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41 lines
1.9 KiB
Markdown
41 lines
1.9 KiB
Markdown
# Request-Reply and Scatter-Gather
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NATS supports two flavors of request reply messaging: point-to-point or one-to-many. Point-to-point involves the fastest or first to respond. In a one-to-many exchange, you can set a limit on the number of responses the requestor may receive or use a timeout to limit on the speed of the response. One-to-many request reply is sometimes called *scatter gather*.
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In a request-response exchange the publish request operation publishes a message with a reply subject expecting a response on that reply subject. Many libraries allow you to use a function that will automatically wait for a response with a timeout. You can also handle that waiting process yourself.
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The common pattern used by the libraries is that the request creates a unique inbox and performs a request call with the inbox reply and returns the first reply received. This is optimized in the case of multiple responses by ignoring later responses automatically.
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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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sub2 [shape=box, style="rounded", label="Subscriber"];
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sub3 [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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reply -> sub3 [style="invis", weight=2];
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subject -> sub1 [label="msg1", style="dotted"];
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subject -> sub2 [label="msg1", style="dotted"];
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subject -> sub3 [label="msg1"];
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sub3 -> reply;
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reply -> publisher;
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}
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</code></div>
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Try NATS request reply on your own, using a live server by walking through the [request/reply tutorial](../tutorials/reqreply.md).
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