Acknowledgements
In a system with at-most-once semantics, there are times when messages are lost. If your application is doing request-reply then it can simply use timeouts to handle network and application failures. When you are using one-way messaging the easiest way to insure message delivery is to turn it into a request-reply with the concept of an acknowledgement message, or ACKS. In NATS an ACK can simply be an empty message, a message with no body.
digraph nats_request_reply {
rankdir=LR
subgraph {
publisher [shape=box, style="rounded", label="Publisher"];
}
subgraph {
subject [shape=circle, label="Subject"];
reply [shape=circle, label="Reply"];
{rank = same subject reply}
}
subgraph {
sub1[shape=box, style="rounded", label="Subscriber"];
}
publisher -> subject [label="msg1"];
publisher -> reply [style="invis", weight=2];
subject -> sub1 [label="msg1"];
sub1 -> reply [label="ack"];
reply -> publisher;
}
Because the ACK can be empty it can take up very little network bandwidth, but the idea of the ACK turns a simple fire-and-forget into a fire-and-know world where the sender can be sure that the message was received by the other side, or with scatter-gather, several other sides.