within the second instance. I guess it's because the first instance is doing the commits, and there needs to be a way to tell the second instance to recreate its IndexSearcher.
One way would be to signal this using a file create/update in conjunction with a FileSystemWatcher, but first wondered if there was anything in Lucene.Net that I could utilise?

Answer 1

The only thing I can think of that might be helpful for you is IndexReader.Reopen(). This will refresh the IndexReader, but only if the index has changed since the reader was originally opened. It should cause minimal disk access in the case where the index hasn't been updated, and in the case where it has, it tries to only load segments that were changed or added.
One thing to note about the API: Reopen returns an IndexReader. In the case where the index hasn't changed, it returns the same instance; otherwise it returns a new one. The original index reader is not disposed, so you'll need to do it manually:
IndexReader reader = /* ... */;
IndexReader newReader = reader.Reopen();
if(newReader != reader)
{
// Only close the old reader if we got a new one
reader.Dispose();
}
reader = newReader;
I can't find the .NET docs right now, but here are the Java docs for Lucene 3.0.3 that explain the API.

Answer 2

If both instance have their own IndexWriter opened on the same directory, you're in for a world of pain and intermittent bad behaviour.
an IW expects and requires exclusive control of the index directory. This is the reason for the lock file.
If the second instance can detect that there is a existing instance, then you might be able to just open an IndexReader/Searcher on the folder and reopen when the directory changes.
But then what happens if the first instance closes? The index will nolonger be updated. So the second instance would need to reinitialise, this time with an IW. Perhaps it could do this when the lock file is removed when the first instance closes.
The "better" approach would be to spin up a "service" (just a background process, maybe in the system tray). All instances of the app would then query this service. If the app is started and the service is not detected then spin it up.