Concurrency between IndexReader and IndexWriter

My application batch adds documents to the index using IndexWriter.addDocument.
Another thread handles searchers, creating new ones as needed, based on a
policy. These searchers open a new IndexReader and there is currently no
synchronisation between this action and any being performed by my writer threads.

I wanted to use the new writer.deleteDocuments and writer.updateDocument in the
same phase as the addDocument, so I wrote some test cases to check the behaviour
of using these in the same phase and found that an IndexReader opened on the
index during this phase gives some odd values and this has upset my
understanding of the concurrency issue...

For example, the following

------------------------------------
Create IndexWriter
Loop IndexWriter.addDocument * count

Create IndexReader
Check numDocs
Check maxDoc
Close reader

Loop IndexWriter.deleteDocuments * count

Create IndexReader
Check numDocs
Check maxDoc
Close reader

Close IndexWriter
------------------------------------

However, the numDocs shows interesting numbers with different values of count.

count = 2, numDocs = 0 after add, 0 after delete
count = 100, numDocs = 100 after add and 100 after delete
count = 127, numDocs = 120 after add, 120 after delete
count = 150, numDocs = 150 after add and 150 after delete
count = 1000, numDocs = 1000 after add and 0 after delete

I then checked how terms returned via a TermEnum were affected and these too
also do not reflect the current state of a deleted document.

I know these numbers are affected by the DEFAULT_MAX_BUFFERED_DELETE_TERMS and
DEFAULT_MERGE_FACTOR.

My question is therefore: how can actions by a reader determine the real state
of a Document or Term if a writer is currently updating the index. Using
reader.isDeleted(docNum) shows an item is not deleted even though it has been,
but not flushed. reader.hasDeletions() also shows false.

My index app never actually uses reader.document() as it collects and caches Id
terms using TermEnum when opening a reader and stale Ids are handled elsewhere,
however, as it stands, the following logic

if (!reader.isDeleted(n))
doc = reader.document(n)

can fail with an IllegalArgumentException if the concurrent writer flushes in
between the test and read.

Thanks
Antony



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