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Transport Next Generation
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Author: Bela Ban
The next version of the transport (org.jgroups.protocols.TP) should be NIO based: TP has an NIO selector, and subclasses
such as TCP or UDP only register NIO channels with TP. For example, UDP would create 2 NIO channels, a unicast and a
multicast channel and register them with TP. TCP would do an accept() in a loop and, whenever a new peer connects,
register the client's NIO socket channel with TP's selector.
TP would therefore be a multiplexer which handles a number of connections, be they TCP or UDP connections. Therefore,
the connection table functionality of TCP would be largely removed, because this is now handled by TP itself.
This requires JDK 7, because NetworkChannel.open() does not yet exist in prior JDKs. To be more precise, it does exist,
but only for datagram sockets (DatagramChannel.open()), not for multicast sockets (no MulticastChannel.open()).
Transport NetGen should be combined with the copyless stack (https://jira.jboss.org/jira/browse/JGRP-809). On the
receive side, this is done by passing the selection key to a thread from the thread pool. If the key has an attachment,
it is the ByteBuffer previously created by a (possibly different) thread to receive the message. The receiver thread
will then simply call read() (or receive()) on the input NIO channel and - when all bytes have been received (defined
by the initial length field of a message) - unmarshall the buffer and pass the resulting message up the stack.
When there is no attachment, the receiver thread creates one (according to the length field which prefixes each message)
and reads the bytes available from the NIO channel into it. If the number of bytes read is equals to the expected length,
the thread proceeds to unmarshalling and passing the message up. Otherwise, it attaches the ByteBuffer to the selection
key and returns. Later, a (potentially) different thread will complete reading the full message and then finish the job.
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