Message delivery semantics between microservices.

The semantics of message delivery between microservices describes the guarantees that the system provides when transmitting messages:

• At most once — the message may be delivered once or not at all (loss is possible). Used when message loss is not critical.

• At least once — the message will be delivered at least once, but duplication is possible. Requires idempotent processing.

• Exactly once — the message is delivered exactly once without duplicates or loss. The most complex semantics, requiring complex acknowledgment mechanisms and transactions.

The choice of semantics depends on the reliability and performance requirements of the system. For example, "exactly once" is important for financial transactions, while "at most once" may be sufficient for logging.