When APM index documents, are the master nodes who writes them into the correct index? In that case, wouldn't it be more effective to aim directly to master nodes?
You should not send any client traffic to your master nodes if possible. Use the coordinating nodes.
It's not essential, but it does save a network hop. Quoting this article:
Since Elasticsearch will automatically reroute ingest requests to ingest nodes, you don't have to specify or configure any routing information. However, if you're doing heavy ingestion and have dedicated ingest nodes, it makes sense to send index requests to these nodes directly, to avoid any extra hops in the cluster.
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