RethinkDB (Document/Realtime): Replaced Riak and CouchDB, chosen for its innovative real-time push capabilities. 2025 Relevance: Low (historical). Unfortunately, RethinkDB as a commercial entity ceased operations, though its open-source project continues. This highlights the inherent risk in technical books covering specific rapidly evolving technologies.
Cassandra (Column-Family): Replaced HBase, reflecting Cassandra's growing enterprise adoption for its always-on architecture and impressive horizontal scalability. 2025 Relevance: High. for extremely large, high-throughput, and always-on data systems.
Datomic (Immutable/Time-Travel): An innovative database with an taiwan phone number list immutable data model, allowing "time-travel" queries. 2025 Relevance: Moderate (niche/academic interest). While conceptually fascinating and influential, its adoption is still relatively niche compared to other mainstream databases.
The shift in the lineup between editions perfectly illustrates the book's commitment to capturing the contemporary landscape while simultaneously demonstrating how quickly specific implementations can rise and fall in prominence.