[LUCENENET-600] Creating an IndexWriter with a RAMDirectory causes two exceptions to be thrown

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Won't Fix
• Affects Version/s: Lucene.Net 4.8.0
• Component/s: Lucene.Net Core
• Assignee: Unassigned
• Reporter: Howard van Rooijen
• Created: 11/Apr/18 16:10
• Updated: 21/Jan/19 08:13
• Resolved: 21/Jan/19 08:13

Description

I have a document scoring algorithm built on top of Lucene. I've just upgraded it to the 4.8.0-beta00005 packages (great job by the way).

We essentially create an in memory index for a single document in order to do some parsing / processing / scoring / classification.

I noticed while running our test suite that the CPU was spiking and also noticed that a large number of first chance exceptions were being generated by these two lines of code:

var directory = new RAMDirectory();
var indexWriter = new IndexWriter(directory, new IndexWriterConfig(LuceneVersion.LUCENE_48, new ScorableDocumentAnalyzer(LuceneVersion.LUCENE_48)));

The first exception is:

'System.IO.FileNotFoundException' in Lucene.Net.dll ("segments.gen"). 

The second exception is:

'Lucene.Net.Index.IndexNotFoundException' in Lucene.Net.dll ("no segments* file found in RAMDirectory@21af1a5 lockFactory=Lucene.Net.Store.SingleInstanceLockFactory:

Based on reading / research, I believer this is because the RAMDirectory is initialised to be null, and when the IndexWriter is created it tries to query the RAMDirectory and FileNotFoundException is thrown.

Is it possible to either initialized as empty rather than null - i.e. reading the directory would not throw an exception - this might involve trying to add an "segments.gen" entry and a matching "segments_n" segmentinfo entry, alternatively is it possible not to throw an exception in this use case? 

Or do you have a suggestion for how it would be possible to manually initialise the RAMDirectory before passing it to the IndexWriter?

Because these two lines are being called per request - we're seeing 2 exceptions per request - this seems like an expensive way of initialising an IndexWriter. We've already had to replace QueryParser with SimpleQueryParser because QueryParser was throwing 50+ exception internally when being instantiated.

If anyone can point me in the right direction, I'd be more than happy to try and create a fix / PR. But I'm wondering as RAMDirectory is often used for unit testing scenarios - does anyone have any deep knowledge about why this current behaviour is the default behaviour? 

Many Thanks,

Howard