Polyglot persistence

Store each kind of data in the database whose guarantees it needs: strongly consistent SQL for money, a wide-column store for high-volume events, a search engine for text. One database for everything imposes the most expensive guarantees, and their cost, on all of it.

Where it shows up in real systems

  • Distributed SQL

    Each write is committed through a consensus protocol (Raft or Paxos), so it is never lost and every read is up to date, even across regions. The price is latency: a write must reach a majority of replicas, and across regions that means tens of milliseconds.

    In production: Google Cloud Spanner, CockroachDB, YugabyteDB, TiDB, Amazon Aurora DSQL

  • Wide-column store

    Every node accepts writes and each row is stored on RF replicas. The consistency level decides how many replicas must answer: ONE is fast and stays available when nodes fail but can return stale data; QUORUM on both reads and writes guarantees you read your latest write, at the cost of latency and availability. This is PACELC in practice.

    In production: Apache Cassandra, ScyllaDB, Amazon Keyspaces, Azure Managed Instance for Cassandra

See it break, then fix it

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