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1. Usually, when clients send a request, they will ask the network client to send the request to a specific node. In these cases, the connection.setup.timeout won’t matter too much because the client doesn’t want to try other nodes for that specific request. The request level timeout would be enough. The metadata fetcher fetches the status of the nodes periodically so the clients will reassign the timeout request correspondingly to a different node.
2. Consumer, producer, and AdminClient are all using leastLoadedNode() for metadata fetches, where the connection setup timeout can play an important role. Unlike other requests can refer to the metadata for node condition, the metadata requests can only blindly choose a node for retry in the worst scenario. We want to make sure the client can get the metadata smoothly and as soon as possible. As a result, we need this connection.setup.timeout.
3. Implementing the timeout in NetworkClient.poll() or anywhere else might need an extra iteration of all nodes, which might downgrade the network client performance.
4. Clients fetch metadata periodically which means leastLoadedNode() will get called frequently. So the timeout checking frequency is guaranteed. 

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