What is the API Composition pattern in microservices?

API Composition solves the problem of a query that needs data spread across multiple services' own databases — a composer (often the API Gateway or a dedicated aggregator service) calls each relevant service, then combines their responses into a single result for the client. It's the straightforward alternative to CQRS's dedicated read-model approach, simple to implement but less efficient for very large datasets or complex joins across many services.