See typesense as C++ implemented alternative to elasticsearch and algolia (and some claims of it being a candidate for use as vector-databases)
Story about HorizonDB — How we replaced Elasticsearch and MongoDB with Rust and RocksDB. I don’t think the product is Open Source, but it is interesting to see all the different libraries they used: tantivy, fasttext, rust, apache-spark, LightGBM, fst, RocksDB ..
Elasticsearch Was Never a Database or “Elasticsearch: Search Index, Not System of Record” by paradedb devs.