AtLeastOnce

Atleastonce is a delivery guarantee used in messaging systems to ensure that each message is delivered to a consumer at least once, though duplicates may occur. It is one of several delivery guarantees, alongside at-most-once and exactly-once. In at-least-once semantics, retries and failures can cause the same message to be delivered multiple times if acknowledgments are not durably recorded or when a consumer restarts.

Operationally, messages are stored durably and delivered to consumers with acknowledgments. If a failure happens before the consumer acknowledges, the message is redelivered. Because delivery can be retried, consumers may process duplicates unless they implement idempotent handling or deduplication. Many messaging systems offer at-least-once by default or in configurable delivery modes. Examples include Apache Kafka (with its retry and offset semantics), RabbitMQ, and AWS SQS (standard queue mode). Each system may require different acknowledgment and redelivery behavior to achieve at-least-once.

Handling duplicates is a design concern: implement idempotent processing, assign unique message identifiers, and store processed IDs to prevent double effects. External systems should be written in an idempotent fashion, or a deduplication cache can be used. Practically, at-least-once reduces the risk of data loss at the cost of potential duplicate work and side effects.

In summary, at-least-once provides reliability through retries, accepting duplication as a trade-off. It is well-suited for systems prioritizing availability and durability, provided that processing can tolerate or mitigate duplicate deliveries.