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Both the initiator and follower of a stream session knows how many
transfer task and receive task the stream session contains in the
preparation phase. They use the _transfers and _receivers map to track
the tasks, like below:
std::map<UUID, stream_transfer_task> _transfers;
std::map<UUID, stream_receive_task> _receivers;
A stream_transfer_task will send STREAM_MUTATION verb to transfer data
with frozen_mutation, when all the STREAM_MUTATIONs are sent, it will
send STREAM_MUTATION_DONE to tell the peer the stream_transfer_task is
completed and remove the stream_transfer_task from _transfers map. The
peer will remove the corresponding stream_receive_task in _receivers.
We do not really need the COMPLETE_MESSAGE verb to notify the peer we
have completed sending. It makes the session completion logic much
simpler and cleaner if we do not depend on COMPLETE_MESSAGE verb.
However, to be compatible with older version, we always send a
COMPLETE_MESSAGE message and do nothing in the COMPLETE_MESSAGE handler
and replies a ready future even if the stream_session is closed already.
This way, node with older version will get a COMPLETE_MESSAGE message
and manage to send a COMPLETE_MESSAGE message to new node as before.
Message-Id: <1458540564-34277-2-git-send-email-asias@scylladb.com>