The code sample suggests it's a network interface at localhost, but it's not 2M syscalls or connections. It's just one big buffered write and it doesn't look like they're waiting for responses to those messages - so essentially this might just be a (pretty good?) stream processing app.
It is not one big write, but optimizations around msgs/write using buffering are used in clients, with obvious care to balance latency and throughput. In the benchmark, the write buffer is 16k, so it is flushed automatically via Go's bufio when it hits that mark, and then I flush it again when the loop is complete, flushing the remainder of the outbound buffer. I then use a PING/PONG, which is part of the NATS protocol, to only stop timing when the PONG returns, and I know all messages have been processed. NATS does have a verbose protocol flag that has all protocol frames ack'd with either +OK or -ERR.
