[LUCENE-7891] Default LRUType of LruTaxonomyWriterCache should be guaranteed to be correct

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 6.4, 6.4.1, 6.4.2, 6.5, 6.5.1, 6.6
• Fix Version/s: 7.1, 8.0
• Component/s: core/other
• Lucene Fields: New, Patch Available
• Assignee: Unassigned
• Reporter: Jan-Willem van den Broek
• Created: 29/Jun/17 11:42
• Updated: 28/Aug/22 15:17
• Resolved: 05/Sep/17 14:15

Description

LruTaxonomyWriterCache uses LRUType.LRU_HASHED by default. This has a very small but non-zero chance of producing incorrect results due to collisions in the longHashCode of FacetLabel. If such a collision occurs, then an affected document will get an incorrect facet.

This has happened to us in production. While it is a rare occurrence, the consequences could be significant, and it was not immediately obvious what caused the problem. Therefore I think it is better if the default is changed to LRUType.LRU_STRING, which is guaranteed to be correct.

I will add a patch containing this change as well as a test for the issue.