REST works well for resource-oriented public APIs and benefits from familiar HTTP semantics. RPC exposes operations such as ReserveInventory; it can express domain commands clearly and may use generated contracts. GraphQL lets clients request a shaped graph of data and can reduce over-fetching, but needs query-cost controls, authorization at field boundaries, and caching strategy.

Asynchronous event interfaces decouple producers from consumers in time. A producer publishes an event such as OrderPlaced; consumers process it independently. This improves isolation and absorbs bursts, but introduces eventual consistency, duplicate delivery, ordering questions, and operational complexity.

Choose an interface based on the interaction. A synchronous checkout command may need an immediate acceptance result, while sending an email can be asynchronous. Mixing styles is normal when each boundary has a clear purpose.