MongoT is MongoDB's open-source Java project that wraps Apache Lucene to power Atlas Search. The post walks through the MongoT codebase, explains how $search aggregation queries flow from MongoDB driver through MongoD to MongoT via gRPC, and covers local debugging with IntelliJ and Bazel. It explores text search, faceted search, and vector search with automatic embeddings via Voyage AI. Performance benchmarks using K6 show median end-to-end response times around 12ms, with detailed breakdowns showing BSON parsing and materialization dominating over actual Lucene query time. The post also maps out MongoT's Java package structure and explains why Lucene's inverted index, multi-index intersection, and vector capabilities complement MongoDB's native B-tree indexes.
