How to ensure that a message from the queue is processed exactly once if there are multiple workers?

To ensure that a message from the queue is processed exactly once when multiple workers are present, mechanisms that guarantee atomicity and processing control should be used:

• Use of transactions and acknowledgements: Workers should acknowledge message processing only after successful completion. If a worker crashes before acknowledgment, the message is returned to the queue.

• Message locking: The queue should support a message locking mechanism during processing so that other workers cannot take it.

• Idempotency of processing: Even with redelivery, processing should be safe and not lead to duplicated effects.

• Use of specialized queues: For example, RabbitMQ with acknowledgments, Kafka with offset control and commit, or Redis Streams with consumer groups.

Example with RabbitMQ in Go (using the streadway/amqp library):

msgs, err := ch.Consume(
    queueName, "", false, false, false, false, nil,
)

for d := range msgs {
    // Process message
    err := processMessage(d.Body)
    if err == nil {
        d.Ack(false) // Acknowledge processing
    } else {
        d.Nack(false, true) // Message will be returned to the queue
    }
}

Thus, only one worker will acknowledge the message, and it will not be processed again.