ImplementaItion Notes
NodeChannel
_cur_batch -> _pending_batches: when _cur_batch is filled up, move it to _pending_batches.
add_row() just produce batches.
try_send_and_fetch_status() tries to consume one pending batch. If has in flight packet, skip send in this round.
So we can add one sender thread to be in charge of all node channels try_send.
IndexChannel
init(), open() stay the same.
Use for_each_node_channel() to expose the detailed changes of NodeChannel.(It's more easy to read & modify)
Sender thread
See func OlapTableSink::_send_batch_process()
Why use polling？
If we use wait/notify, it will notify when generate a new batch. We can't skip sending this batch, coz it won't notify the same batch again. So wait/notify can't avoid blocking simply.
So I choose polling.
It's wasting to continuously try_send(), but it's difficult to set the suitable polling interval. Thus, I add std::this_thread::yield() to give up the time slice, give priority to other process/threads (if there are other process/threads waiting in the queue).
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