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31 lines
1.4 KiB
Markdown
31 lines
1.4 KiB
Markdown
# Acknowledgements
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In a system with at-most-once semantics, there are times when messages can be lost. If your application is doing request-reply it should use timeouts to handle any network or application failures. It is always a good idea to place a timeout on a requests and have code that deals with timeouts. When you are publishing an event or data stream, one 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 payload.
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<div class="graphviz"><code data-viz="dot">
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digraph nats_request_reply {
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rankdir=LR
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subgraph {
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publisher [shape=box, style="rounded", label="Publisher"];
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}
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subgraph {
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subject [shape=circle, label="Subject"];
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reply [shape=circle, label="Reply"];
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{rank = same subject reply}
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}
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subgraph {
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sub1[shape=box, style="rounded", label="Subscriber"];
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}
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publisher -> subject [label="msg1"];
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publisher -> reply [style="invis", weight=2];
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subject -> sub1 [label="msg1"];
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sub1 -> reply [label="ack"];
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reply -> publisher;
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}
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</code></div>
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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 a [scatter-gather pattern](reqreply.md), several other sides. |