InfluxDB

InfluxDB is a time-series database built for metrics, events, and IoT data, handling millions of writes per second with built-in downsampling and retention policies. Regular databases can store time series data, but InfluxDB is optimized for high write throughput and fast range queries like "show me CPU usage for the last 24 hours."

Apache 2.0, recently rewritten from Go to Rust (v3). The query language is SQL-compatible now (v3), dropping the custom InfluxQL and Flux languages from earlier versions. Handles millions of writes per second. Built-in downsampling, retention policies, and continuous queries.

InfluxDB Cloud has a free tier: limited writes and queries, 30-day retention. Self-hosting is free with no feature restrictions. Cloud paid plans start at ~$0.002/MB written. Self-hosting v3 requires some infrastructure knowledge but isn't awful: single binary, configure storage. The ops burden is moderate: you need to manage retention policies, backups, and storage growth. Time series data accumulates fast. Solo: cloud free tier for prototyping, self-host for production. Small teams: self-host, $10-30/mo in compute. Medium to large: evaluate InfluxDB Cloud vs self-hosted ops cost.

The catch: InfluxDB has gone through three major architecture rewrites (Go monolith → Go with TSM → Rust with Apache Arrow). Each version had a different query language and different trade-offs. The ecosystem documentation is fragmented; make sure you're reading v3 docs, not v1 or v2 guides.