Google Cloud Platform for Developers

Nacking messages

In many cases, a service can quickly identify that a given message should not be acknowledged. This may occur when a precondition is not met or a client is overloaded. In such cases, the client may nack the message. In Cloud Pub/Sub, a nack is performed by setting the message acknowledgment deadline to zero. Doing so will make the message immediately available for redelivery, which is useful for subscriptions with long default ack deadlines. For pull subscriptions, nacking messages requires an additional API call to Cloud Pub/Sub. For push subscriptions, the service needs to simply return any non-successful HTTP status code.