What is polyglot persistence?

Polyglot persistence is the practice of using different database technologies for different microservices, choosing the best tool for each service's specific data needs rather than forcing all services to use one shared database. For example: the product catalog service uses Elasticsearch for full-text search, the session service uses Redis for fast key-value lookups, the order service uses PostgreSQL for ACID-compliant relational data, and the activity feed service uses Cassandra for high-write time-series data. This approach maximizes efficiency but requires operational expertise across multiple database technologies and makes cross-service queries (which should be avoided anyway) impossible via SQL joins.