Flatten graph token filter

The flatten_graph token filter accepts an arbitrary graph token stream, such as that produced by Synonym graph, and flattens it into a single linear chain of tokens suitable for indexing.

This is a lossy process, as separate side paths are squashed on top of one another, but it is necessary if you use a graph token stream during indexing because a Lucene index cannot currently represent a graph. For this reason, it’s best to apply graph analyzers only at search time because that preserves the full graph structure and gives correct matches for proximity queries.

For more information on this topic and its various complexities, please read the Lucene’s TokenStreams are actually graphs blog post.