RethinkDB

Open-source NoSQL database built for the realtime web. Stores JSON documents and pushes changes to applications as they happen, instead of requiring polling or manual refresh. Designed to make building and scaling realtime apps simpler.

Hosted under the Linux Foundation since 2017 after CNCF contributed the source under Apache 2.0. RethinkDB stands out with native changefeeds: clients subscribe to changes on tables, documents, or arbitrary ReQL query results and receive pushed updates. Unlike MongoDB or traditional document stores, changefeeds are built into the query language rather than bolted on. Official drivers for Python, Ruby, Node.js, Java and others pair with realtime stacks like Socket.io or SignalR.

Key capabilities:

• Changefeeds for tables, single documents, or filtered and aggregated query results
• ReQL, a functional query language with joins, map-reduce, and geospatial support
• Cluster management API for sharding and replication via web UI or programmatic control
• Secondary indexes, table joins, and nested document access
• Web-based administration tools and deployment options for Linux and macOS

Suited for collaborative editors, live dashboards, chat applications, multiplayer game state, and realtime analytics. Developers subscribe to change streams with a few lines of ReQL, integrate with existing web frameworks, and scale horizontally when needed. Geospatial queries support location-aware apps. Integrations exist for Elasticsearch, RabbitMQ, and Rails.